The success of the forging processes requires high tonnage of the press machine and high strength/toughness of tools/dies to deform the materials to the required shape or dimension. These result in high production costs and limits in manufacturing some of the part configurations.
To reduce the production cost, the reduction in the forming load and the tooling costs is a big and major challenge for engineers and researchers nowadays. In other words, the lower forming load could enhance the service life of the forming tools and lower press machine capacity. Many techniques were developed and introduced to overcome these problems by focusing on the design of billets, toolings, and processes. Many techniques for the forming load reduction are reviewed and some of them are instanced . Relief hole and axis is a method of the divided flow technique proposed to reduce the forming load in a cold forging process of gears proposed by Kondo et al .The material flow is split into 2 parts to fill the tooth tips located at the outer gear teeth and/or flow either radially inwards toward the center axis or axially within a central hole simultaneously . In addition, the optimization of the preform shape and modification of the final forged shape is a technique that can reduce the forming load and also facilitate the die filling, improve material yield, and eliminate forging defects. A modern forging process, which is the combination of the advantages of sheet forming and cold forging, was introduced, namely Flow Control Forming (FCF)  or Sheet-bulk metal forming (SBMF) . The upsetting, ironing, and extrusion process are integrated into the conventional sheet metal forming which is typically the sheet forming consisting of the blanking, drawing, and bending processes. Therefore, the sheet material is forced to change shape at all 3-dimensions similar to the material flow in the bulk forming process. It is mentioned that this FCF or SBMF is mainly to reduce the material waste as well as the forming load . Furthermore, a technique for reducing the number of toolings for producing a similar product by only the few dimension differences, namely Common Single-Die Exchange Technique or C-SDET, is proposed . The idea is to determine the representative preform that could be used to produce various product models. In other words, this design aims to develop common generalized preforms by using the same component configuration/shape but varying the dimensions in some places to decrease the number of preforms and tooling installation time (downtime of the machine to install the tooling for each model).
The design of the optimum preforms and tooling shapes requires specific knowledge and past experience of the engineers which have been done for particular products. The optimum preform shape can lead to defect free parts with minimum required tonnage and waste material . Currently, the computational algorithms are developed and applied to design the optimum preform shape of very complex parts, such as the turbine blade. For example, the backward tracing algorithms together with the advanced finite element simulation were developed to determine the optimum intermediate shape in a shell nosing process to achieve the uniform wall thickness of the nosing part . Furthermore, sensitivity analysis based algorithm of the optimum preform design for single stage forming process was developed. The initial shape of the billet is determined by measuring the sensitivity of design criteria, billet height, and width on the final shape of the forging to achieve complete die filling .
Currently, the tendency to reduce the weight of the machine elements is a big challenge. The weight reduction provides important benefits, especially for the cost reduction, but the parts must maintain good mechanical properties, lifetime service, and reliability. Therefore, achieving the lightweight components can be practically done in two ways; a) utilizing lightweight materials, such as aluminum alloys or magnesium alloys, and b) optimizing part design, such as hollow design .
The utilization of the hollow components often results in material and energy savings throughout the manufacturing process. Metalworking procedures are used to create hollow machine components in which the workpiece is hollowed out over its whole length (with some stock allowed for finishing). This strategy provides practical advantages in addition to the economic benefits.
In such a case of a semi-hollow stepped shaft, as shown in Figure 1, the hot backward extrusion is conventionally applied to produce such part . This technique provides various advantages in terms of product quality, manufacturing rate, and cost. However, higher forming loads, lubrication costs, limiting shape complexity are the disadvantages of the backward extrusion . Moreover, the backward extrusion is not commonly employed at the high temperature, because the hot backward extrusion process causes the decrease of the tool life especially at the punch corners because of the high contacting stress .
The forming load and the uniformity of the wall thickness are also concerned mostly in the backward extrusion. It is mainly controlled by the punch profile. The influence of the punch face slope and the punch fillet radius on the lateral and axial force was investigated in . The results show that by decreasing the punch fillet radius and the punch slope, the lateral force is reduced. In contrast, the smaller punch fillet causes the increase of the axial force. Therefore, the proper punch geometry needs to be determined. The uniformity of the wall thickness between straight and circular punch land was investigated by Danckert (2004) . The different length of the punch land causes in changing the contact conditions between the punch land and the cup wall. It results in the punch off-center and variation in the cup wall thickness. Therefore, the utilization of the circular punch land reduced the maximum wall thickness variation by almost 18%. Moreover, the wall thickness uniformity is also affected by the elastic deflection of the die and container including the buckling of the punch . It was recommended that the punch should be short as much as possible, and/or the forming load should be reduced below the buckling threshold of the punch during the backward extrusion to prevent buckling . However, in some forging parts, the internal shape is controlled by the punch profile which depends on the geometry requirement. It is difficult to avoid or change such cases in some parts, especially for a long semi-hollow part. Therefore, the forming technique to overcome this problem needs to be developed without significantly modifying the internal punch shapes.
As seen in Figure 1, this part normally was manufactured by the hot backward extrusion. The preliminary simulation results obtained by the FEM show that the maximum forming load is about 1,500 tons, as seen in Figure 2. This such a high forming load could lead to some damages on the forming tools and the press machine. Therefore, this research aims to propose a new technique for producing the semi-hollow stepped shafts, namely, a combined bulging-piercing technique. It is composed of two main steps; bulging, and piercing- coining steps. The combined bulging-piercing technique is mainly developed to reduce the forming load in manufacturing of the long semi-hollow stepped shafts (Figure 1) and still maintain the concentricity of the parts. Three main process parameters, namely the bulging stroke (Sb), the counter-punch lifting displacement (Lc), and the friction value (m), were investigated to determine the effect on the forming load and the die filling by the FEM simulation. Only the case with the minimum forming load was selected to implement experimentally to avoid any damage which might happen in the production press and tools. Then, the die filling, wall thickness measurement, and macro etching (flowline analysis) were performed to validate the design concept and outcomes, outcomes of the load reduction, and accurate geometries.