Hot and cold flashless forging in vertical split dies of a stepped shaft


 Flashless forging is classified as a precise metal forming technology. The main advantages of this technology are the reduction of the flash allowance and the shortening of the manufacturing time by eliminating the flash trimming operation. The article presents the process of one-step forging of a stepped shaft made of aluminum with the use of split dies. The process was carried out in cold and hot metal forming conditions. The forging process was analyzed numerically using the Simufact.Forming 15.0 software. The geometrical parameters of the obtained product were analyzed, the distribution of effective strain, temperature and the standardized cracking criterion were determined. The process force parameters were also determined. Numerical tests were verified in real conditions with the use of a specially designed device for forging in vertical split dies. Comparison of hot and cold forging in vertical split dies is presented.


Introduction
The shape and dimensions of precision forgings are largely similar to the shape and dimensions of finished parts. Precision forging is a near net-shape technology. It uses closed dies to prevent the formation of flash. The elimination of flash forming in the forging process reduces the consumption of material, energy (used for heating the billet in hot forging), and time (spent on flash trimming). In addition, flashless forging preserves the continuity of metal fibers, increasing in this way the strength of parts. Initially, flashless forging was used only to produce axisymmetric parts with simple shapes. Nowadays, however, the technology is increasingly used to produce flashless forgings with complex shapes. Song and Im [1], for example, present a technology of flashless forging of bevel gears for use in the automotive industry. The process they describe involves forging gears from AISI4130 steel under cold working conditions. Song and Im [1] have shown that the initial dimensions of the billet have a significant impact on the forging operation, i.e. correct filling of the die cavity, the value of forces acting on the tools, and tool strength. Flashless forging of parts with complex shapes has also been discussed by Vazquez and Altan [2], who used the example of a connecting rod. They observed that flashless forging required strict control of the volume of the preform and that, in practice, a thin axial fin of flash might form between the moving parts of the forging tools. A research team from Hanover has proposed several concepts for forging precision parts for the automotive industry [3]. The authors from Hanover of the article focus mainly on solutions for precision forging of crankshafts. Studies [4][5][6][7] report information on an innovative process of extruding precision products in a device with a moving container. In this process, the material is extruded radially into a cavity which is expanded during the process owing to the use of the moving container, which moves in the direction opposite to the movement of the punch. The concepts of flashless forging reported in the literature require the use of specialized machines, consisting of several tools effected by a system of wedges, and dies locked with the use of a spring mechanism.
The overview of the literature indicates that the main area of application of precision forgings is the automotive industry [8][9][10].
The technology of vertical split-die forging has been known for a long time now, but it has rarely been used in industry. This technology has been most widely applied in the production of plumbing fittings and parts. Currently, split dies are being replaced with special devices with two dies 3 and a system of punches driven by lever systems. Forging presses of this type permit to forge tees, cross tees and other fittings with pre-formed holes [11]. Vertical split dies can also be used to produce inner races of CV joints [12]. In the cold forging process, the greatest problem is posed by the low durability of the tools. The stock material must also be specially prepared to reduce its hardness; for this purpose it is subjected to spheroidizing annealing (softening). In order to minimize the forging forces, the material is also coated with phosphate or molybdenum disulfide. Split dies can also be used for forging aluminum in isothermal conditions [13] or for extruding machining tools, such as twist drill bits [14,15].
The traditional hot and cold forging process is widely used in the industry. The hot and cold forging process in split dies has not been compared so far. The article compares cold and hot forging in vertical split dies. A comparative analysis was carried out on the example of forging a stepped shaft. The analysis was performed in order to acquire new knowledge about the forging process in in vertical split dies. The decision to carry out this analysis was made on the basis of the wide interest in flashless forging technologies. Fig. 1 shows the geometry of the die inserts used and the dimensions of the die cavity, whose shape corresponds to the shape of the stepped shaft which is the subject of this analysis. The total length of the forged shaft was 125 mm. Two die inserts were used in the forging operation. The die inserts had a conical shape, with the generatrix inclined at an angle of 12°. They were guided in the device's grooves by guide poles. The dies were opened using a hydraulic ejector with a lever system.

