## Calibration of Material Parameters for an Iron Powder Blend

The procedures of the previous section are applied below to the calibration of the two-state variable model introduced earlier. The data is for a powder blend comprising 99.5% by weight of Distalloy AE, 0.5% by weight of graphite, and 1% wax Hoechst micropulver. This last component is admixed as internal lubricant. Distalloy AE is a diffusion alloyed iron powder with composition 4 wt% Ni, 1.5 wt% Cu, and 0.5 wt% Mo. Particle sizes for this powder range from 20 to 180

m. The apparent density of the powder is 3.04 g/cm3; the pore free density of the material is 7.33 g/cm3. The data used are from experiments reported by Pavier and Doremus (Ref 14).

Consider the yield function of the model introduced in the section "A Constitutive Model for Metallic Powders with Ductile Particles" :

To fully determine the yield function F the functions b(D) and c(D) must be identified. Figure 14 illustrates the yield surface for a lubricated atomized iron powder (Hoeganaes Ancorsteel 1000). The surface is plotted for different values of relative density and particle yield stress pairs. For this powder, the functions b(D) and c(D) were determined by Trasorras et al. (Ref 40) to be:

0 13D

The constitutive model has two-state variables and requires two different kinds of tests for its calibration.

I 50

I 50

0=0.9. cry - 375 MPa | |||

COrj |
= 350 MPa |
• | |

■A |
\ | ||

0.7, oy = 300 MPa |
V | ||

•D = |
Pressure (P), MPa Pressure (P), MPa Fig. 14 Yield surface for lubricated atomized iron powder (Hoeganaes Ancorsteel 1000). Surface plotted for different values of relative density and particle hardening Triaxial Consolidation Test for 6(D). The state of stress in the test specimen is assumed to be uniform throughout (homogeneous). This assumption leads to the result that the radial and tangential stresses are equal and given by the cell pressure. If the specimen is cylindrical in shape, the components of stress in the powder aggregate can then be appropriately defined using a cylindrical coordinate system as follows: The negative signs are assigned to indicate the compressive nature of the stresses in the aggregate. The deviatoric and pressure components are then: The increment of plastic strain given by Eq 61 is: The deviatoric and volumetric components of plastic strain increment become: Eliminating A A obtain, Thus, the function b(D) may be determined using data from the triaxial tests for values of axial stress Ez, axial plastic |

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