Modelling and Optimization of the Precision Hot Forging/Extrusion Process of an Asymmetric C45E/1.1191 Carbon Steel Bearing Element ^†

Abstract

Precision extrusion forging is an innovative manufacturing process for trouble-free production of high-quality components with an accurate shape. The process provides a reduced technological chain and high production efficiency, as only certain surfaces need additional processing. This study used QForm software as an environment for simulating precision extrusion forging. The main goal of this research was to present a brief overview of the latest research on the simulation of precision extrusion forging, with an emphasis on the production cycle rather than on mathematical description. This article examines the processes of simulation modeling of precision extrusion forging with newly designed tooling for the manufacture of a newly introduced asymmetric load-bearing facade element patented by Braykov. With the help of simulation modeling, appropriate modes for specific production were established, and were later implemented. The production process itself is briefly presented at the end of this article.

1. Introduction

In recent decades, growing demand at the industrial level for high quality, defect-free manufactured products with high mechanical properties has led to a dramatic development of forging processes, particularly extrusion forging processes. In addition to the advantages of extrusion forging [1], reduced die life is a significant challenge [2,3]. Extrusion forging processes are among the most difficult manufacturing techniques. Although this technology has been mastered, the proper production of extrusion-forged products with complex shapes that meet the high-quality expectations of customers requires significant experience from designers, technologists, and machine operators [4,5]. The introduction of new extrusion-forged products, the ongoing continuous optimization of existing technologies, the large number of factors influencing the correctness of the entire process, and their mutual interactions make the process very difficult to analyze [2]. In each stage of the forging production process there is a risk of error leading to a defect, which is usually not recoverable; reasons for this can be of very different natures: low or high temperatures, improperly made tools [6], worn or defective tools, improperly selected deformation speeds, as well inadequate lubrication of tools.

With the progress of automation and the robotization of forging-metal forming processes, more and more advanced lubrication and cooling systems are being developed [7,8]. Furthermore, thanks to the use of manipulators, completely flexible systems can be built that allow control of all important lubrication parameters, such as nozzle position, application time, lubricant composition, etc. [9]. Moreover, such solutions are fully synchronized with the operation of the forging aggregate, thus eliminating the human factor [10]. It should be noted that the implementation of automated lubrication systems in hot forging processes is difficult due to extreme operating conditions, including cyclic mechanical and thermal loads, as well as the short deformation time during forging of 0.2–0.5 s. Therefore, in the reference sources there are many discussions of lubrication systems in cold forging processes [11] or cold and hot forging [12,13].

In the forging process, hot working refines the grains, increasing both strength and plasticity; also, forged components very rarely have internal defects, unlike castings. Parameters such as internal defects can affect the fatigue design [14,15]. Due to these listed and other factors, there is currently an intensive search for new technological solutions designed for specific applications of extrusion forging, which should provide optimal conditions with relatively low costs while being reliable and easy-to-implement technologies in the blacksmith shop. The anisotropy of material induced in the forging process is the leading cause of improving mechanical properties. To achieve the required texture, it is necessary to plan the forging stages so that the material flow in the tool is appropriate. [16,17]. The present study aimed to validate and optimize a precision extrusion forging process for an asymmetric load-bearing element for curtain walling through simulation based on QForm 7.2.4 software.

Precision extrusion forging is a process that resembles flashless forging and can be considered a special case of extrusion. The difference between extrusion and hot stamping is that during hot stamping the entire workpiece is turned into a semi-finished product without a residue, as in extrusion, but the articles are relatively short. In both processes, there are three stages of deformation: first, flattening the billet in conditions similar to open die stamping; second, filling the die cavity; and third, extrusion of the metal into the deformation space (die).

In most real processes the first and second stages are very short-lived (ΔH1 and ΔH2 << ΔH3) and their presence is assessed by ƒ = F (H) [18] (p. 172). Moreover, these three stages are sharply separated because leakage of the metal into the die (nozzle) starts in the first and second stages and is more intense the smaller the H3/D3 ratio is. This suggests that three-dimensional pressure is already established at the beginning of the process, which allows the maximum ductility of the metal to emerge; this is why precision extrusion forging can be realized both hot and cold.

