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The crankshaft is a key component in automobile engines, having a significant impact on the overall performance of the engine; its quality directly affects the lifespan of the vehicle. The forging and casting processes can no longer meet the automotive industry’s requirements for high precision, high performance, and low cost in double-crankshaft applications. Using internal high-pressure forming bulging technology to manufacture double-crankshafts offers advantages such as overall forming, continuous flow line distribution, dense microstructure, high strength, light weight, excellent comprehensive mechanical properties, low cost, and the ability to facilitate automated production. In the field of engineering, vehicle lightweighting is achieved mainly from two aspects. Materials such as aluminum-magnesium alloys, composite materials and high-strength steels with high specific strength are commonly used. However, lightweight materials and high-strength steels have poor plasticity during cold forming, allowing only the manufacture of parts with relatively simple structures; they are not suitable for complex components with bending axes and variable cross-sections. Structurally, integrally formed hollow components can be used in place of traditionally welded or cast components, which helps to reduce the weight of the vehicle body while ensuring the precision, load-bearing capacity and safety of the parts. Therefore, structural optimization of parts and improvement of forming processes represent important approaches to achieving vehicle lightweighting. The internal high-pressure integral forming process is relatively difficult to implement, and it requires a precise matching of process parameters. For the internal high-pressure forming process of double-curved cranks, the main influencing factors include the loading path, such as the relationship between pressure inside the tube and time, the relationship between axial feed at the left and right ends and time, as well as the lubrication conditions of the tube material. By performing finite element simulations of the internal high-pressure forming bulging process for crankshafts with double bends, and analyzing the influence of different loading paths on the bulging effect of such crankshafts, the optimal process parameters for their bulging can be determined. Then, based on the numerical simulation results, actual forming tests were conducted on the double-camber crankshaft, yielding good test results. Part dimensions and process analysis: The double-crankshaft features a hollow structure with a variable cross-section, whose shape is circular. The overall dimensions of the double-crankshaft are shown in the figure below: Typical cross-sectional positions of the double-crankshaft’s overall dimensions. The initial diameter of the shaft is 38 mm, the wall thickness is 1 mm, and the length is 140 mm. At section A-A, the diameter changes from 38 mm to 50.5 mm, resulting in a maximum expansion rate of 33%. The double-crankshaft is a component used for transmitting torque and force; therefore, SS304 austenitic stainless steel, which boasts high strength and good toughness, is chosen as the material for its fabrication. The typical cross-sectional perimeter of the double-crankshaft’s geometry is shown in the table below: Typical cross-sectional perimeter and tube blank diameter. The key process parameters for manufacturing the double-crankshaft include the internal pressure level and the feeding amounts of the left and right punches. Internal high-pressure forming is a complex forming process that involves the combined action of internal pressure and axial feeding. Therefore, the matching relationship between these two factors has a significant impact on the forming quality of pipes. If the internal pressure is low and the axial feed rate is too high, the material fed axially cannot flow to the corners in time, resulting in accumulation at the corners and at the ends of the tube blank, which causes wrinkles. If the internal pressure is high and the axial feed rate is too low, the material cannot be supplied to the corners in a timely manner, causing the corners to become increasingly thin until they break. Establishment of the finite element model: The finite element model for high-pressure bulging of a double-crankshaft is shown in the figure below. The model consists of 4 components: the die, the tube blank, the left punch, and the right punch. The die and punches are classified as rigid elements; the anisotropy of the sheet metal is taken into account during numerical simulation, and the material model No. 36 from the material database is used. The tube blank material is stainless steel SS304, with a friction factor of 0.1, Poisson’s ratio of 0.28, yield strength of 245 MPa, tensile strength of 408 MPa, material density of 7850 kg/m3, hardening index n of 0.32, and strengthening coefficient K of 537 MPa; the material’s constitutive relationship is as follows. Finite element model for high-pressure forming of SS304 stainless steel double-crankshafts. Influence of loading path on wall thickness distribution and bulging height of double-crankshaft walls. During the forming process of double-crankshafts, the relationship between internal pressure and axial feed is a key factor determining whether forming will occur or not; therefore, when conducting numerical simulations and actual forming tests, high-pressure loading must follow these guidelines: the peak loading value should not exceed the material’s maximum formability pressure, which is generally 1/3 to 1/10 of the material’s yield strength. At the beginning of the forming process, the pressure should be increased as quickly as possible to reach the material’s yield strength, in order to prevent wrinkling defects. In the later stages of forming, the pressure should be maintained for a certain period of time, so that the tube blank can adhere fully to the mold under high internal pressure. Based on the above design principles, 5 loading paths are established for the high-pressure forming process of the double-crankshaft, and these 5 load paths are defined as Path 1 to Path 5 respectively. The hydraulic and forging steps vary for the 5 loading paths, while the final shaping pressure is the same. The hydraulic bulging pressure for loading path 1 is 20 MPa, and this value remains constant during the subsequent forging stage; the hydraulic bulging force for loading path 2 is 40 MPa, and it stays unchanged during the subsequent forging stage; the hydraulic