August 2022 Volume 4

MATERIALS

run a die is affected by cyclic mechanical deformation and the accompanying fatigue. Abrasive tool wear is the most common for hot forging. It is simulated by applying the Archard model and occurs primarily on those contact surfaces where significant movements of metal take place. The scale formed on the workpiece surface mixes with lubricant to form an active abrasive medium. Accurately simulating die behavior involves consideration of the die material’s mechanical and thermal properties, including Young’s modulus, Poisson’s ratio, yield strength, tensile strength (proportional to hardness), thermal conductivity and other thermomechanical parameters. Additionally, boundary conditions, such as how the dies are held in place, must be considered. Metal fatigue leads to premature damage in a die that is subjected to multiple loads well below the static yield strength of the material. The fracture process has three main phases: nucleation, spread, and catastrophic fracture. Nucleation is the development and initial growth of microscopic cracks. The total crack life from nucleation to fracture is the spread (or propagation) phase. Catastrophic fracture results from a rapid increase in crack propagation velocity. Also, dies in production are affected by thermal stresses and thermal fatigue. If, during production, a die overheats, the die material could locally form ‘hot spots’ and undergo a phase transformation that would adversely affect die performance and die life. Computerized simulations of parts and dies have been used for decades, but more recently simulations of forged parts and dies have been coupled, whereby mechanical and/or thermal simulations are run on the forging and on the die concurrently. These are far more complex and time consuming to run than uncoupled simulations with rigid dies but yield superior results. Ellinghausen estimates that about 10 percent of the simulations run by industrial users are mechanically coupled. The power of this technology is demonstrated by finding the optimal preform shape for the pictured U-joint cross forging. In the original technology, an upset billet was preformed in dies that were designed according to traditional guidelines with increased drafts and radii, then forged in the finish dies (Figure 1). We can see that a lap occurs in the finish forging (Figures 1b, 1c). A new optimal preform die was then developed using QForm Direct that didn’t show any defect in the finish forging (Fig. 2), either in the simulation or in the actual trial forgings. Moreover, this modification of the preform die reduced the required billet volume by 6.7 percent and significantly reduced the die wear. --Courtesy of Forge Technology Inc., Woodstock, IL

Wear pressure field shows abrasive die wear based on the Archard model.

Figure 1. 1a (left) Traditionally designed preform; 1b (center) Defect in finished forging; 1c (right) Defect magnified

Figure 2. 2a (left) QForm Direct preform; 2b (right) No defect in finished forging

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