metals
Article
Reduction of Die Wear and Structural Defects of Railway Screw
Spike Heads Estimated by FEM
Jackeline Alcázar 1, Germán Abate 1,2, Nazareno Antunez 1,2, Alejandro Simoncelli 1,2, Antonio J. Sánchez Egea 3 ,
Daniel Martinez Krahmer 1,2 and Norberto López de Lacalle 4,*


Citation: Alcázar, J.; Abate, G.;
Antunez, N.; Simoncelli, A.;
Egea, A.J.S.; Krahmer, D.M.;
López de Lacalle, N. Reduction of Die
Wear and Structural Defects of
Railway Screw Spike Heads
Estimated by FEM. Metals 2021, 11,
1834. https://doi.org/10.3390/
met11111834
Academic Editors: Koh-ichi Sugimoto
and Badis Haddag
Received: 26 September 2021
Accepted: 12 November 2021
Published: 15 November 2021
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Copyright: © 2021 by the authors.
Licensee MDPI, Basel, Switzerland.
This article is an open access article
distributed under the terms and
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Attribution (CC BY) license (https://
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4.0/).
1 Faculty of Engineering, Universidad Nacional de Lomas de Zamora, Juan XXIII y Camino de Cintura,
Buenos Aires 1832, Argentina; jalcazar@ingenieria.unlz.edu.ar (J.A.); gabate@inti.gob.ar (G.A.);
nantunez@inti.gob.ar (N.A.); asimoncelli@inti.gob.ar (A.S.); dmartinez@inti.gob.ar (D.M.K.)
2 Center for Research and Development in Mechanics, National Institute of Industrial Technology (INTI),
Avenida General Paz 5445, Buenos Aires 1650, Argentina
3 Department of Mechanical Engineering (EEBE), Universitat Politècnica de Catalunya,
Av. D’Eduard Maristany, 16, 08019 Barcelona, Spain; antonio.egea@upc.edu
4 Department of Mechanical Engineering, University of the Basque Country,
Escuela Superior de Ingenieros Alameda de Urquijo s/n., 48013 Bilbao, Spain
* Correspondence: norberto.lzlacalle@ehu.eus
Abstract: Railway spike screws are manufactured by hot forging on a massive scale, due to each
kilometer of railway track needing 8600 spike screws. These components have a low market value,
so the head must be formed in a single die stroke. The service life of the dies is directly related
to the amount of energy required to form a single screw. The existing standard for spike screws
specifies only the required tolerances for the head dimensions, particularly the angle of the hub faces
and the radius of agreement of the hub with the cap. Both geometrical variables of the head and
process conditions (as-received material diameter and flash thickness) are critical parameters in spike
production. This work focuses on minimizing the energy required for forming the head of a railway
spike screw by computational simulation. The variables with the highest degree of incidence on
the energy, forging load, and filling of the die are ordered statistically. The results show that flash
thickness is the variable with the most significant influence on forming energy and forming load, as
well as on die filling. Specifically, the minimum forming energy was obtained for combining of a hub
wall angle of 1.3 an as-received material diameter of 23.54 mm and a flash thickness of 2.25 mm.
Flash thickness generates a lack of filling at the top vertices of the hub, although this defect does not
affect the functionality of the part or its serviceability. Finally, the wear is mainly concentrated on the
die splice radii, where the highest contact pressure is concentrated according to the computational
simulation results.
Keywords: screw spike; hot forging; die wear; defects; computational simulation
1. Introduction
The railway transport system is one of the main transport systems used worldwide,
and railways transport both people and cargo. Its importance is due to its strong influence
on many countries’ social, economic, and industrial development [1]. Many components
of the railway system require forging, such as railway wheels, axles, crankshafts, large
and small connecting rods, disk brakes, chassis components, connection couplings, and
sleeper screws [2]. These parts require superior strength and toughness [3], given their
demanding service conditions. A critical component of the railway track is spike screws.
These parts are mass-produced because 8600 of them are used for each kilometer of track [4].
These components are manufactured by forging, which involves a press [5], dies [6], and a
lubrication system [7].
The spike screws connect the rails to the sleepers and, together with the pads, elastic
clips and guide plates constitute the fastening system [8]. The spike screw links the parts
Metals 2021, 11, 1834. https://doi.org/10.3390/met11111834 https://www.mdpi.com/journal/metals

Metals 2021, 11, 1834 2 of 11
of the track and has a safety aspect since the failure of these parts in service conditions
can cause accidents. These failures, caused mainly by the spike screw fracture, can result
from over-tightening, under-tightening, or fatigue [9]. Faria et al. [9] studied the spike
screws commonly used by the Brazilian railway sector due to their recurrent failures of
these parts during service conditions. A computational simulation helps to propose screw
spikes geometries modifications that can result in improved theoretical fatigue resistance.
Moreover, Moreira et al. [4] deployed experimental tests and computational simulation
intending to improve the screw behavior in fatigue conditions, proposing changes in the
material and the thermal treatments of the spike screws. The spike screws are manufactured
in two steps, firstly by heating one part of the blank to forge the head of the screw. Then,
the other part is heated to create the thread by rolling using three dies. It is a standard
procedure used by the forging industry in Argentina [10]. Some tests have been done by
heating both sides of the blank and forming the two sides of spike screws simultaneously,
thus increasing productivity. For example, Gontarz et al. [11] developed a mechanical press
that forges the head at each end of the blank (double configuration) and then synchronizes
this equipment with a linear wedge forging machine that threads the body of both screws.
In the end, that machine could manufacture two pieces at a time. Regarding the internal
defects produced by the wedge forging machine, Van Hai and Hong Hue [12] studied a hot
tapping process of AISI 1045 spike screws because their internal defects can significantly
reduce their fatigue strength. They were able to reproduce the defects as mentioned
above by computational simulation, a situation that enables them to carry out a future
improvement on the design of mechanical testing of these components.
The energy consumption to form this kind of component directly impacts die wear [13].
It is one of the reasons why the blockers are used in forging processes [14,15]. However,
blockers are not always possible to apply due to the low market price of these parts. Due to
these limitations, different manufacturing options must be sought, like the one suggested by
Hu et al. [16]. They proposed a multi-objective optimization approach on A309 aluminum
alloy through the design of experiments, simulation and hydraulic press forming. They
found that by combining the operational parameters, the lifespan of the dies could be
increased by 23.5% compared to the original process condition. Additionally, replacing
the forging dies significantly impacts the process costs, which can reach up to 30% [17].
In particular, wear is responsible for 70% of die failures during service conditions [18].
Many studies have focused on reducing the friction and wear conditions to increase the
lifespan of the die. For example, Behrens et al. [19] studied the relationship between
surface topography and wear. Accordingly, they fabricated a series of similar dies, whose
final surfaces were obtained by turning, milling, and blasting. After forming 500 forged
gearwheel specimens by each method, they determined that the slightest geometrical
deviation was achieved with the milled die, while the most significant deviation occurred
in the turned die [19]. Likewise, Krawczyk et al. [20] studied surface thermal softening in
forging dies. They measured surface temperatures in the order of 600 C, which produced a
softening up to a depth of 0.3 mm. The thermal impact led to a decrease in the die hardness,
which causes accelerated wear due to abrasion and plastic deformation.
The German company Hirschvogel [21], which specializes in the forging of parts for
the automotive sector, details the reasons why it is very important to simulate forging
processes: reducing the time of research and development, early detection of failure zones,
a decrease of the cutting weight, a better understanding of the process, among others. In
this way, the general procedure is to design the forging parts in CAD programs and then
study the mechanical behavior with finite element simulation programs. The objective is to
calibrate the process to reduce design failures or defects in the material due to excessive
deformations. Once manufactured, the component’s mechanical properties are studied
and its surface properties are analyzed by non-destructive testing to find cracks, folds, or
other defects. In this sense, Prabhu et al. [22] analyzed forged parts for the aeronautical
sector by combining simulation and experimental techniques to carry out its development.
After producing parts with the adjusted process, they did not present surface defects and

