Elsevier

Powder Technology

Volume 368, 15 May 2020, Pages 130-136
Powder Technology

Variation in particle size fraction to optimize metal injection molding of water atomized 17–4PH stainless steel feedstocks

https://doi.org/10.1016/j.powtec.2020.04.058Get rights and content

Highlights

  • 17–4 PH MIM feedstocks tailored to three different powder volume fractions

  • powder shape quantified using dynamic image analyzer

  • injection molding parameters optimized for complex-shaped item

  • variation in coarse size fraction influences flow stability of feedstocks

  • variation in coarse size fraction considerably affects dimensional tolerances

Abstract

This work reports on an essential effect of rather slight size differences in water atomized 17–4PH stainless steel powder on resulting microstructure and dimensional tolerances of metal injection molding (MIM) parts. The powders of round to irregular shapes prepared in three different powder volume fractions were admixed into the paraffin wax/HDPE (50/50) binder in a double sigma mixer. After determination of critical solid loading, the rheological, molding, and sintering performance of 66 vol% solid loading feedstocks was tested on a complex-shaped MIM component. The slight differences in size fractions are reflected also in the powder shape, which was quantified through sphericity factor and aspect ratio by dynamic image analysis. The results indicate an increase in viscosity, flow instability and injection pressure, lower sintered density with enhanced dimensional deformations for feedstocks having higher amount of coarser (10–20) μm fraction accompanied with higher shape irregularity determined from sphericity factor and aspect ratio.

Introduction

Metal Injection Molding (MIM) technology inherits the advantages of plastic injection molding and powder metallurgy for manufacturing small-to-medium size precise metal components from different alloying composition such as steels (Fe2Ni, Fe7Ni, FeCr), stainless steels (SS 17–4PH, SS 420, SS 316 L), high temperature alloys (HK30, Inconel 713) and high wear resistance alloys (cemented carbides). In general, any alloy available in a powder size ranging between 0.5 and 20 μm should be processable with MIM.

Technology involves consequent processing steps starting from identifying suitable powder and binder system for successful mixing and injection molding. Molded (green) components then undergo debinding, where a binder is removed through solvent, thermal or catalytic approach. After a complete binder removal, metal particles are sintered to form a dense structure of a metal body with density and mechanical properties similar to a wrought metal.

Selection of suitable powder type plays a key role in MIM, governing molding and sintering conditions. Gas atomized powders can be packed to high densities with suitable viscosity during injection molding, whereas their spherical shape limits the component strength during debinding and sintering [1,2]. However, lower oxygen in the gas atomized powders with enhanced powder packing results in a higher densification during sintering. In contrast, rounded or irregularly shaped water atomized powders exhibit higher resistance to flow and lower packing density due to the presence of oxides inducing porosity, and thus lower sintered density [[3], [4], [5], [6]]. A comparison study of the 14 μm gas and water atomized powders after sintering in a hydrogen atmosphere at 1300 °C resulted in the sintered density of 98.9% with the tensile strength of 1280 MPa for the spherically shaped powder, and 97.2% with the tensile strength of 1080 MPa for the irregularly shaped one [7]. However, the difference in the obtained strength may be to some extent eliminated by sintering of water atomized powder at higher temperatures. Often, the cost effective mixtures of gas and water atomized powders are used to improve the interparticle friction to resist the slumping during debinding [7,8].

Claudel et al. [9] investigated the effect of particle size distribution (PSD) of Inconel 718 feedstock on the rheological properties. The results indicate that lower the average particle size is, higher the critical solid loading can be achieved. The similar study on 316 L feedstock by Sotomayor et al. [10] reports the increase in feedstock viscosity for finer size fractions in the powder. Further, shear sensitivity was found to decrease with particle size, whereas activation energy remained unchanged. Seerane et al. [11] studied the influence of PSD on the SS 17–4PH sintered parts and found out that the coarser PSD (12–50 μm) yielded minimum shrinkage and inferior sintered density, whereas fine PSD (2–8 μm) resulted in higher shrinkage, lower sintered densities due to agglomerations, but superior mechanical properties. A study provided on four gas atomized Inconel 718 powders ranging from fine (<22 μm) to coarser fractions (65–212 μm) by Contreras et al. [12] stresses the advantages of broad PSD in terms of better packing and lower viscosity.

Regardless of a considerable research attention to demonstrate the influence of the powder characteristics as particle size distribution (PSD), mean size and shape, majority of them lack the possibility to distinguish between the effect of the shape and the size. In our recent work [13], this issue was considered and investigated for the set of gas (spherical) and water (irregular) atomized powders having the same mean sizes. It was found out that for coarse particles the processability in terms of rheological behavior is better in case of gas atomized powders in accordance with previous findings, but in case of fine powders, water atomized powders showed higher performance.

Also, the research has been so far focused mainly on advantages/disadvantages of finer and coarser particle size on a flow performance of feedstocks, and there are only rare papers [e.g. 4,5] devoted to optimize overall MIM process to obtain desirable mechanical performance and dimensional tolerances. Thus, in this work the emphasis is given to tailor particle size fractions to produce a defect free complex-shaped MIM parts from water atomized 17–4PH stainless steel powder.

Section snippets

Materials and their characterization

Stainless Steel (SS) 17–4PH is a ferrous alloy containing (12–17) wt% chromium, (4–8) wt% nickel, and (0–4) wt% copper with possible maximum additions of molybdenum (0.5 wt%), silicon (1 wt%), manganese (1 wt%), and niobium (0.4 wt%) and carbon (0.07 wt%). Three different tailor-made particle size fractions were chosen for the feedstocks´ preparation. Table 1 summarizes their key properties such as particle size distribution (PSD), pycnometer density (ρpycn), tap density (ρtap), and particle

Evaluation of powder characteristics

The mean particle size (D50) of the chosen water atomized powders is in the range of (8–9) μm (Table 1). Table 2 shows the tailored particle size distribution from fine (lower than 5 μm) fraction to coarse one (up to 35 μm). It is observed that the WA_24, WA_34 and WA_44 powders have the (10–20 μm) particle size fraction (highlighted in bold in Table 2) corresponding to 24, 34 and 44%, respectively, while the coarsest volume fraction (20–35 μm) is kept similar. The SEM micrographs of powders (

Conclusion

The results obtained within this study indicate that it is desirable to control and tailor the powder characteristics in terms of particular size fractions and as well as its shape in order to produce quality MIM components. The critical parameters include the particle size distribution and sphericity/aspect ratio of powder, stability of viscosity, injection molding parameters, and sintering deformation. The increase in the population of coarser particle fraction accompanied with relatively

Acknowledgement

This work was performed with the financial support of the Ministry of Education, Youth and Sports of the Czech Republic – Program NPU I (LO1504).

Author contributions

Berenika Hausnerova: conceptualization; methodology; planning and supervision; writing and revision of the manuscript; funding acquisition.

Mukunda Bishmena Naharaj: resources; investigation; formal analysis of the results; writing of the draft of the manuscript.

Both authors have read and approved the final manuscript.

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