May 2021 Volume 3

FORGING RESEARCH

of the flashes on two sides of the forging might not be the same – the flash on the front face was longer than that on the back face. These observations were confirmed by comparing them to the actual forging, as shown in Figure 21b.

Figure 19. Solid model of a designed forming roll The finite element modeling helped the designers to gain a better understanding of the forming mechanism, to identify the root cause of the forming defects and to provide solutions to these problems. Both the design and product development time were reduced significantly. For example, after a forming roll was designed, a finite element model was built to simulate the forming process, which gave the design engineers an opportunity to observe the evolution of the dimensions and geometries of the workpiece second by second. Figure 20 displays a few examples of products whose designs were enhanced by finite element analysis. Cases where the dimensions of the simulated part were smaller than the design indicated that more material was needed to get the parts in the correct dimensions. When the simulated dimensions were larger than the design, on the other hand, oversized parts were not always indicated. Sometimes, triangulated or out-of-round parts were implied because of the excessive material in the forming roll caliber. If the difference was small, the target part dimensions could be achieved by adjusting the rolling mill settings. But if a substantial difference existed, the forming roll design had to be revised. An experienced designer can determine how much change in the forming roll design should be made based on the extent of the difference.

a b Figure 21. Simulated flash shape (a) and flash in the actual part (b) The profile of the bore surface in a forging in which the part had a large rib OD was often a place where ID underfill occurred. The metal flow along the inner bore was very sensitive to many factors. Design was certainly a factor affecting the ID profile, but it was not the only one. The rolling conditions might also have contributed significantly to this problem, as an ID underfill could be either created or eliminated just by changing the mill settings and tube shell dimensions. In the part shown in Figure 22a, the ID underfill in Area A occurred at the beginning of rolling. Then, during the deformation, another ID underfill was formed in Area B. In the final stage of the simulation, the second ID underfill was flattened but the first one retained its size. Compared to the actual forging in Figure 22b, the simulated part showed the same pattern of ID profile, although the ID underfill in Area A was greater than that of the actual part. In a limited number of studies of cases with an ID underfill, it was seen that the normal pressure on the lower corners of both sides of the large rib tended to be high and the acceleration gradient in the ID underfill region tended to be large. This explains how the ID underfill was formed. This also explains why, in production, sometimes the ID underfill can be exacerbated by increasing the feed angle; under a large feed angle, more material is pulled into the caliber, which causes the higher pressures in the problem areas.

Figure 20. Examples of products enhanced by finite element analysis- assisted design Finite element simulations also helped identify imperfections in the rolled parts caused by improper design of the forming rolls. In the simulation shown in Figure 21a, a slightly distorted mesh and varying plastic strain distribution (yellow to red colors) in the connecting area (bottom of the groove) might imply unevenly separated flashes. An offset plastic strain revealed that the length

a b Figure 22. Simulated ID underfill (a) and ID underfill in the actual part (b) 6.2 Finite element analysis-assisted design of experience This example demonstrates how finite element analysis was applied

80

FIA MAGAZINE | MAY 2021