Metals 2021, 11, 1834 3 of 11
their mechanical properties were 10% higher than those specified in the technical drawing.
Additionally, Behrens and coworkers [23] developed a finite element model describing scale
behavior to be incorporated into commercial software used to simulate these processes,
given the influence of scale formation on friction and material flow in forging die cavities.
Generally, the validation of the mechanical behavior of components in railway lines,
such as the spike screws, requires compliance with specific regulations. The legislation
establishes the nominal dimensions and tolerances for the hub angle, hub to cap radius,
and the starting material’s diameter. The aim of this research is to deepen the head forming
process by hot-forging with experiments to determine the geometric combination that
generates the lowest energy and correlate the contact pressures obtained by computational
simulation with higher wear areas at the head of the spike screws measured with a 3D scan.
Therefore, this work’s industrial interest is optimizing the energy to form the head of a spike
screw, which is a product of low market value due to its mass production requirement. In
this sense, it is possible to produce safe rail fasteners that comply with current regulations
and can be formed with a process that reduces production costs, increases the life of the
dies, and reduces energy consumption.
2. Materials and Methods
2.1. Dimension and Tolerances of the Screw Spike
This work aims to analyze the influence of the tolerances present in the IRAM-FAT L
7 012 Standard after hot forging the head of the spike screws used for fastening the railroad
rails. This work focuses on analyzing the forming process of the head (Figure 1a). This
part can be divided into two parts: hub and cap. The standardized measurements of these
parts of the head and their respective tolerances are shown in Figure 1b, extracted from the
IRAM-FAT L 7 012 Standard.
Metals 2021, 11, x FOR PEER REVIEW 3 of 12


other defects. In this sense, Prabhu et al. [22] analyzed forged parts for the aeronautical
sector by combining simulation and experimental techniques to carry out its develop-
ment. After producing parts with the adjusted process, they did not present surface de-
fects and their mechanical properties were 10% higher than those specified in the technical
drawing. Additionally, Behrens and coworkers [23] developed a finite element model de-
scribing scale behavior to be incorporated into commercial software used to simulate these
processes, given the influence of scale formation on friction and material flow in forging
die cavities.
Generally, the validation of the mechanical behavior of components in railway lines,
such as the spike screws, requires compliance with specific regulations. The legislation
establi hes the nominal dimensions and tolerances for the hub angle, hub to cap radius,
and the starting material’s diameter. The aim of this research is to deepen the head form-
ing process by hot-forging with experiments to determine the geometric combination that
generates the lowest energy and correlate the contact pressures obtained by computa-
tional simulation with hi her wear areas at the head of the spike screws measured with a
3D scan. Therefore, this work’s industrial interest is optimizing the energy to form the
head of a spike screw, which is a product of low market value due to its mass pr duction
requirement. In this sense, it is possible to produce safe rail fasteners that comply with
current regulations and can be formed with a process that reduces production costs, in-
creases the life of the dies, and reduces energy consumption.
2. Materials and Methods
2.1. Dimension and Tolerances of the Screw Spike
This work aims to analyze the influence of the tolerances present in the IRAM-FAT
L 7 012 Standard after hot forging the head of the spike screws used for fastening the
railroad rails. This work focuses on analyzing the forming process of the head (Figure 1a).
This part can be divided into two parts: hub and cap. The standardized measurements of
these parts of the head and their respective tolerances are shown in Figure 1b, extracted
from the IRAM-FAT L 7 012 Standard.

Figure 1. Railway fasteners: (a) components of the head and (b) main geometric variables.
The influence of the hub wall angle and the hub to cap radius was analyzed. The hub
wall angle comes from the dimensions of the base D2 and the top face D1. Table 1 shows
the minimum and maximum values corresponding to both variables, D1 and D2 and the
hub to cap radius R.
Table 1. CAD dimensions of the head of the spike screw.
Geometry D2 min.–D2 max. D1 min.–D1 max. R min.–R max.
CAD dimensions (mm) 22–23 20–21 2.5–5
Figur . Railwa fasteners: (a) components of the hea an ( ) mai geometric variables.
The influence of the hub wall angle and the hub to cap radius was analyzed. The hub
wall angle comes fro the dimensions of the base D2 and the top face D1. Table 1 shows
the minimum and maximum values corresponding to both variables, D1 and D2 and the
hub to cap radius R.
Table 1. CAD dimensions of the head of the spike scre .
Geometry D2 min.–D2 max. D1 min.–D1 max. R min.–R max.
CAD dimensions (mm) 22–23 20–21 2.5–5
Combining the two hub angles and the two hub and cap joint radii, four die geometries
of the screw head can be obtained, as shown in Figure 2. These combinations required a
dimensional increase of 1.4% of their cavities due to steel shrinkage [24]. The die for the
bottom was the same in all cases.

Metals 2021, 11, 1834 4 of 11
Metals 2021, 11, x FOR PEER REVIEW 4 of 12


Combining the two hub angles and the two hub and cap joint radii, four die geome-
tries of the screw head can be obtained, as shown in Figure 2. These combinations required
a dimensional increase of 1.4% of their cavities due to steel shrinkage [24]. The die for the
bottom was the same in all cases.

Figure 2. Head forming dies resulting from the hub dimensions in mm.
Regarding the diameter of the as-received material, the manufacturing tolerances
published by AINDAR for hot-rolled SAE 1030 steel bars with a nominal diameter of 23.6
mm, whose tolerance is +/−0.26 mm, were used. The nominal size of these bars is the one
used by Argentine companies to make railroad lag bolts, because it fits the threaded sector
of this product. The flash thickness was taken into two different values, 1.50 mm and 2.25
mm, resulting from the specialized literature analyzed as a function of the weight of the
piece. Additionally, the flash gap of the die considered for all simulations was 6.3 mm
[24]. Since the four resulting dies present different volumes, in order to carry out a com-
parative analysis, a burr percentage of 17% was established for all cases with a flash thick-
ness of 1.50 mm. Then, a total head height of 31 mm was set (minimum height according
to the manufacturing drawing) so that when using a 2.25 mm flash, the head would not
be out of tolerance.
2.2. Material of the Screw Spike
Table 2 shows the average chemical composition by weight of the samples analyzed.
This chemical composition is compatible with an AISI 1030 steel and is in the mid-range
of carbon steels used in the manufacture of railway screws [4,9,12]. Table 3 shows the
mechanical properties of an SAE 1030 steel, which is similar to the chemical composition
found in screw spikes.
Table 2. Chemical composition of the material of the tested screw spikes.
Sample %C %Mn %Si %P %S
Screw spike 0.31 ± 0.02 0.71 ± 0.06 0.17 ± 0.06 < 0.010 < 0.009
Table 3. Mechanical properties of SAE 1030 [25].
Material
Yield Strength
(MPa)
Ultimate Tensile
Strength (MPa)
Total strain
(%)
Brinell Hardness
(HB)
SAE 1030 345 550 32 179




Combination 1 Combination 2
Combination 3 Combination 4
Lower Die
20
R2.5
23
20
R5
23
21
R5
22
21
R2.5
22
Figure 2. Head forming dies resulting from the hub dimensions in mm.
Regarding the diameter of the as-received material, the manufacturing tolerances
published by AINDAR for hot-rolled SAE 1030 steel bars with a nominal diameter of
23.6 mm, whose tolerance is +/ 0.26 mm, were used. The nominal size of these bars
is the one used by Argentine companies to make railroad lag bolts, because it fits the
threaded sector of this product. The flash thickness was taken into two different values,
1.50 mm and 2.25 mm, resulting from the specialized literature analyzed as a function of
the weight of the piece. Additionally, the flash gap of the die considered for all simulations
was 6.3 mm [24]. Since the four resulting dies present different volumes, in order to carry
out a comparative analysis, a burr percentage of 17% was established for all cases with a
flash thickness of 1.50 mm. Then, a total head height of 31 mm was set (minimum height
according to the manufacturing drawing) so that when using a 2.25 mm flash, the head
would not be out of tolerance.
2.2. Material of the Screw Spike
Table 2 shows the average chemical composition by weight of the samples analyzed.
This chemical composition is compatible with an AISI 1030 steel and is in the mid-range
of carbon steels used in the manufactur of railway screws [4,9,12]. Table 3 shows the
mechanical properties of an SAE 1030 steel, which is similar to the chemical composition
found i screw spikes.
Table 2. Chemical composition of the material of the tested screw spikes.
Sample %C %Mn %Si %P %S
Screw spike 0.31 0.02 0.71 0.06 0.17 0.06 <0.010 <0.009
Table 3. Mechanical properties of SAE 1030 [25].
Material
Yield Strength
(MPa)
Ultimate Tensile
Strength (MPa)
Total strain
(%)
Brinell
Hardness (HB)
SAE 1030 345 550 32 179
2.3. DOE of the FEM Analysis
A factorial Design of Experiments (DOE) was carried out, resulting from eight com-
putational simulations of the head forming process, which arise from combining the four
die geometries with two diameters of the as-received material. Table 4 shows the sixteen
variants computed when considering two flash thicknesses.

