16 MPR March/April 201016 MPR March/April 2010 0026-0657/10 ©2010 Elsevier Ltd. All rights reserved.

PIM2010

Gas atomised powders cut energy costsMIM’s net shape forming ability has enabled it to expand significantly in the past decade. Keith Murray and Martin Kearns of the Sandvik Osprey powders group say that gas-atomised powders give producers an edge in terms of quality and efficiency...

The metal injection moulding (MIM) market has demon-strated significant growth over the last 5-10 years. The tech-

nology’s net shape forming capability has helped it supplant investment casting and other traditional metal forming methods as the preferred manufacturing route for an increasingly diverse range of medium to high volume applications and market segments. The use of gas-atomised metal powders as the basis for MIM products can be demonstrated to deliver value-added ben-efits over water-atomised powders in terms of enhanced product attributes such as superior surface finish, improved mechani-cal properties and tighter dimensional tol-erances. Gas-atomised powders can also support lower production costs through reduced binder consumption, energy con-sumption during sintering and process sta-bility leading to improved consistency.

It is perhaps not surprising to observe that the global MIM industry has been adversely impacted by the global eco-nomic slowdown in 2008-9, but the extent of the decline does appear to vary across the different market segments and geo-graphical regions. While manufacturing in Europe has been affected by the slow-down in automotive activity and Asia has also been affected by a sharp drop in demand for consumer electronics, strong demand from the medical and firearms sectors appears to have insulated the

MIM industry in North America to some extent [1]. The competitive advantage of MIM in terms of net shape capability has helped support sustained market growth of 10-15% per annum since the begin-ning of the decade [2]. One result is that current annual demand for MIM powders is approximately 6000 tonnes [3]. With a market share of around 50%, stainless steels remain the core alloy type used in the industry. However the portfolio of alloys continues to grow as the adoption of MIM technology expands into new areas. This diversification can be seen in the sales mix at Sandvik Osprey as nickel-based alloys, tool steels and low-alloy steels are becoming an increasingly signif-icant proportion of MIM powder sales.

It is also Sandvik Osprey’s experience that there appears to be a divergence in the powder particle sizes being used in MIM. While overall demand for MIM powders is increasing, there is evidence to suggest that the relative proportions of coarser powders eg -32μm, 80% -22μm and also finer powders eg 90% -16μm, 90% -10μm appear to be increasing. The selection of powder size reflects the trade-off between part cost and part precision. Coarser pow-ders are being used increasingly for high volume, larger parts where material costs represent a more significant proportion of the overall part cost and where the dimen-sional tolerances and surface finish are not as demanding. Finer powders are, in turn, being used increasingly in the manufacture

Figure 1. Evolution of sales mix for MIM Powders.

Martin Kearns Keith Murray

March/April 2010 MPR 17metal-powder.net

of high-precision parts for such as medi-cal applications, which demand improved dimensional tolerances, or decorative parts which require a superior surface finish.

The presence of oxygen in MIM pow-ders, either in the form of inclusions or surface oxides, can have an adverse effect on the quality and performance of the finished part. Inclusions will worsen mechanical properties and impair surface finish, while surface oxides retard the rate of sintering. This can result in either insufficient densi-fication, leading to residual porosity in the finished component, or require the use of an extended sintering cycle and increased energy costs. Water-atomised powders are particularly prone to such problems as they have an intrinsically high oxygen content (typically several thousands of parts per mil-lion) which originates from the atomising medium itself. However, the careful control of key process variables during gas atomis-ing will produce a powder with a significant-ly lower oxygen content as shown in Table 1.

Figure 3 contains SEM surface images of two as-sintered components. One part was manufactured using water atomised 316L pre-alloy whilst the other was manu-factured using gas-atomised powder of 316L pre-alloy. Both components were processed using the same sintering cycle. The parts were manufactured during the development of a consumer electronics application, where a good surface finish was required.

Qualitative analysis of the two imag-es clearly reveals that the level of imper-fections on the surface of the part manu-factured using water-atomised powder is significantly higher than that of the

part manufactured using gas-atomised powder. EDAX analysis of the surface features on the water-atomised sample reveals that a number are rich in silica. Table 2 contains the results of the quan-titative analysis of the same samples.

Examination of these results confirms that a significantly lower level of porosity can be achieved in the final sintered part by using gas-atomised powder rather than water-atomised powder with an equivalent particle size. The use of a more expensive

Figure 2. Trends in popularity of MIM powder sizes.

Table 1. Typical Oxygen levels in gas atomised metal powders.