bulging force for loading path 3 is 30 MPa, with no change during the subsequent forging stage; the hydraulic bulging force for loading path 4 is 35 MPa, and it remains unchanged during the subsequent forging stage; the hydraulic bulging force for loading path 5 is 35 MPa, and it increases linearly to 40 MPa during the subsequent forging stage. The axial feed amount for paths 1 to 5 is the same, at 17 mm. The 5 loading paths during simulation: In loading path 1, due to the low pressures during the hydraulic bulging and forging stages, the tube blank material cannot expand circumferentially; as axial feeding continues, the wall thickness gradually increases, leading to wrinkling on the inner side and folding. In severe cases, the wrinkles cannot be eliminated even with high pressure from subsequent plastic surgery. The folding in the deformation zone at the bottom of the corner can be clearly seen in the figure below. The simulation results and FLD for loading path 1. For loading path 2, the pressure is too high during the hydraulic bulging stage, and since the axial feed is low at this time, it is not sufficient to compensate for the circumferential deformation; as a result, the wall thickness at the top of the corner becomes increasingly thin, causing the top of the corner to crack before it can make contact with the mold. Simulation results for loading path 2 and FLD It can be seen from the simulation results of loading path 1 and loading path 2 that the pressure during the hydraulic bulging stage should not be too high or too low; a reasonable bulging pressure should lie between 30 and 35 MPa. Three sets of loading paths were established using this pressure range. The simulation results for loading path 5 show that the top of the crank elbow is the thinnest, while the bottom of the elbow and the ends of the main shaft are the thickest; moreover, the rate of thinning remains within the allowable range, the top of the elbow fits well against the mold, and the formed part is qualified. Simulation results for loading path 5: The bulging height and the maximum wall thickness reduction vary depending on the loading path used. When loading path 3 is selected, the bulging height reaches 31.1 mm, which is the minimum value; whereas for loading paths 4 and 5, the bulging height is 31.5 mm in both cases, which is the maximum value. As can be seen from the table below, under a constant axial feed rate, as the bulging pressure increases, the bulging height increases. However, once the pressure reaches a certain value, the bulging height remains constant, while the thinning rate continues to increase as the bulging pressure rises. Although under loading path 3, a lower forming pressure results in a smaller wall thickness reduction rate, the bulging height does not meet the requirements; therefore, loading path 5 was ultimately chosen as the loading method. Bulging height and maximum wall thickness reduction rate under different loading paths – Experimental study on internal high-pressure forming of double-crankshafts. The actual forming process for double-crankshafts involves first cutting a 304 stainless steel tube to obtain a tube blank of 200 mm in length using a saw; then applying a specialized surface coating as a lubricant to the outer surface of the tube blank. After that, the tube blank is placed inside the mold cavity and the mold is closed. The process parameters are set based on finite element simulations, and while high-pressure oil is injected into the tube blank, a punch is used to compress it. Once the testing procedure is completed, the formed double-crankshaft component is removed. Die for high-pressure bulging test of double-crankshafts Four-column high-pressure forming equipment The test results using loading paths 1, 2, and 5 are shown in the figure below. The bulging result when the bulging pressure and axial feed are appropriately matched is shown in Figure a (path 5). As can be seen from Figure a, the double-crankshaft exhibited neither wrinkling nor cracking. As can be seen from Figure b, when the bulging height is very low, the double-crankshaft structure fractures. The main reason for this fracture is excessive pressure in the early stages, along with insufficient axial feeding, which prevents enough material from being available for circumferential expansion; this leads to excessive thinning until fracture occurs. As can be seen from Figure c, the wrinkling occurs at the transition area of the crankshaft’s radii. The main reason for this wrinkling is the low pressure in the initial stage, which results in slow metal flow; as a result, the axial deformation cannot be converted into circumferential deformation, leading to material accumulation at the root area and thus wrinkling. Test results of the double-crankshaft under different loading conditions. The wall thickness of part A was analyzed further: after cleaning the oil contamination from the surface of the part, the surface was smooth, free of pits and scratches, indicating good surface quality. The double-crankshaft was cut along the axial direction using wire cutting to enable thickness measurement; 12 points on the cross-section were measured, and these values were compared with the simulation results. Comparison shows that there is a significant variation in the wall thickness distribution; the wall thickness is thinnest at the top of the crankshaft elbow (point 5), gradually increasing as one moves axially toward the sides, and is thickest at the bottom of the crankshaft elbow (point 1). In terms of thickness distribution, the experimental values are in good agreement with the simulated values. Comparison of wall thickness distribution along the crankshaft axis direction: Comparison of simulation and experimental results shows that the bulging height and length of the crankshaft journals in the actually manufactured parts meet the design specifications. Comparison of simulation and experimental results Simulation and experimental results The high-pressure forming process parameters for double-crankshaft parts were optimized through numerical simulation, with loading path 5 identified as the best option: the internal pressure was 50 MPa, the advancement distance of the left and right punches was 17 mm, and the friction factor was 0.1. Using this optimized loading path prevented defects such as wrinkling and cracking in the double-crankshaft parts, resulting in parts with a bulging height of 31.5 mm and a uniform wall thickness distribution; the experimental results were generally consistent with those of the simulation.