Metals 2021, 11, 1834 5 of 11
Table 4. Simulations of the forming process of the head of the screw spike.
Simulations Dies
As-Received
Material
Dmin. (mm)
As-Received
Material
Dmax. (mm)
Min. Burr
Thickness
(mm)
Max. Burr
Thickness
(mm)
16 4 23.54 24.06 1.50 2.25
The simulations were carried out using Simufact-Forming (version 15, Hexagon,
Hamburg, Germany) to establish the forging load and the forging energy and verify the
matrix’s filling for each combination. Before the simulation, a mesh validation process was
performed. The 3D model was used on 14 of the geometry, using finite volumes with first
order and high order mesh quality, with triangular surface mesh sizes elements [26] of
0.7 mm, 1 mm, and 1.8 mm. Five simulations were carried out to study the mesh to find
the minimum mesh size that would solve the model in a time no longer than 20 h (stop
criterion). Firstly, the whole geometry of the component was considered. In the end, using
a quarter of the geometry was enough due to the symmetry of the product. A balance
between processing time and accuracy of the results was found with a high order mesh
quality and size of 1 mm with a total of 11,176 elements. Table 5 shows the sequence of
simulations carried out to balance the quality of results and processing time.
Table 5. Convergence analysis to establish meshing conditions.
Simulation
Response
Complete Geometry 1/4 Geometry
First Order
1.0 mm
High Order
1.8 mm
High Order
1.0 mm
High Order
0.7 mm
High Order
1.0 mm
Configuration
Metals 2021, 11, x FOR PEER REVIEW 5 of 12


2.3. DOE of the FEM Analysis
A factorial Design of Experiments (DOE) was carried out, resulting from eight com-
putational simulations of the head forming process, which arise from combining the four
die geometries with two diameters of the as-received material. Table 4 shows the sixteen
variants computed when considering two flash thicknesses.
Table 4. Simulations of the forming process of the head of the screw spike.
Simulations Dies
As-Received
Material
Dmin. (mm)
As-Received
Material
Dmax. (mm)
Min. Burr
Thickness
(mm)
Max. Burr
Thickness
(mm)
16 4 23.54 24.06 1.50 2.25
The simulations were carried out using Simufact-Forming (version 15, Hexagon,
Hamburg, Germany) to establish the forging load and the forging energy and verify the
matrix’s filling for each combination. Before the simulation, a mesh validation process was
performed. The 3D model was used on ¼ of the geometry, using finite volumes with first
order and high order mesh quality, with triangular surface mesh sizes elements [26] of 0.7
mm, 1 mm, and 1.8 mm. Five simulations were carried out to study the mesh to find the
minimum mesh size that would solve the model in a time no longer than 20 h (stop crite-
rion). Firstly, the whole geometry of the component was considered. In the end, using a
quarter of the geometry was enough due to the symmetry of the product. A balance be-
tween processing time and accuracy of the results was found with a high order mesh qual-
ity and size of 1 mm with a total of 11,176 elements. Table 5 shows the sequence of simu-
lations carried out to balance the quality of results and processing time.
Table 5. Convergence analysis to establish meshing conditions.
Simulation Re-
sponse
Complete Geometry ¼ Geometry
First Order
1.
High Order
1.8 m
High Order
1.0 m
High Order
0.7 mm
High Order
1.0 mm
Configuration

Force (t) 158.5 181.9 (*) (**) 187.2
Simulation time (h) 4.2 5.6 11.5 17.7 15.9
Elements 23,076 7158 23,076 46,500 11,176
(*) The simulation found the stop criterion (20 h) with a progress of 19.6%. (**) The simulation
found the stop criterion (20 h) with a progress of 7.4%.
The mechanical press corresponds to a 630 t of capacity with a rod length of 1000
mm, a crankshaft radius of 175 mm, and a stroke frequency of 30 rpm. The material tem-
perature was 1150 °C and the shear friction coefficient was set as 0.4 [18]. These values
come from the library of the software for medium carbon steel–steel interaction during
hot working. To determine the type of steel to be used during the simulations, chemical
analyses were carried out by spark optical emission spectroscopy (OES-spark spectrome-
ter model Q4 Tasman, Bruker, Buenos Aires, Argetina) on samples of spike screws avail-
able at the Machining and Forming Processes laboratory of INTI-Mecánica (Buenos Aires,
Argentina). During the simulations, in some combinations, the formation of folds oc-
curred. A series of penetrant ink tests were carried out to verify if these folds were found
Metals 2021, 1, x FOR PEER REVIEW 5 of 12


2.3. DOE of the FEM Analysis
A factorial Design of Experiments (DOE) was ca ried out, resulting from eight com-
putational simulations of the head forming proce s, which arise from combining the four
die geometries with two diameters of the as-received material. Table 4 shows the sixteen
variants computed when considering two flash thicknesses.
Table 4. Simulations of the forming proce s of the head of the screw spike.
Simulations Dies
As-Received
Material
Dmin. (mm)
As-Received
Material
Dmax. (mm)
Min. Bu r
Thickne s
(mm)
Max. Bu r
Thickne s
(mm)
16 4 23.54 24.06 1.50 2.25
The simulations were ca ried out using Simufact-Forming (version 15, Hexagon,
Hamburg, Germany) to establish the forging load and the forging energy and verify the
matrix’s filling for each combination. Before the simulation, a mesh validation proce s was
performed. The 3D model was used on ¼ of the geometry, using finite volumes with first
order and high order mesh quality, with triangular surface mesh sizes elements [26] of 0.7
mm, 1 mm, and 1.8 mm. Five simulations were ca ried out to study the mesh to find the
minimum mesh size that would solve the model in a time no longer than 20 h (stop crite-
rion). Firstly, the whole geometry of the component was considered. In the end, using a
quarter of the geometry was enough due to the symmetry of the product. A balance be-
tw en proce sing time and a curacy of the results was found with a high order mesh qual-
ity and size of 1 mm with a total of 1,176 elements. Table 5 shows the sequence of simu-
lations ca ried out to balance the quality of results and proce sing time.
Table 5. Convergence analysis to establish meshing conditions.
Simulation Re-
sponse
Complete Geometry ¼ Geometry
First Order
1.0 m
High Order
1.
High Order
1.0 mm
High Order
0.7 mm
High Order
1.0 mm
Configuratio

Force (t) 158.5 181.9 (*) ( *) 187.2
Simulation time (h) 4.2 5.6 1.5 17.7 15.9
Elements 23,076 7158 23,076 46,5 0 1,176
(*) The simulation found the stop criterion (20 h) with a progre s of 19.6%. (**) The simulation
found the stop criterion (20 h) with a progre s of 7.4%.
The mechanical press co responds to a 630 t of capacity with a rod length of 1 0
mm, a crankshaft radius of 175 mm, and a stroke frequency of 30 rpm. The material tem-
perature was 150 °C and the shear friction coe ficient was set as 0.4 [18]. These values
come from the library of the software for medium carbon st el–st el interaction during
hot working. To determine the type of st el to be used during the simulations, chemical
analyses were ca ried out by spark optical emi sion spectroscopy (OES-spark spectrome-
ter model Q4 Tasman, Bruker, Buenos Aires, Argetina) on samples of spike screws avail-
able at the Machining and Forming Proce ses laboratory of INTI-Mecánica (Buenos Aires,
Argentina). During the simulations, in some combinations, the formation of folds oc-
cu red. A series of penetrant ink tests were ca ried out to verify if these folds were found
Metals 2021, 11, x FOR PEE REVIEW 5 of 12