Main Characteristics of the Process

The most common formulas that relate the cross section before deformation S_o to the cross section after deformation S₁ are as follows:

• Extrusion ratio

$${\mathrm{e}}_{\mathrm{p}}={\displaystyle \frac{({\mathrm{S}}_{\mathrm{o}}-{\mathrm{S}}_1)}{{\mathrm{S}}_{\mathrm{o}}}};$$

(1)

• True strain

$$\mathsf{\epsilon }=\mathrm{l}\mathrm{n}{\displaystyle \frac{({\mathrm{S}}_{\mathrm{o}}-{\mathrm{S}}_1)}{{\mathrm{S}}_{\mathrm{o}}}};$$

(2)

• Degree of reduction

$$\lambda ={\displaystyle \frac{{\mathrm{S}}_1}{{\mathrm{S}}_{\mathrm{o}}}}$$

(3)

Depending on the type of metal and the shape of the press die, the degrees of deformation used can range from 5–40 for steels and up to 300 and more for non-ferrous metals.

2. Methods and Materials

An analysis of the durability of extrusion forging tools used to stamp elements to the level of suspended facades from C45 steel was performed. Such elements are used to level suspended facades in tall buildings to compensate for linear deviations. The requirements were that it should have high operational properties, which are associated with the correct flow of metal during precision extrusion forging and the absence of internal and surface defects, as well as high quality and precision of the shape. The element was produced in one pass from a cylindrical starting workpiece above the recrystallization temperature. The selected steels are given in Table 1, and the detailed chemical composition is described in Table 2. Figure 1 shows a schematic of the simulated forging tool, which was used in the QForm 7.2.4 environment. The die container was made from steel 1.2379 on a wire EDM machine. The tool was heat treated to a hardness of 52–54 HRC, after which the surface layer was ion nitrided to increase hardness and wear resistance. The die container was pre-strain heated between 200 and 250 °C before starting the process to minimize scrap. The punch, on the other hand, was made of steel 1.2343, heat treated to 56–58 HRC.

Table 1. Steels used, according to world standards.

	Standarts	DIN	AISI	JIS	GOST	Grade (UNS)
Tool 1	X155CrVMo12-1	1.2379	D2	SKD11	Х12МФ	D2
Tool 2	X37CrMoV5-1	1.2343	H11	SKD6	4ХМФ6C	H11
Workpiece	C45	1.0503	C45	1.0503	C45	1.0503

Table 1. Steels used, according to world standards.

	Standarts	DIN	AISI	JIS	GOST	Grade (UNS)
Tool 1	X155CrVMo12-1	1.2379	D2	SKD11	Х12МФ	D2
Tool 2	X37CrMoV5-1	1.2343	H11	SKD6	4ХМФ6C	H11
Workpiece	C45	1.0503	C45	1.0503	C45	1.0503

Table 2. Chemical compositions (in percentages).

Standard	Steel Number (Name)	C	Si	Mn	P, ≤	S, ≤	Cr	Ni	Mo	V	Cr + Mo + Ni
EN ISO 4957 [19]	1.2343 (X37CrMoV5-1)	0.33–0.41	0.90–1.20	0.25–0.50	0.03	0.02	4.8–5.5		1.10–1.50	0.30–0.50
EN 10083-2 [20]	1.0503 (C45)	0.42–0.50	0.40	0.50–0.80	0.045	0.045	0.40	0.40	0.10		0.63

Table 2. Chemical compositions (in percentages).

Standard	Steel Number (Name)	C	Si	Mn	P, ≤	S, ≤	Cr	Ni	Mo	V	Cr + Mo + Ni
EN ISO 4957 [19]	1.2343 (X37CrMoV5-1)	0.33–0.41	0.90–1.20	0.25–0.50	0.03	0.02	4.8–5.5		1.10–1.50	0.30–0.50
EN 10083-2 [20]	1.0503 (C45)	0.42–0.50	0.40	0.50–0.80	0.045	0.045	0.40	0.40	0.10		0.63

The entire tooling was developed using the CAD software SolidWorks 2014, based on which preliminary simulation modeling was also conducted using the specialized software QForm 7.2.4 to simulate deformation processes. The punch was set tool steel with a hardness of 58 HRC, the workpiece was C45 steel, and the die was tool steel with a hardness of 53 HRC, according to the available libraries in QForm.