2. FE modeling and experimental testing conditions
Split-die forging was simulated by finite element method (FEM) using Simufact.Forming software. The experimental tests were carried out under cold and hot working conditions. The 5 material for cold forging was modeled as an elastic-plastic body, and the material for hot forging was modeled as a rigid-plastic body. The tools for both processes were modeled as rigid bodies. Billet was modelled by hex element with 8 nodes. Rheological model of AW-6060 aluminum it was assumed that the value of flow stress σ F depends on: strain ε, strain rate ̇ and temperature T. For the calculations the von Mises yield criterion and the associated flow law was employed. It was assumed that 90% of the work of plastic deformation and friction is converted into heat. The value of the flow stress of AW-6060 aluminum resulted from the following equation: where: σ F is the flow stress [MPa], ε is the effective strain, ̇ is the strain rate [1/s], T is the .Thermal parameters of AW-6060 aluminum are shown in Fig. 3.

Fig. 3. Thermal parameters of AW-6060 aluminum used in FEM modeling
The friction conditions were described by the constant friction model. The following friction factors were adopted: m = 0.15 [16] for hot forging, and m = 0.10 for cold forging [17]. These friction factors corresponded to lubrication conditions in which a molybdenum disulfide lubricant and mineral oil were used for hot forging and cold forging, respectively. The thermal conditions were described by the billet-tools heat transfer coefficient (8 kW/m 2 K) and the billet-environment heat transfer coefficient (0.5 kW/m 2 K) [18]. During forging, the punch moved at a constant rate of 10 mm/s. The experimental tests were conducted using ∅ 30 × 169 mm bars as stock material. The cold forging material was subjected to softening annealing at 415 °C for 5 hours before forging and cooled down to ambient temperature with the furnace. Hot forging was carried out at the initial billet temperature of 450 °C. In the cold forging operation, at 25% completion, the material is upset evenly over the entire length.
Half way into the process (50%), the material fills the part of the die cavity with a diameter ∅ 34 mm.
In the second half of the process, the conical die cavity is being filled gradually. At the end of the operation (100% completion), the die cavity is completely filled with the material.  During hot forging, the material is upset in a different way than during cold forging. In the first quarter of the process, the course of upsetting is almost identical to that for cold forging. At 50% completion, however, a distinct barrel shape is formed in the conical area of the cavity. In the lower part of the cylindrical die cavity with a diameter ∅ 34 mm, the material is in contact with the dies, but in the upper part, there is a gap between the billet and cavity walls. As the material is further upset, it fills the conical die cavity and the gap in the cylindrical part of the cavity is reduced. At 90% completion, the gap in the cylindrical portion of the die cavity is still not filled with material. It is only when the operation is fully completed (100%) that the gap in the cylindrical part of the cavity is completely filled. However, at this stage, a gap is visible in the conical part of the die cavity. The fact that the material flow pattern is different during hot forging may be due to temperature changes. In the case of cold forging, the temperature changes are not so pronounced.    Table 1 compares the dimensions of the shaft obtained in the experiments and that designed in the CAD system. The hot-forged shaft has a much better dimensional accuracy. This is because the radial forces acting on the dies during forging were lower than in the cold-forging operation. The results indicate that the dies opened more widely under the influence of the radial force in the upper part of the die, which is confirmed by larger differences in dimensions ∅ A and ∅ B for both forging operations.

Conclusions
The present study describes the processes of cold and hot flashless forging of a stepped shaft made of AW-6060 aluminum alloy. The tests confirm that this type of shaft can be forged in vertical split dies. The analysis of the results leads to the following conclusions and observations: • In the proposed forging method, the material flow patterns observed during cold and hot forging are completely different.
• The different flow patterns lead to differences in the distribution of plastic strains.
• The risk of cracking is similar for hot and cold forging.
• The cold forging force is twice that of hot forging. The radial force observed during cold forging is four times greater than the radial force for hot forging.
• Cold forging creates a greater risk of the dies opening during the forging operation.
• Hot-forged parts have a better dimensional accuracy and surface quality.
• The hot forging process in vertical split dies is more advantageous than the cold forging process.