2.3. DOE of the FEM Analy is
A factorial Design of Experiments (DOE) was carried out, resulting from eight com-
putational simulations of the head forming process, which arise from combi ing the four
die geometries with two diameters of the as-r ceived material. Table 4 shows the sixteen
v riants computed when considering two flas thicknesses.
Table 4. Simulations of the forming process of t e head of the screw spike.
Simulations Dies
As-R ceived
Material
Dmin. ( m)
As-R ceived
Material
Dmax. ( m)
Min. Burr
Thickness
( m)
Max. Burr
Thickness
( m)
16 4 23.54 24.06 1.50 .25
The simulations were carried out using Simufact-Forming (version 15, Hexagon,
Hamburg, Germany) to establis the forging load and the forging energy and verify the
matrix’s filling for each combination. Before the simulation, a mesh validation process was
performed. The 3D model was used on ¼ of the geometry, using finite volumes with first
order and high order mesh quality, wi h triangular surface mesh sizes elements [26] of 0.7
m, 1 m, and 1.8 m. Five simulations were carried out to study the mesh to find the
minimu mesh size that would solve the model in a time n longer than 20 h (stop crite-
rion). Firstly, the whole geometry of the component was consid red. In the end, using a
quarter of the geometry was enough due to the sy metry of the product. A balance be-
tw en processing time and accuracy of th results was found with a high order mesh qual-
ity and size of 1 m with a total of ,176 elements. Table 5 shows the sequence of simu-
lations carried out to balance the quality of results and processing time.
Table 5. Convergence naly is to establish meshing cond tions.
Simulation Re-
sponse
Complete Geometry ¼ Geometry
First Order
1.0 m
High Order
1.8 m
High Order
1.
High Order
0.7
High Order
1.0 m
Configuration

Force (t) 158.5 81.9 (*) (**) 187.2
Simulation time (h) 4.2 5.6 1.5 1 .7 15.9
Elements 23,076 7158 23,076 46,5 0 ,176
(*) The simulation found the stop criterion (20 h) with a progress of 19.6%. (**) The simulation
found the stop criterion (20 h) with a progress of 7.4%.
The mechanical press corresponds to a 630 t of c pacity with a rod length of 1 0
m, a crankshaft radius of 175 m, and a stroke frequency of 30 rpm. The material tem-
perature was 150 °C and the shea friction coefficient was set as 0.4 [18]. These values
come from the library of the software for medium carbon st el–st el interaction during
hot working. To determine the type of st el to be use during the simulations, chemical
nalyses were carried out by spark optical emission spectroscopy (OES-spark spectrome-
ter model Q4 Tasman, Bruker, Buenos Aires, Argetina) on samples of spike screws vail-
able a the Machi ing and Forming Processes laboratory of INTI-Mecánica (Buenos Aires,
Arge tina). During the simulations, in some combinations, the formation of folds oc-
curred. A series of penetrant ink tests were carried out to verify if these folds were found
Metals 2021, 11, x FOR PEER REVIEW 5 of 12


2.3. DOE of the FEM Analysis
A factorial Design of Experiments (DOE) w s carried out, resulting from eight com-
putational simulations of the head forming process, which arise fr combi ing the four
die geometries with two diameters of the as-r ceived m teri l. Table 4 shows the sixteen
variants computed when considering two flas thicknesses.
Table 4. Simulati ns o the forming process of the head of the screw spike.
Simulations Dies
As-R ceived
M terial
D in. ( m)
As-R ceived
M terial
Dmax. ( m)
Min. Burr
Thickness
( m)
Max. Burr
Thickness
( m)
16 4 23.54 24.06 1.50 2.25
The simulations were carried out using Simufact-Forming (version 15, Hexagon,
Hamburg, Germany) to establish the for ing loa and the for ing energy and verify the
matr x’s f lling for each combination. Befor the simulation, a mesh v lidation proces was
performed. The 3D model was used on ¼ of the geometry, us g finite volumes with first
o der and igh o der mesh quality, with triangular surface mesh sizes elements [26] of 0.7
, 1 m, and 1.8 m. F ve simulations were carried ut o study th mesh to find the
ini u mesh size that would solv the model in a time n longer than 20 h (stop crite-
rion). Firstly, t e whole geometry of the c mponent was consi red. In the end, using a
quarter of the geometry was enough due to the sy metry of the product. A balance be-
tween process ng time nd ac uracy of th result was found wit a igh o d r mesh qual-
ity and size of 1 m wi h tal of 11,176 elements. Table 5 shows the sequence of simu-
lations carried ut to balanc the quality of re ults and process ng time.
Table 5. Converge ce nalysi to establish meshi g conditions.
Simulation Re-
sponse
Compl t G ometry ¼ G ometry
First O der
1.0 m
High O der
1.8 m
High O der
1.0 mm
High O der
0.
High O der
1.0
Configuration

Force (t) 158.5 81.9 (*) (**) 187.2
Simula on time (h) 4.2 5.6 11.5 17.7 15.9
Elements 23,076 7158 23,076 46,500 11,176
(*) The simulati n found the stop criterion (20 h) with a progress of 19.6%. (**) The simulation
found the stop criterion (20 h) with a progress of 7.4%.
The mechanical press corresponds to a 630 t of pacity with a rod length of 1 00
m, crankshaft radius of 175 m, nd a stroke frequency of 30 rpm. The m terial tem-
perature was 1150 °C and t e shea fricti n coefficient was set as 0.4 [18]. These values
come from the library of the softwa e for medium carbon ste l–steel interaction during
h t working. To determin the type of steel to be use during the simulations, chemical
nalyses were carried out by spark optical emission spectr scopy (OES-spark spectrome-
ter model Q4 Tasman, Bruker, Buenos Aires, Argetina) on samples of spike screws vail-
ble at the Machi i g and Forming Processes l borat ry of INTI-Mecánica (Buenos Aires,
Argentina). During the simulatio s, in some combinations, the formati n f folds oc-
curred. A series of penetra t ink tests were carried ut to verify if these folds were found
Metals 2021, 1, x FO PEER R VIEW 5 of 12


2.3. DOE of the FEM Analysis
A f ctorial Design of Experiments (DOE) was ca ried out, resulting from eight com-
put tional simulati ns of t head forming proce s, wh ch arise fro combining the four
di geometr es ith two diameters of th as-r c ived m teri l. Table 4 how the sixteen
v riants computed when considering two flash thicknesses.
Table 4. Simulations o the forming pr ce s of t e head of the screw spike.
Simulations Dies
As-R c ived
M terial
D in. (mm)
As-R c ived
M terial
D ax. (mm)
Min. Bu r
Thickne s
(mm)
Max. Bu r
Thickne s
(mm)
16 4 23.54 24.06 1.50 2.25
The simulations were ca ried out us ng Simufact-Forming (version 15, Hexagon,
Hamburg, Germany) to establis the for ing lo and the for ing energy and verify the
matrix’s fi ling for each combi ation. Before the simulation, a mesh validati n proce was
performed. The 3D model was used n ¼ of the geometry, us g finite volumes with first
order and igh order mesh quality, with tri ngular surface mesh sizes elements [26] of 0.7
m , 1 mm, and 1.8 mm. F ve simulations were ca ried ut to s udy the mesh to find the
ini u mesh size hat would solve the model in a time o longer than 20 h (stop crite-
rion). Firstly, t e whol geometry of the c mponent wa consi ered. In the end, using a
quarter of the geometry was enough due to the symmetry of the product. A balanc be-
tw en proce s ng time nd uracy of the result was found wit a igh ord r mesh qual-
ity and size of 1 mm wi h t tal of 1,176 elements. Table 5 hows the sequence of simu-
lations ca ried ut to balance the quality of results and proce s ng time.
Table 5. Conv rg ce analysi to establi meshing c ditions.
Simulation Re-
sponse
Compl te G ometry ¼ G ometry
Fi st Order
1.0 mm
High Order
1.8 mm
High Order
1.0 mm
High Order
0.7 mm
High Order
1.
Configuration

