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Research Papers

Processing Defects and Resulting Mechanical Properties After Metal Injection Molding

[+] Author and Article Information
J. C. Gelin1

Department of Applied Mechanics, FEMTO-ST Institute, ENSMM Besançon, 26 Rue de l’Epitaphe, 25030 Besançon, Francejean-claude.gelin@ens2m.fr

Th. Barriere, J. Song

Department of Applied Mechanics, FEMTO-ST Institute, ENSMM Besançon, 26 Rue de l’Epitaphe, 25030 Besançon, France

1

Corresponding author

J. Eng. Mater. Technol 132(1), 011017 (Dec 09, 2009) (9 pages) doi:10.1115/1.2931155 History: Received August 23, 2007; Revised February 08, 2008; Published December 09, 2009; Online December 09, 2009

The paper is concerned with occurrence of processing defects and resulting mechanical properties associated with material processing by metal injection molding (MIM). MIM process is a multistep one that consists first in the injection of metallic powders mixed with a thermoplastic binder, followed by a debinding stage that permits to evacuate the polymeric binder, and then followed by a sintering stage by solid state diffusion that normally leads to a nearly dense component. The main defects arising during MIM processing are associated with powder segregation during injection molding, and uncompleted or heterogeneous mechanical properties resulting from solid state diffusion. The paper first describes a biphasic fluid flow approach that can accurately predict powder volume fraction after injection molding and consequently the associated segregation defects. This analysis is followed and continued by a proper sintering model based on an elastic-viscous analogy that predicts the resulting local densities after sintering and also associated defects. So, from the two subsequent models, it becomes possible to get the final powder densities after processing and to localize the possible resulting defects. This analysis is completed by an analysis using a porous material model to get the final resultant mechanical properties after processing.

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Figures

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Figure 1

SEM photograph of the MIM feedstock composed of gas-atomized 316L stainless steel powders and wax-based thermoplastic binders

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Figure 2

Jetting phenomena arising in mold cavity during the filling stage with 316L stainless steel based feedstock: (a) original feedstock and (b) recycled feedstock

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Figure 3

Inhomogeneous green density in the molded components due to segregation defects occurring during injection step of MIM for 316L stainless steel: (a) injection molded components, (b) density contours of tensile test specimen, and (c) density contours of the wheel component through segmented parts

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Figure 4

Cracks and distortion defects occurring during the debinding of 316L stainless steel MIM hip implants: (a) injection molded and debinded components, (b) cracks occurring during debinding, and (c) distortion occurring during debinding

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Figure 5

Comparison of the geometries of the 316L stainless steel tensile test specimens after injection molding and sintering indicating the uneven shrinkage in various directions

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Figure 6

Distortion of the bending test specimen (316L stainless steel) occurring in sintering due to the defects associated with the injection molding step

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Figure 7

Viscosity curves obtained on the 316L stainless steel based feedstock and the associated viscous behaviors for each phase obtained from modeling

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Figure 8

The filling states and powder volume fraction variations during the injection molding at different filling stages, obtained from biphasic injection simulations for 316L stainless steel feedstock

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Figure 9

The uniaxial viscosity and shrinkage obtained by the proposed model with the identified parameters for the 316L stainless steel: (a) uniaxial viscosity and (b) shrinkage

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Figure 10

Comparison of the geometries of the wheel part before and after sintering obtained by the numerical simulation

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Figure 11

Uneven shrinkage of the 316L stainless steel wheel occurring after sintering in three orthogonal directions issued from numerical simulation by considering green inhomogeneity, friction, and gravity effects

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Figure 12

Final relative density of the sintered wheel in 316L stainless steel resulting from numerical simulation

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Figure 13

The dimensional variations of the wheel components in 316L stainless steel: (a) after injection molding and (b) after sintering

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Figure 14

Numerical prediction and the experimental data for yield strength and ultimate tensile strength of the 316L stainless steel sintered parts

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