Alloy

Oxygen content (ppm)

Average Standard Deviation

17-4PH 700 150

316L 660 100

420 460 130

CoCrMo 250 70

Fe7Si 460 50

440C 490 140

4340 580 150

Figure 3. Surface images of a 316L MIM part manufactured using (i) water atomised powder and (ii) gas atomised powder under the same conditions (the dark spots are pores)

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finer grade of water atomised powder can help reduce the level of porosity, but the levels achieved were still found to be infe-rior to the coarser gas-atomised product.

In addition to having low oxygen con-tent, gas-atomised powders can also be characterised as having a spherical mor-phology and high packing density; char-acteristics that have been shown to deliver benefits in terms of overall production costs as well as the mechanical properties of the finished MIM part. The work of Gulsoy et al [4] provides a thorough comparison of gas- and water-atomised stainless steel powder. In this study the processing char-acteristics (feedstock preparation), sinter-ing behaviour and final material properties after heat treatment were evaluated for both gas- and water-atomised 17-4PH powder, one of the most widely used alloys within MIM today. The average particle size for the water-atomised powder used was 8.75μm

while the average particle size for the gas-atomised powder used was 10.65μm.

Gulsoy observed that the tensile prop-erties (H1050) and hardness values of test pieces made from gas-atomised pow-der were typically 10% and 5% high-er respectively than pieces made using water-atomised (see Figure 4). Although the ductility results are not reported here, it was found that the elongation levels achieved using gas-atomised pow-der were typically 5-10% higher than those achieved using water-atomised powder. This superior performance can be related to the higher sintered den-sity that was achieved with gas-atomised powder, typically 1.5% higher than the water-atomised equivalent across the range of sintering temperatures evalu-ated. This in turn is related to the higher intrinsic packing density of spherical gas-atomised particles compared with

the more irregular morphology of water-atomised products.

It can also be concluded that, in order to achieve equivalent strength, hardness and ductility levels, a lower sintering tem-perature could be used if water-atomised powder were to be substituted with gas-atomised powder of a similar, or even coarser, particle size. This substitution could deliver both environmental benefits in terms of lower energy consumption and economic benefits in the form of reduced energy costs. Similar observations were also made by Tai & Liang [5] in their study of 316L austenitic stainless steel, another popular MIM alloy. Not only do such benefits translate into superior mechani-cal properties and surface finish, but they can also yield lower total MIM processing costs, and lower total raw material costs, as well as enabling designers to achieve part designs not possible for parts based on water-atomised powders.

Looking beyond the field of stainless steels, there are examples of other alloy families demonstrating the benefits of using gas-atomised powders. Medical components are traditionally manufactured using an investment casting route. However it has been recognised that the net shape form-ing capability of MIM could potentially displace casting as the favoured method of production for higher volume products. One of the commonly used surgical implant alloy compositions today is Co-28Cr-6Mo, typically referred to as F75, the ASTM des-ignation for the cast version, as it offers a combination of high strength coupled with good corrosion resistance and biocompat-ibility. The work of Johnson & Heaney [6] provides a comprehensive evaluation of the

Figure 4. Tensile Strength and Hardness of 17-4PH (H1050) made from Gas and Water Atomised powders, Gulsoy et al. [4].

Table 2. Porosity measurements for water and gas atomised 316L parts.

Powder Type Particle Size (μm) Test Porosity (%) Average Porosity (%)

Water atom-ised

90% -24

1 3.07

3.62 3.78

3 4.08

4 3.36

Water atom-ised

90% -13

1 3.86

1.72 1.39

3 1.69

4 2.03

Gas atomised 90% -22

1 0.49

0.52 0.50

3 0.53

4 0.59

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performance of both gas and water-atom-ised Co-28Cr-6Mo powders processed by MIM. In this study the sintered components were also hot isostatically pressed (HIPped) to remove any residual porosity and then solution heat treated to minimise the forma-tion of undesirable grain boundary carbides.

As was the case in the evaluation of 17-4PH powders discussed previously, Johnson & Heaney also observed that a higher sintered density was achieved using gas-atomised powder. The optimal sintering temperature for the gas-atomised samples was also found to be some 40ºC lower than that of the water-atomised samples. Subsequent testing confirmed that mechani-cal properties exceeding the ASTM F1537 requirements for wrought material were achieved after HIPping of the water atom-ised samples, and after HIPping and heat treatment of the gas-atomised samples.