Force (t) 1 8.5 181.9 (*) ( *) 187.2
Simula on time (h) 4.2 5.6 1.5 17.7 15.9
Elements 23,076 7158 23,076 46,5 0 1,176
(*) The simulation found he stop c terion (20 ) with a p ogre s of 19.6%. (**) The simulation
found he stop c terion (20 ) with a p ogre s of 7.4%.
The me h nical press co responds to a 630 t of apac ty with a rod length of 1 0
mm, a cranksh ft radius of 175 mm, nd a stroke frequency of 30 rpm. The m terial tem-
perature was 150 °C and t shear fricti n oe ficient was set as 0.4 [18]. These values
come from the library of the software for medium carbon steel–st el interaction during
h t working. To determine the type of s el to be used during the simulations, chemical
analyses were ca ried out by spark optical emi sion spectr scopy (OES-spark spectrome-
ter model Q4 T sman, Bruker, Buenos Aires, Argeti a) on samples of spike screws avail-
ble at the Machini g and Forming Proce ses l b ratory of INTI-Me ánica (Buenos Aires,
Arge tina). During the simulatio s, in some combi ations, the formation of f lds oc-
cu red. A series of pe etra t ink tests were ca ried ut to ver y if these folds were found
Force (t) 158.5 181.9 (*) (**) 187.2
Simulation
time (h)
4. 5. 11.5 17.7 15.9
Elements 23,076 715 23,076 46,500 11,176
(*) The simulation found the stop criterion (20 ) wit a progress of 19.6%. (**) The simulation found the stop
criterion (20 h) with a progress of 7.4%.
The mechanical press corresponds to a 630 t of capacity with a rod length of 1000 mm,
a crankshaft radius of 175 mm, and a stroke frequency of 30 rpm. The material temperature
was 1150 C and the shear friction coefficient was set as 0.4 [18]. These values come from
the library of the software for medium carbon steel–steel interaction during hot working.
To determine the type of steel to be used during the simulations, chemical analyses were
carried out by spark optical emission spectroscopy (OES-spark spectrometer model Q4
Tasman, Bruker, Buenos Aires, Argetina) on samples of spike screws available at the
Machining and Forming Processes laboratory of INTI-Mecánica (Buenos Aires, Argentina).
During the simulations, in some combinations, the formation of folds occurred. A series
of penetrant ink tests were carried out to verify if these folds were found in some of the
available samples of spike screws. According to IRAM NM ISO 1972-ASTM E 165, the
method was applied using the visible water technique. The final cleaning of the samples
was done each time in two stages: washing with solvent and brushing and then drying with
a hot air projector. The drying temperature was below 70 C and a Magnaflux penetrant
liquid type SKL-WP with a penetration time of 15 min was used, while the excess liquid
was removed by spraying. The developer used was wet non-aqueous SKC-S2.

Metals 2021, 11, 1834 6 of 11
Two spike screws were scanned, one manufactured from a worn matrix and the other
without apparent wear marks, using a structured light 3D scanner (HP, Barcelona, Spain).
Subsequently, a study was carried out to determine the measurement deviation with respect
to the CAD design using GOM Inspect software (Zeiss, Buenos Aires, Argentina). The
results obtained from the computational simulations were evaluated applying the ANOVA
statistical tool using Minitab (version 15, Addlink, Madrid, Spain) Box and Pareto plots
were utilized to determine the relative importance of each factor and the affected responses
and extract the corresponding values of the p-values.
3. Results
The computational simulations for the different geometrical-dimensional combina-
tions and process variables used for head forming are performed. The forming energy,
forming loads and contact pressures, the matrix filling, and the appearance of defects (folds)
play a crucial role in the screw spike production. Penetrant inks verified the appearance of
defects at the surface and the worn areas were evaluated by 3D scanning and validated
with the desired CAD part.
3.1. Computational Simulations
Using a 2.25 mm thick burr channel resulted in a lack of filling at the hub vertices in
all cases. Figure 3 shows the simulation of that lack of proper filling at the corner of the
head of the spike. All simulations used 1.50 mm burr channel thickness showed complete
filling of the forming die.
Metals 2021, 11, x FOR PEER REVIEW 6 of 12


in some of the available samples of spike screws. According to IRAM NM ISO 1972-ASTM
E 165, the method was applied using the visible water technique. The final cleaning of the
samples was done each time in two stages: washing with solvent and brushing and then
drying with a hot air projector. The drying temperature was below 70 °C and a Magnaflux
penetrant liquid type SKL-WP with a penetration time of 15 min was used, while the ex-
cess liquid was removed by spraying. The developer used was wet non-aqueous SKC-S2.
Two spike screws were scanned, one manufactured from a worn matrix and the other
without apparent wear marks, using a structured light 3D scanner (HP, Barcelona, Spain).
Subsequently, a study was carried out to determine the measurement deviation with re-
spect to the CAD design using GOM Inspect software (Zeiss, Buenos Aires, Argentina).
The results obtained from the computational simulations were evaluated applying the
ANOVA statistical tool using Minitab (version 15, Addlink, Madrid, Spain) Box and Pa-
reto plots were utilized to determine the relative importance of each factor and the af-
fected responses and extract the corresponding values of the p-values.
3. Results
The computational simulations for the different geometrical-dimensional combina-
tions and process variables used for head forming are performed. The forming energy,
forming loads and contact pressures, the matrix filling, and the appearance of defects
(folds) play a crucial role in the screw spike production. Penetrant inks verified the ap-
pearance of defects at the surface and the worn areas were evaluated by 3D scanning an
validated with the desired CAD part.
3.1. Computational Simulation
Usin 2.2 thick bur channel esulte i lack f fillin at the hu vertices in
all cases. Figur show th simulatio f that lack f ope fillin t t corne f the
hea f th spike. All simulations 1.5 burr channel thickness showe complete
filling f the forming die.

Figure 3. Screw spike with a vertex underfill for 2.25 mm burr cannel.
Figures 4 and 5 show the corresponding box plots for load and energy results, segre-
gated by head angle, hub to cap radius, flash thickness, and starting material diameter,
respectively. The forming energy and forming load as a function of head angle and hub
to cap radius show no significant differences. Additionally, the combination of energy as
a function of head angle presents a slight difference in the average of about 9.2%.
Figure 3. Screw spike with a vertex underfill for 2.25 mm burr cannel.
Figur an sho the corresponding box plots for load and energy results, seg
regated by head angle, hub to cap radius, flash thickness, an startin materia diamete
espectively. The forming energy and forming load as a function of head angle and hub to
cap radius show no significant differences. Additionally, the combination of energy as a
function of head angle presents a slight difference in the average of about 9.2%.
Metals 2021, 11, x FOR PEER REVIEW 7 of 12


4.291.30
200
175
150
125
100
75
50
4.291.30
4.0
3.5
3.0
2.5
2.0
Force (t)
Head angle
Energy (kJ)
Box plot - Force (t), Energy (kJ)
5.02.5
200
175
150
125
100
75
50
5.02.5
4.0
3.5
3.0
2.5
2.0
Force (t)
Radius corner splice
Energy (kJ)
Box plot - Force (t), Energy (kJ)
Figure 4. Box plots for geometric parameters according to the standard.
2.251.50
200
75
150
125
100
75
50
2.251.50
4.0
3.5
3.0
2.5
2.0
Force (t)
Flash
Energy (kJ)
Box plot - Force (t), Energy (kJ)
24.0623.54
200
75
150
125
100
75
50
24.0623.54
4.0
3.5
3.0
2.5
2.0
Force (t)
Starting material diameter
Energy (kJ)
Box plot - Force (t), Energy (kJ)

Figure 5. Box plots for the analyzed process parameters.
Additionally, the statistical analysis of the combination of forming energy and form-
ing load regarding flash thickness shows very significant differences. In particular, as
thicker the flash is used, the lower the load and energy required to form the head. More-
over, the diameter of the as-received material presents a lower impact in the process, alt-
hough it is denoted that as the diameter increases, both load and energy increase. An
ANOVA analysis was performed to evaluate more precisely the influence of each param-
eter on the load and energy values using the p-value with a 0.05 level of confidence. The
results of the ANOVA are listed in Table 6.
Table 6. Results of the p-value for the analyzed forging parameters.
Factor
p-Value
Load (t) Energy (kJ)
Head angle 0.233 0.000
Hub to cap radius 0.569 0.551
As-received material diameter 0.000 0.000
Flash thickness 0.000 0.000
The ANOVA exhibits that both the flash thickness and the as-received material di-
ameter have a significant influence on the load and energy values and the hub angle that
influences the forming energy. On the contrary, the hub to cap radius does not influence
the response variables. To determine the order of influence, Figure 6 shows the Pareto
diagrams for force and energy.
Figure 4. Box plots for geometric parameters according to the standard.

