Densification Behavior of Iron Powder during Cold Stepped Compaction

An Investigation For Optimal Tool Kinematics In Powder Compaction Cycle To Minimize Density Gradient In Green Powder Compacts

“…One of the major disadvantages of PM is the density variation throughout the green powder compacts. During the compaction cycle, due to the existence of friction at powder-tool interfaces, the contact surfaces experience a non-uniform stress distribution mainly having to do with a variable friction coefficient and tool kinematics of a press system, consequently resulting in the density gradient throughout a compact body [10,15,20,43,44,45,49,48,70,80,[88][89][90]101,102]. Density variation in a green powder compact represents a serious problem in terms of its reliability and performance as it may contribute to a crack-defect generation during the compaction cycle, depending on part's geometrical complexity, and more importantly a non-uniform shrinkage of a powder compact during the sintering process [4, 10, 20, 26, 44, 48, 49, 70, 88-90, 101, 102].…”

“…Unlike, isotropic material which only yields due to deviatoric stress tensor, the yielding of powder is caused by both deviatoric and hydrostatic stress tensors. Consequently, material parameter β represents the degree of influence of hydrostatic stress component on the onset of yielding of powder (or porous) material, whereas γ represents the ratio of the apparent stress applied to the powder (or porous) body and the yield stress, σ , applied to the powder particles (or matrix of porous material) [43,106 ]; see Figure 10.…”

“…where ε ij e and ε ij p are elastic and plastic strain rate tensors, respectively [17,43,45,50]. The plastic strain rate tensor ε ij p is expressed as ε ij p = λ ∂Φ ∂σ ij (5) or in alternative form as…”

“…where λ is a positive proportionality constant, to be determined later; Φ is Shima and Oyane's yield function for powder (or porous) material; and η is the shear viscosity for powder constitutive (or matrix of porous) material [17,43,45,50,75].…”

“…where D ijkl e is a fourth-order elastic modulus tensor [17,43,45,50 ]. Further, it is assumed that plastic work done on powder (or porous) body is equivalent to that of the matrix material, such…”