Metallographic analysis of the HIPped and heat-treated samples, however, revealed the presence of oxide inclusions in the water-atomised product, while the gas-atomised product was found to be free of such inclu-sions. This can be directly related to the oxygen content of the initial powders, 0.26% in the case of the water-atomised product compared to <0.02% in the gas-atomised powder. Johnson and Heaney commented that, in addition to the material specifica-tions for static mechanical properties, many Co-28Cr-6Mo medical implants also have specific fatigue requirements and therefore full density is essential. They concluded that, while typical fatigue property requirements can be met by the HIPped and heat-treated gas-atomised product, the inclusions that originate from the impurities (oxides) in the water atomised powder may have a detri-mental effect on fatigue strength.

It is widely accepted that in a well-controlled MIM operation, dimensional tolerances of under 0.5% are achievable. While it is recognised that the primary source of dimensional variability occurs during moulding, the situation can also be exacerbated during sintering. In their study of 17-4PH stainless steel Muterlle et al. [7] showed that densification at a given sinter-ing temperature is inversely proportional to carbon content. Carbon level in the part is itself a function of the oxygen content, as oxygen will react with carbon during sintering to generate both carbon monox-ide and carbon dioxide gas. Consequently the expected level of variability in carbon content for gas-atomised powders after sintering (ΔCGA) should be lower than the equivalent water-atomised powder (ΔCWA), given the typical oxygen levels of the two powder types identified previously.

Figure 6. Schematic of effect of carbon variability on alloy melting point and sintered density.

Figure 5. Sintered Density and Tensile Strength results for Co-28Cr-6Mo made from Gas and Water Atomised powder, Johnson & Heaney [6].

20 MPR March/April 2010 metal-powder.net

Figure 6 above contains a schematic which shows the effect of carbon variability on alloy melting temperature. Assuming that ΔCWA is greater than ΔCGA the expected variability in melting temperature for water-atomised powder (ΔTWA) is greater than that for a gas-atomised product (ΔTGA). This will mean that variability in sintered density, and therefore in dimensional tolerances, will be lower for gas-atomised powders (ΔρGA) than for the equivalent water-atomised product

(ΔρWA). This behaviour will be particularly evident in alloys with high carbon contents such as tool steels.

In his evaluation of the processing of SKD-11 tool steel by MIM Lin [8] has identified a possible solution to improve dimensional control and consistency. In this study the shrinkage behaviour of samples manufactured using gas-atomised SKD-11 powder with two different parti-cle size distributions, 90% -31μm (typical

D10 ~5μm, D50 ~13.5μm, D90 ~30μm) and 90% -16μm (typical D10 ~4μm, D50 ~8.5μm, D90 ~15.5μm) was measured across a range of sintering temperatures from 1220ºC to 1340ºC.

Examination of the data in Figure 7 shows a marked difference in part shrink-age across the range of sintering tempera-tures evaluated between the two particle size distributions that were tested. The relative improvement can be explained by the fact that, for a constant solids loading, the increased powder surface area per unit volume in the finer powder fraction will support a faster initial rate of sintering and therefore a greater level of densifica-tion across the temperature range.

References[1] M Bulger, J Hayashi and M Kearns: Regional Perspectives on NAFTA, Asia and

European MIM markets respectively, PIM2009, Orlando, Florida 2009.[2] R German, ‘PIM breaks the $1bn barrier’ Metal Powder Report, Vol. 63 No.3,

March 2008[3] W Heinke, ‘PIM Technology in 3C Applications’, PMAsia 2009, Shanghai,

April 2009.[4] H O Gulsoy, S Ozbek and T Baykara, ‘Microstructural & Mechanical Properties

of Injection Moulded Gas and Water Atomised 17-4PH Stainless Steel Powder’. Powder Metallurgy Vol 50(2) 2007 120-126.

[5] C KTai & C H Liang, ‘Study of Powder Characteristics on Mechanical Properties of Metal Injection Moulding (MIM) Product’, PowderMet2007, USA

[6] J L Johnson & D F Heaney, Center for Innovative Sintered Products, Penn State University, USA

[7] P V Muterlle, M Zendron, M. Perina, R. Bardini and A. Molinari, ‘Influence of Carbon content on Microstructure and Tensile Properties of the 17-4PH stainless steel produced by MIM’. PIM International., Vol. 2 No. 4, December 2008.

[8] Prof S Lin, National University of Taiwan, PMAROC 2008, Taiwan

Figure 7. Shrinkage behaviour of 90% -31μm and 90% -16μm SKD-11 Gas Atomised Metal Powder, Lin [8].

The AuthorsThis article was developed from Review of Developments in Gas Atomised Alloy Metal Powders for MIM Applications, a paper by Keith Murray and Martin Kearns, who both work for Sandvik Osprey Ltd. The original was given at the European Powder Metallurgy Association’s EuroPM 2009 confer-ence and Exhibition at Copenhagen.