Metals 2021, 11, 1834 7 of 11
Metals 2021, 11, x FOR PEER REVIEW 7 of 12


4.291.30
200
175
150
125
100
75
50
4.291.30
4.0
3.5
3.0
2.5
2.0
Force (t)
Head angle
Energy (kJ)
Box plot - Force (t), Energy (kJ)
5.02.5
200
175
150
125
100
75
50
5.02.5
4.0
3.5
3.0
2.5
2.0
Force (t)
Radius corner splice
Energy (kJ)
Box plot - Force (t), Energy (kJ)
Figure 4. Box plots for geometric parameters according to the standard.
2.251.50
200
75
150
125
100
75
50
2.251.50
4.0
3.5
3.0
2.5
2.0
Force (t)
Flash
Energy (kJ)
Box plot - Force (t), Energy (kJ)
24.0623.54
200
75
150
125
100
75
50
24.0623.54
4.0
3.5
3.0
2.5
2.0
Force (t)
Starting material diameter
Energy (kJ)
Box plot - Force (t), Energy (kJ)

Figure 5. Box plots for the analyzed process parameters.
Additionally, the statistical analysis of the combination of forming energy and form-
ing load regarding flash thickness shows very significant differences. In particular, as
thicker the flash is used, the lower the load and energy required to form the head. More-
over, the diameter of the as-received material presents a lower impact in the process, alt-
hough it is denoted that as the diameter increases, both load and energy increase. An
ANOVA analysis was performed to evaluate more precisely the influence of each param-
eter on the load and energy values using the p-value with a 0.05 level of confidence. The
results of the ANOVA are listed in Table 6.
Table 6. Results of the p-value for the analyzed forging parameters.
Factor
p-Value
Load (t) Energy (kJ)
Head angle 0.233 0.000
Hub to cap radius 0.569 0.551
As-received material diameter 0.000 0.000
Flash thickness 0.000 0.000
The ANOVA exhibits that both the flash thickness and the as-received material di-
ameter have a significant influence on the load and energy values and the hub angle that
influences the forming energy. On the contrary, the hub to cap radius does not influence
the response variables. To determine the order of influence, Figure 6 shows the Pareto
diagrams for force and energy.
Figure 5. Box plots for the analyzed process parameters.
Additionally, the statistical analysis of the combination of forming energy and forming
load regarding flash thickness shows very significant differences. In particular, as thicker
the flash is used, the lower the load and energy required to form the head. Moreover, the
diameter of the as-received material presents a lower impact in the process, although it is
denoted that as the diameter increases, both load and energy increase. An ANOVA analysis
was performed to evaluate more precisely the influence of each parameter on the load and
energy values using the p-value with a 0.05 level of confidence. The results of the ANOVA
are listed in Table 6.
Table 6. Results of the p-value for the analyze fo gin parameters.
Factor
p-Value
Load (t) Energy (kJ)
Head angle 0.233 0.000
Hub to cap radius 0.569 0.551
As-received material diameter 0.000 0.000
Flash thickness 0.000 0.000
The ANOVA exhibits that both the flash thickness and the as-received material di-
ameter have a significant influence on the load and energy values and the hub angle that
influences the forming energy. On the contrary, the hub to cap radius does not influence
the response variables. To determine the order of influence, Figure 6 shows the Pareto
diagrams for force and energy.
Metals 2021, 11, x FOR PEER REVIEW 8 of 12


Radius corner splice
H ad ang e
Starting material diameter
Flash
2520151050
Te
rm
Standardized Effect
2.20
Pareto Chart of the Standardized Effect
(response in Force (t), Alfa = 0.05)
Radius corner splice
Head angle
Starting material diameter
Flash
20151050
Te
rm
Standardized Effect
2.20
Pareto Chart of th Standardized Effect
(response in Energy (kJ), Alfa = 0.05)

Figure 6. Influence order of the operating parameters on load and energy of the forming process.
The Pareto diagrams results indicate that the forming load shows the following in-
fluence: flash thickness followed by the as-received material diameter. While the forming
energy presents the following order of influence: flash thickness, the as-received material
diameter and, also, hub-to-cap radius. Note that when using the largest as-received mate-
rial diameter, i.e., 24.06 mm, folds appeared during the plastic forming simulation (this
was a finding that we had not visualized on the screw spikes samples). Figure 7 shows
how these folds are created during the forming process.

Figure 7. Sequence of fold formation at the cap of the head of the screw spike.
Several samples of screws spikes were analyzed with penetrant inks to validate the
computational simulations. Figure 8 demonstrates the result of the penetrant inks show-
ing the circular-shaped fold on the cap of the screw.

Figure 8. Folds exposed by penetrating inks on the cap of the screw.
Figure 6. Influence order of the operating parameters on load and energy of the forming process.
The Pareto diagrams results indicate that the forming load shows the following
influence: flash thickness followed by the as-received material diameter. While the forming
energy presents the following order of influence: flash thickness, the as-received material
diameter and, also, hub-to-cap radius. Note that when using the largest as-received
material diameter, i.e., 24.06 mm, folds appeared during the plastic forming simulation

Metals 2021, 11, 1834 8 of 11
(this was a finding that we had not visualized on the screw spikes samples). Figure 7 shows
how these folds are created during the forming process.
Metals 2021, 11, x FOR PEER REVIEW 8 of 12


Radius corner splice
Head angle
Starting material diameter
Flash
2520151050
Te
rm
Standardized Effect
2.20
Pareto Chart of the Standardized Effect
(response in Force (t), Alfa = 0.05)
Radius corner splice
Head angle
Starting material diameter
Flash
20151050
Te
rm
Standardized Effect
2.20
Pareto Chart of th Standardized Effect
(response in Energy (kJ), Alfa = 0.05)

Figure 6. Influence order of the operating parameters on load and energy of the forming process.
The Pareto diagrams results indicate that the forming load shows the following in-
fluence: flash thickness followed by the as-received material diameter. While the forming
energy presents the following order of influence: flash thickness, the as-received material
diameter and, also, hub-to-cap radius. Note that when using the largest as-received mate-
rial diameter, i.e., 24.06 mm, folds appeared during the plastic forming simulation (this
was a finding that we had not visualized on the screw spikes amples). Figure 7 show
how thes folds eate during th forming p ocess

Figure 7. Sequence of fold formation at the cap of the head of the screw spike.
Several samples of screws spikes were analyzed with penetrant inks to validate the
computational simulations. Figure 8 demonstrates the result of the penetrant inks show-
ing the circular-shaped fold on the cap of the screw.

Figure 8. Folds exposed by penetrating inks on the cap of the screw.
Figure 7. Sequence of fold formation at the cap of the head of the screw spike.
Several samples of screws spikes were analyzed with penetrant inks to validate the
computational simulations. Figure 8 demonstrates the result of the penetrant inks showing
the circular-shaped fold on the cap of the screw.
Metals 2021, 11, x FOR PEER REVIEW 8 of 12


Radius corner splice
Head angle
Starting material diameter
Flash
2520151050
Te
rm
Standardized Effect
2.20
Pareto Chart of the Standardized Effect
(response in Force (t), Alfa = 0.05)
Radius corner splice
Head angle
Starting material diameter
Flash
20151050
Te
rm
Standardized Effect
2.20
Pareto Chart of th Standardized Effect
(response in Energy (kJ), Alfa = 0.05)

Figure 6. Influence order of the operating parameters on load and energy of the forming process.
The Pareto diagrams results indicate that the forming load shows the following in-
fluence: flash thickness followed by the as-received material diameter. While the forming
energy presents the following order of influence: flash thickness, the as-received material
diameter and, also, hub-to-cap radius. Note that when using the largest as-received mate-
rial diameter, i.e., 24.06 mm, folds appeared during the plastic forming simulation (this
was a finding that we had not visualized on the screw spikes samples). Figure 7 shows
how these folds are created during the forming process.