3. Simulation

A virtual 3D tool (Figure 2a) was developed to simulate the precision extrusion forging of the specific asymmetric element consisting of a die and a punch (Figure 2a). A three-dimensional mesh was created along X, Y, Z—14,1,7 (Figure 2b), with the help of the three-dimensional displacement of the metal flow, as has been described. From the simulation conducted, it was established that there was good filling of the deformation space and a strong unevenness of the distribution of the effective deformations in the volume of the blank (Figure 2c). Discrete points were introduced according to the software algorithm with coordinates X, Y, Z—7,1,1 (Figure 2d), as in each of them it was possible to track parameters during simulation.

Chart 1 represents stress–strain curves for selected discrete points during the extrusion forging process: effective stress—σ_tr, plastic strain—ε_tr.

In Chart 2, the three main stages of metal leakage in the area of the matrix are clearly expressed. The first stage was up to 0.86 mm displacement of the punch. After that, there was a sharp increase in force until the second stage (3.37 mm) was noted. During the third stage, the required force increased slightly, with a sharper increase at the end, which was due to the approach of the punch to the matrix.

3.1. Initial Boundary Conditions for Numerical Modeling of an Asymmetric Model Using the Precision Extrusion Forging Method

3.1.1. Limit Values for Speed Based on the Specific Speed of the Punch

-

Punch speed ν = 0.15 m/s

-

Speed of the deformed workpiece W = (S_o/S₁)ν [m/s]

With the help of the software and additional processing based on the specified speeds, we constructed graphs, shown in Chart 3, for the change in the speed of metal flow during the deformation process, both for the workpiece in the die container and for the already deformed part of the workpiece, respectively. Both constructed graphs were approximated to straight lines with the same correlation coefficient R² = 0.3661 between speed and time. This coefficient corresponded to a weak correlation (0.3 < |R| ≤ 0.5), but it was the same, which is why there was no change in the speeds between them (Chart 4).

3.1.2. Limit Values for Temperatures and Tribological Conditions

In order to determine the limit values of the influence of temperature and tribological conditions on the course of extrusion forging, numerical simulations were conducted with the QForm 7.2.4 software product. To realize simulation processes, boundary conditions based on known deformation processes implemented in industrial conditions were adopted as follows:

-

Nominal charging temperatures 850 °C, 950 °C, 1050 °C, and 1150 °C.

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Ambient temperature 20 °C.

-

Tool temperature 250 °C.

-

Tool thermal conductivity 30 W/m.K.

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Hydraulic press with a nominal value of 120 MN—selected from the software database.

-

Friction coefficients 0.15 glass, 0.4 graphite and water, and 0.7 saline solution.

-

Levanov coefficient 1.25.

To implement simulation modeling in QForm 7.2.4, 3D tools were introduced that met the geometric requirements for the development of the specific part; these tools were previously prepared with the SolidWorks 2014 software product. The multifactor modeling itself was divided into two modules in order to limit the multivariate nature of the results obtained and their more accurate analysis. In Module 1, the influence of the temperature of the workpiece was considered at a friction coefficient of 0.4 graphite and water, and other boundary conditions were not changed.

-

850 °C—min. temperature for the specific steel for hot deformation.

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950 °C—step 100 °C.

-

1050 °C—step 100 °C.

-

1150 °C—max. temperature for the specific steel for hot deformation.

In Module 2, the influence of the friction coefficient was considered at selected temperatures, and other boundary conditions were not changed.

-

0.15 glass, 850 °C/1050 °C.

-

0.4 graphite and water, 850 °C/1050 °C.

-

0.7 salt solution, 850 °C/1050 °C.

Based on data obtained from simulations conducted with set boundary conditions from Module 1 (Figure 3), the following conclusions could be made:

-

An increase in the maximum temperature was observed at all four temperatures of the initial billet (

Table 3. Variations in temperature.

Temperature	850	950	1050	1150	850
Maximum	961.55	1018.73	1091.2	1177.91	961.55
Minimum	542.33	579.56	614.9	661.71	542.33

), with the highest increase occurring at the workpiece with the lowest temperature of 850 °C, which was most likely due to the difficult passage of the metal into the deformation zone [21] and slower recrystallization processes.

