Effect of powder loading on metal injection molding stainless steels

https://doi.org/10.1016/j.jmatprotec.2006.10.039Get rights and content

Abstract

Powder loading is one of the most critical factors which has important influence on metal injection molding processes. Using the gas atomized spherical 17-4 PH stainless steel powder and the binder of 65% PW + 30% EVA + 5% SA, four kinds of feedstocks were prepared at the powder loading of 60, 64, 68 and 72%, respectively. The effects of the powder loading on the feedstock rheological properties, the compact distortion and tolerance control as well as mechanical properties and microstructures were investigated. It is proved that 68% powder loading is the optimal one. The feedstock with 68% powder loading can be injection molded with a comparatively low viscosity on a relatively wide temperature range, and it is best to get quick powder re-packing and binder molecule orientation during injection molding. From the standpoint of compact shape retention and dimension tolerance control, the optimal powder loading of 68% is also the best. Furthermore, the compact of 68% powder loading is easy to get sinter densification and has superior mechanical properties and microstructures.

Introduction

Metal injection molding (MIM) developed very fast as a kind of powder metallurgy net-shaping process in recent years. Introducing the idea of plastic injection molding, the binder is added into metal powder particles as the vehicle of flowability to get molded parts of the desired shape [1], [2], [3], [4], [5], [6]. The mixture of powder and binder is termed the feedstock. In the feedstock mixture, it is expected that each powder particle, which should be enveloped by a very thin film of binder, has a tight contact with each other. At the same time, all pores among powder particles are filled with binder. But it is very difficult to get that ideal powder–binder mixture. The volume ratio of solid powder to the total volume of powder and binder is defined as the powder loading. In general, it is expected that MIM feedstock has higher powder loading. A large excess of binder is unacceptable because the excess binder separates from the powder during molding, leading to flashing or inhomogeneities in molded parts. Most importantly, a large binder excess leads to compact slumping during debinding, since the particles are not held in place as the binder is removed. Higher powder loading means smaller compact volume shrinkage and easier dimension tolerance control, which is very important for the mass production of complex and delicate MIM parts. But too high powder loading is also unacceptable because it will lead to too high feedstock viscosity and result in the failure of injection molding.

In recent years, there are a number of investigations concerning the effect of powder loading on the injection molding of metal and ceramic powders. Huzzard and Blackburn [7] studied the influence of the powder loading on the rheological behavior of alumina aqueous pastes at five levels of powder loadings. Allaire et al. [8] found that a feedstock with too higher powder loading had a viscosity greater than 1000 Pa s and exhibited a higher sensitivity to changes in temperature than feedstocks containing lower concentrations of zirconia powder. De Souza et al. [9] got a higher powder loading with deagglomerated tungsten powder prepared by rod-milled as-received agglomerated tungsten powder. Supati et al. [10] studied the influence of powder loading on the quality of the feedstock during the mixing process using a capillary rheometer. Dropmann et al. [11] increased the powder loading to 75% using comparatively coarse spherical Ni-base alloy powders combined with an appropriate low viscosity binder. They got a conclusion that the minimum amount of binder used leads to a reduced shrinkage after solidification, less distortion, a reduced level of contamination and a shorter debinding time. Li et al. [12] investigated the effect of the powder loading on the critical compact thickness during binder removal, concluded that the critical thickness was inversely proportional to the powder loading. German et al. [13] studied the effect of powder loading on the in situ dimensional change during solvent debinding of powder injection molded components. Liu and Tseng [14] studied the influence of powder loading on the green microstructure evolution and sintering behavior of molded articles. Tseng [15] evaluated the influence of powder loading on the dimensional control in ceramic injection molding using statistical analysis. All of the previous works help a lot to understand the effect of powder loading on the injection molding of metal and ceramic powders. However, the effect of the powder loading is not the key point of the investigation in most of these previous reports. There is no comprehensive study of the effect of powder loading on metal injection molding process. Actually, there are three important issues related to the powder loading in metal injection molding process. The first is the excellent feedstock rheological properties for successful molding of intricate and delicate parts. The second is the small compact distortion and tight dimension tolerance control. The last is the good mechanical properties. In present paper, the effects of the powder loading on the feedstock rheological properties, the compact distortion and tolerance control as well as mechanical properties and microstructures through injection molded 17-4PH stainless steel powder were investigated.

Section snippets

Materials

The metal powder used in this study is the gas atomized spherical 17-4 PH stainless steel powder with the pycnometer density of 7.89 g/cm3. The chemical composition and the powder morphology are shown in Table 1 and Fig. 1, respectively. The particle size distribution is as follows: d10 = 5, d50 = 12 and d80 = 22 μm. The apparent and tap density of the powder is 3.92 and 4.70 g/cm3, respectively. A binder system based on paraffin wax was prepared. The minor component is poly(ethylene vinyl acetate)

Effect of powder loading on the feedstock rheological properties

In MIM process, the feedstock rheological properties are key features which influence the steady flow and the uniform filling into the mold. The evaluation of the feedstock rheological properties is based on the viscosity and its shear sensitivity and temperature sensitivity [16]. The viscosities of the feedstock with different powder loadings at different shear rates and the same temperature of 135 °C are shown in Table 3. The viscosity data indicate the flowability of the feedstock. The lower

Discussion

The effects of the powder loading on the feedstock rheological properties, the compact distortion and tolerance control as well as mechanical properties and microstructures have been studied in the above section. In general, a higher powder loading is beneficial for all these properties. But the powder loading cannot be increased unlimitedly. MIM feedstock represents a balanced mixture of powder and binder. The amount of binder depends on the powder particle packing, since filling all of the

Conclusions

There is an optimal powder loading which is just slightly below the critical powder loading for any given powder–binder system. A feedstock with the optimal powder loading will have good rheological properties for molding, small distortion and good mechanical properties after debinding and sintering. For the circumstances discussed in present paper, four levels of powder loading of 60, 64, 68 and 72% for the gas atomized 17-4 PH stainless steel powder were studied. From the investigation of the

Acknowledgements

This work was financially supported by 863 plan Foundation (contract no. 2001AA337050), HuoYinDong Education Foundation (81041), National Excellent dissertation Foundation (200135).

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