Figure 7. Sequence of fold formation at the cap of the head of the screw spike.
Several samples of screws spikes were analyzed with penetrant inks to validate the
computational simulations. Figure 8 demonstrates the result of the penetrant inks show-
ing the circular-shaped fold on the cap of the screw.

Figure 8. Folds exposed by penetrating inks on the cap of the screw. Figure 8. Folds exposed by penetrating inks on the cap of the screw.
In Figure 9, it is shown in detail the progress of the contact pressure distribution in the
different phases of the forming process. At the step of the head forming, it can be seen that
the outer diameter of the material first makes contact with the sides of the tooling cavity.
The highest contact pressure is found at the radius of agreement between the cap and the
head of the screw spike. The maximum contact pressures were found to be in the order of
884 MPa, while the yield stress of a quenched and tempered AISI H13 steel at 45 HRC was
1280 MPa [26].
Metals 2021, 11, x FOR PEER REVIEW 9 of 12


In Figure 9, it is shown in detail the progress of the contact pressure distribution in
the different phases of the forming process. At the step of the head forming, it can be seen
that the outer diameter of the material first makes contact with the sides of the tooling
cavity. The highest contact pressure is found at the radius of agreement between the cap
and the head of the screw spike. The maximum contact pressures were found to be in the
order of 884 MPa, while the yield stress of a quenched and tempered AISI H13 steel at 45
HRC was 1280 MPa [26].

Figure 9. Contact pressure along the forming process of the head of the screw spike.
3.2. Screw Spike Surface Scanning
The surfaces were scanned to determine the geometric deviations and validate the
results of the simulations. Accordingly, Figure 10a exhibits the geometrical deviations of
the head of a screw spike formed in an unworn matrix concerning the CAD with the min-
imum dimensional tolerances. Figure 10b shows the geometrical deviations of the head of
a screw spike formed in a worn matrix with respect to the CAD with the minimum di-
mensional tolerances. Figure 10c compares the deviations between the two previous
scanned samples (Figure 10a,b). Finally, Figure 10d corresponds to the worn actual cavity
forming the head of a screw spike.

Figure 10. The 3D scan images were analyzed with GOM Inspector to determine the di-
mensional deviation between samples formed on new and worn dies.
In the screw spike, the wear zones are found in the central part of the sidewalls of the
hub and the hub to cap radius of the screw spike (see Figure 10b). This region is where the
Figur Contac essu alon th formin oces th hea th spike

Metals 2021, 11, 1834 9 of 11
3.2. Screw Spike Surface Scanning
The surfaces were scanned to determine the geometric deviations and validate the
results of the simulations. Accordingly, Figure 10a exhibits the geometrical deviations
of the head of a screw spike formed in an unworn matrix concerning the CAD with the
minimum dimensional tolerances. Figure 10b shows the geometrical deviations of the
head of a screw spike formed in a worn matrix with respect to the CAD with the minimum
dimensional tolerances. Figure 10c compares the deviations between the two previous
scanned samples (Figure 10a,b). Finally, Figure 10d corresponds to the worn actual cavity
forming the head of a screw spike.
Metals 2021, 11, x FOR PEER REVIEW 9 of 12


In Figure 9, it is shown in detail the progress of the contact pressure distribution in
the different phases of the forming process. At the step of the head forming, it can be seen
that the outer diameter of the material first makes contact with the sides of the tooling
cavity. The highest contact pressure is found at the radius of agreement between the cap
and the head of the screw spike. The maximum contact pressures were found to be in the
order of 884 MPa, while the yield stress of a quenched and tempered AISI H13 steel at 45
HRC was 1280 MPa [26].

Figure 9. Contact pressure along the forming process of the head of the screw spike.
3.2. Screw Spike Surface Scanning
The surfaces were scanned to determine the geometric deviations and validate the
results of the simulations. Accordingly, Figure 10a exhibits the geometrical deviations of
the head of a screw spike formed in an unworn matrix concerning the CAD with the min-
imum dimensional tolerances. Figure 10b shows the geometrical deviations of the head of
a screw spike formed in a worn matrix with respect to the CAD with the minimum di-
mensional tolerances. Figure 10c compares the deviations between the two previous
scanned samples (Figure 10a,b). Finally, Figure 10d corresponds to the worn actual cavity
forming the head of a screw spike.

Figure 10. The 3D scan images were analyzed with GOM Inspector to determine the di-
mensional deviation between samples formed on new and worn dies.
In the screw spike, the wear zones are found in the central part of the sidewalls of the
hub and the hub to cap radius of the screw spike (see Figure 10b). This region is where the
Figure 10. The 3D scan images were analyzed with GOM Inspector to determine the dimensional
deviation between samples formed on new and worn dies.
In the screw spike, the wear zones are found in the central part of the sidewalls of the
hub and the hub to cap radius of the screw spike (see Figure 10b). This region is where
the material first makes contact with the die cavity. Additionally, it is where the highest
contact pressures occur during forming, according to the results obtained from the finite
volume simulation. The contact pressures, associated with the magnitude of the load and
forming energy, inversely depend on the flash dimensions (thickness and length) [27]. In
particular, for lower flash thickness, higher plastic strain is determined in the simulation
and, therefore, higher forming energy per cycle is assessed; consequently, higher wear of
the die is expected. Figure 11 shows the maximum contact pressure regarding the flash
thickness and segmented by wear zones (radius and hub face).
Since the deviations of the worn zones are negative, the plastic deformation of the die
hub is reducing its size, as shown in Figure 10d. Other aspects of railway tracks manufac-
turing are related to hole and drilling quality, similar to other cases and applications, so in
further research the hole quality will be also investigated [28].

Metals 2021, 11, 1834 10 of 11
Metals 2021, 11, x FOR PEER REVIEW 10 of 12


material first makes contact with the die cavity. Additionally, it is where the highest con-
tact pressures occur during forming, according to the results obtained from the finite vol-
ume simulation. The contact pressures, associated with the magnitude of the load and
forming energy, inversely depend on the flash dimensions (thickness and length) [27]. In
particular, for lower flash thickness, higher plastic strain is determined in the simulation
and, therefore, higher forming energy per cycle is assessed; consequently, higher wear of
the die is expected. Figure 11 shows the maximum contact pressure regarding the flash
thickness and segmented by wear zones (radius and hub face).