-

A probable reason for the increase in temperature may be high friction, which made it difficult for the deformed flow to pass due to the small radius of roundness of the die.

-

Around the punch and die, in areas where the metal was static, cooling of the part was observed (Figure 3), and in areas where the metal was most intensely deformed, the temperature rose.

-

At the end of these processes, the lower end of the workpiece was the most overcooled in all four simulations.

Extrusion force curves vs. time were obtained after processing the results received from the simulation of the process. Notably, the highest values of the applied force were necessary to deform the billet with the lowest temperature of 850 °C. Increasing the temperature of the workpiece (Chart 5) reduced the force required to implement the extrusion forging process, but at the same time led to an increase in the temperature of the punch and die. On the other hand, at temperatures above 300 °C these would wear out faster due to increased friction and reduced strength characteristics.

Chart 6 presents results of the simulation modeling of Module 2 at friction coefficients of 0.15, 0.4, and 0.7 at selected temperatures of 850 °C and 1050 °C.

The simulation results confirmed that increasing the temperature led to a decrease in the friction coefficient, but to increased heating of the punch and die due to the presence and influence of other parameters, such as mechanical, structural, and strength changes, etc., depending on the different speeds of the metal in the workpiece. Another important parameter is the dosing of the lubricant in the real production process, which was carried out with a dosing nozzle (Figure 4c) that injected the selected lubricant—graphite and water (friction coefficient 0.4)—into the die before placing each workpiece, removing even a small amount of its temperature. Due to the accumulated high temperatures of the die, the lubricant failed to prevent additional heating of the contact surface of the die. Local overheating was possible in these areas, which would have led to a reduction in the stresses on the metal surface and ultimately to an increase in wear in these areas.

4. Results

The typical extrusion process, by forging, was carried out by a mechanical press. First, each workpiece prepared with dimensions Ø40 × 25 mm from steel 1.0503 was heated with an inductor to a set temperature of 850/950 °C, as shown in Figure 4a, and then, with the help of a robot manipulator, it was placed in the die container. The robot freed the space of the mechanical press with a force of 1000 kN; a graphite lubricant was previously applied to the tool.

The real extrusion forging process included the following operations:

-

With the help of a robot, the workpiece was positioned under the inductor (Figure 4a).

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The inductor was in the lower position and heated the workpiece to the desired temperature (Figure 4a).

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A robot took the hot workpiece (Figure 4a) and positioned it in the die (Figure 4b).

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A working stroke and extrusion forging followed.

-

The robot took the finished forging (Figure 4c) and transported it to the container of finished products.

The entire completed process, from taking a blank from the robot, pre-deformation heating, placing it in the extrusion-forging die, removing the forging, and injecting lubricant, had a duration of 70 s.

5. Conclusions

This article examines the results of the development of tooling related to the 3D filling of the deformation space during precision extrusion forging of an asymmetric load-bearing facade element. The simulation conducted showed:

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The specific technology is very complex to implement in industrial conditions, as even small changes in the optimal parameters of the technological process can lead to poor-quality parts.

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The analyses conducted with QForm 7.2.4 established that a change in the charging temperature in the range of 850–1150 °C with a step of 100 °C affected both the power and tribological parameters and facilitated a change in the temperature of the tools.

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The temperature of the tool matrix is a key factor in obtaining defect-free forgings in combination with many factors such as the temperature of the workpiece, extrusion forging speed, flow rate in the deformation space, and lubricant.

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As increasing heat was applied to the workpiece, the temperature of the die increased and the lubricant failed to prevent additional heating of the contact surface of the die, which is why the lowest possible temperature is recommended for the production process.

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The developed technology for precision extrusion forging allows defect-free and waste-free production.

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Simulation results in the QForm 7.2.4 environment proved to be in good agreement with the results of the manufacturing process.

The precision extrusion forging process was successfully implemented in production according to the optimal parameters obtained from the simulation. Future developments include detailed studies of the dimensional accuracy of the finished product, metallographic analyses, strength tests, as well as tool life evaluation.