Figure 11. Maximum contact pressure related to the flash thickness and most degraded areas.
Since the deviations of the worn zones are negative, the plastic deformation of the
die hub is reducing its size, as shown in Figure 10d. Other aspects of railway tracks man-
ufacturing are related to hole and drilling quality, similar to other cases and applications,
so in further research the hole quality will be also investigated [28].
4. Conclusions
This work focuses on minimizing the energy required for metal forming the head of
a railway spike screw by computational simulation. The results show the following main
conclusions:
1. The wear is mainly focused on the die splice radii, where the highest contact pressure
is concentrated according to the computational simulation results. Specifically, the
scanned worn heads showed a transfer of material on the sidewalls of the hub to-
wards the die radius.
2. The main factor that affects the load and forming energy is the flash thickness, which
its value depends on the initial setup. Specifically, the minimum forming energy was
obtained for combining a hub wall angle of 1.3°, a starting material diameter of 23.54
mm, and a flash thickness of 2.25 mm. This flash thickness generates a lack of filling
at the top vertices of the hub, although this defect does not affect the functionality of
the part or its serviceability.
3. An as-received material diameter in the higher range of dimensional tolerance, 24.06
mm, produces folds on the head cap and increases the energy by at least 18%, which
increases the probability of failure of the die in a shortened time.
Author Contributions: Conceptualization, N.L.d.L. and D.M.K.; methodology, A.S., G.A. and
A.J.S.E.; experimental tests, J.A., A.S. and G.A.; formal analysis, all authors (J.A., G.A., N.A., A.S.,
A.J.S.E., D.M.K. and N.L.d.L.); investigation, all authors (J.A., G.A., N.A., A.S., A.J.S.E., D.M.K. and
N.L.d.L.); resources, D.M.K. and N.L.d.L.; writing—original draft preparation, J.A., D.M.K. and
Figure 11. Maximum contact pressure related to the flash thickness and most degraded areas.
4. Conclusions
This work focuses on minimizing the energy required for metal forming the head
of a railway spike screw by computational simulation. The results show the following
main conclusions:
1. The wear is mainly focused on the die splice radii, where the highest contact pressure
is concentrated according to the computational simulation results. Specifically, the
scanned worn heads showed a transfer of material on the sidewalls of the hub towards
the die radius.
2. The main factor that affects the load and forming energy is the flash thickness, which
its value depends on the initial setup. Specifically, the minimum forming energy
was obtained for combining a hub wall angle of 1.3 , a starting material diameter
of 23.54 mm, and a flash thickness of 2.25 mm. This flash thickness generates a
lack of filling at the top vertices of th hub, although this defect does not affect the
functionality of the part or its serviceability.
3 An as-received material diameter i the higher range of dimensional tolerance,
24.06 mm, produces folds on the head cap and increases the energy by at least
18%, which increases the probability of failure of the die in a shortened time.
Author Contributions: Conceptualization, N.L.d.L. an D.M.K.; methodology, A.S., G.A. and A.J.S.E.;
experimental tests, J.A., A.S. and G.A.; formal analysis, all authors (J.A., G.A., N.A., A.S., A.J.S.E.,
D.M.K. and N.L.d.L.); investigation, all authors (J.A., G.A., N.A., A.S., A.J.S.E., D.M.K. and N.L.d.L.);
resources, D.M.K. and N.L.d.L.; writing—original draft preparation, J.A., D.M.K. and A.J.S.E.;
writing—review and editing, all authors; supervision, A.J.S.E. and N.L.d.L.; project administra-
tion, D.M.K. and N.L.d.L.; funding acquisition, N.L.d.L. and D.M.K. All authors have read and
agreed to the published version of the manuscript.
Funding: This work is supported by the Serra Húnter program (Generalitat de Catalunya) reference
number (UPC-LE-304 (2018)) and by the Aeronautics Advanced Manufacturing Center (CFAA).
Thanks also are given to special agreement INTI-Faculty of engineering of Bilbao and to university
group grant IT 1337-19.
Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.
Data Availability Statement: Data sharing is not applicable.
Acknowledgments: The authors acknowledge Mariano Fernández Soler for providing the spike
screw samples, María Emilia Boedo for helping with the chemical analysis, Aurelio Martins Dos
Santos for performing the liquid penetrant tests, and Facundo Riu for the drawings.

Metals 2021, 11, 1834 11 of 11
Conflicts of Interest: All the authors who sign this manuscript do not have any conflict of interest to
declare. Furthermore, the corresponding author certifies that this work has not been submitted to or
published in any other journal.
References
1. Bonnett, C. Practical Railway Engineering, 2nd ed.; Imperial College Press: London, UK, 2008; p. 7.
2. Schulergroup. Available online: www.schulergroup.com/major/download_center/broschueren_railway/download_railway/
railway_broschuere_e.pdf (accessed on 24 May 2021).
3. Euroforge. Available online: www.euroforge.org/ (accessed on 22 May 2021).
4. Moreira, T.; Godefroid, L.; de Faria, G.; da Mota Silveira, R. Computational análisis via FEM of Tirefond screws used in the
fastening system of railroads aiming to avoid a recurrent failure case. Eng. Fail. Anal. 2019, 106, 104186. [CrossRef]
5. Sánchez Egea, A.; Deferrari, N.; Abate, G.; Martínez Krahmer, D.; López de Lacalle, N. Short-cut method to assess a gross
available energy in a medium-load screw friction press. Metals 2018, 8, 173. [CrossRef]
6. Liu, Q.; Guang, X.; Sun, S. The present situation and developing trends of hot closed-die forging. Adv. Mater. Res. 2015, 1094,
365–368. [CrossRef]
7. Sánchez Egea, A.; Martynenko, V.; Abate, G.; Deferrari, N.; Martinez Krahmer, D.; López de Lacalle, N. Friction capabilities of
graphite-based lubricants at room and over 1400 K temperatures. Int. J. Adv. Manuf. Technol. 2019, 102, 1623–1633. [CrossRef]
8. Railway Fasteners. Available online: www.railway-fasteners.com (accessed on 21 May 2021).
9. De Faria, G.; Godefroid, L.; Cândido, L.; Silotti, T. Metallurgical characterization and computational simulation of a screw spike
aiming to improve its performance in railways. Eng. Fail. Anal. 2016, 66, 1–7. [CrossRef]
10. Forja Sudamericana. Available online: www.forjasudamericana.com.ar/ (accessed on 21 May 2021).
11. Gontarz, A.; Pater, Z.; Weroñski, W. Head forging aspects of new forming process of screw spike. J. Mater. Process. Technol. 2004,
153, 736–740. [CrossRef]
12. Van Hai, D.; Hong Hue, D. Finite element simulation and experimental study on internal fracture of railway sleeper screw during
cross wedge rolling process. Mater. Sci. Forum 2015, 804, 311–314. [CrossRef]
13. Tomov, B. A new shape complexity factor. J. Mater. Process. Technol. 1999, 92, 439–443. [CrossRef]
14. Tomov, B.; Rossen, R. Shape complexity factor for closed die forging. Int. J. Mater. Form. 2010, 3, 319–322. [CrossRef]
15. Tomov, B.; Radev, R. An example of determination of preforming steps in hot die forging. J. Mater. Process. Technol. 2004, 157,
617–619. [CrossRef]
16. Hu, X.; Wang, L.; Wu, H.; Liu, S. Multi-objective optimization of swash plate forging process parameters for the die wear/service
life improvement. Mater. Sci. Eng. 2017, 283, 12–18. [CrossRef]
17. Bayramoglu, M.; Polat, H.; Green, N. Coast and performance evaluation of different surface treated dies for hot forging process. J.
Mater. Process. Technol. 2008, 205, 394–403. [CrossRef]
18. Altan, T.; Ngaile, G.; Shen, G. Cold and Hot Forging Fundamentals and Applications, 1st ed.; ASM International: Novelty, OH, USA,
2004.
19. Behrens, B.; Bouguecha, A.; Huskic, A.; Baumer, M.; Paschke, H.; Lippold, L. Increasing the efficiency of forging operations using
adjusted tribological surfaces enhanced by hard coatings. Tribol. Online 2016, 1, 432–443. [CrossRef]
20. Krawczyk, J.; Widomski, P.; Kaszuba, M. Advanced complex analysis of the thermal softening of nitrided layers in tools during
hot die forging. Materials 2021, 14, 355. [CrossRef]
21. Hirschvogel Automotive Group. Available online: https://www.hirschvogel.com/es/compania (accessed on 28 May 2021).
22. Prabhu, T. Simulations and experiments of hot forging design and evaluation of the aircraft landing gear barrel Al alloy structure.
J. Mater. Eng. Perform. 2016, 25, 1257–1268. [CrossRef]
23. Behrens, B.; Chugreev, A.; Awiszus, B.; Graf, M.; Kawalla, R.; Ullmann, M.; Korpala, G.; Wester, H. Sensitivity analysis of oxide
scale influence on general carbon steels during hot forging. Metals 2018, 8, 140. [CrossRef]
24. Bonnemezón, A.; Martinez Krahmer, D. Práctica Industrial de la Forja en Caliente, 1st ed.; Editorial Nueva Librería: Buenos Aires,
Argentina, 2012.
25. Matweb Material Property Data. Available online: www.matweb.com (accessed on 25 September 2021).
26. Hatzenbichler, T.; Harrer, O.; Buchmayr, B.; Planitzer, F. Effect of different contact formulations used in commercial FEM software
packages on the results of hot forging simulations. Metall. Ital. 2009, 102, 11–15.
27. Abate, G.; Perez, D.; Martinez Krahmer, D.; Bigot, R.; Radev, R. Forja en caliente en matriz cerrada: Influencia del flash y el peso
del material de partida sobre la fuerza de conformado y el llenado de la matriz. In Proceedings of the Anales del 15º Congreso de
Metalurgia y Materiales, Concepción, Chile, 17–20 November 2015.
28. Olvera, D.; López de Lacalle, N.; Urbikain, G.; Lamikiz, A.; Rodal, P.; Zamakona, I. Hole making using ball helical milling on
titanium alloys. Mach. Sci. Technol. 2012, 16, 173–188. [CrossRef]