The effects of hot isostatic pressing of platinum alloy castings on mechanical properties and microstructures

In earlier research a better understanding of the solidification characteristics for a number of platinum-based alloys was established. The findings demonstrated a strong tendency toward the formation of shrinkage and gas porosity upon solidification, and hot isostatic pressing (HIP), a high-pressure thermal treatment developed as a densification process, was proven to be an effective method to minimize and/or eliminate this porosity.

While the previous results made it clear that porosity had been significantly reduced following the HIP process, the authors had not yet explored the full range of HIP’s effects in terms of post-processed microstructure and mechanical properties. The goal of this new phase of research is to further our understanding by characterizing the post-HIP effects on platinum based castings with respect to grain size and shape, chemical distribution, and mechanical strength.

Key conclusions from the 2011 study can be summarized as follows:

• A single sprue approach as shown in Figure 1a was not recommended for the geometry tested because the multiple sprue approach alone demonstrated a significant reduction in porosity for all alloys tested.
• Even with use of an optimized and directionally solidified sprue system, none of the alloys were capable of achieving fully dense castings.
• HIP was proven as an effective means to eliminate or nearly eliminate the presence of porosity in all platinum alloys tested.

To better demonstrate how the HIP process works, Figure 3 shows the schematic for a typical HIP unit. The unit contains a high temperature furnace enclosed in a pressure vessel. Parts are typically placed within the chamber in vertical layers with the use of  steel or ceramic shelving to maximize load capacity. During operation the HIP chamber is first placed under vacuum, followed by flooding with an inert gas, usually argon, which is used to apply the isostatic pressure. The temperature and pressure is then ramped up and left to dwell for a specified period of time depending upon the material’s properties. Parts become densified when the material’s yield strength is surpassed, creating a plastic flow that forces internal voids to collapse under differential pressure. The internal surfaces of the voids diffusion bond together, increasing density and thereby improving the material properties.  HIP unit sizes span from small laboratory size up to large-scale industrial. The unit shown in Figure 4 is an example of a smaller scale unit.

Not all metals will HIP effectively and the extent to which an alloy will respond to HIP is a function of its creep resistance. Creep is a solid material’s tendency to move slowly and deform permanently under stress. In metals, creep increases with temperature and starts at approximately 40 to 50% of an alloy’s melt temperature.[i] The rate of creep is a function of temperature, the material’s properties, and the amount of pressure that is applied.  In order to achieve optimum material properties, the parameters used in a HIP cycle must be precisely dialed in according to the needs of the alloy. Typical parameters including both metals and ceramics will generally fall into the ranges shown in Table 1:

The casting parameters and conditions for our trials are shown in Table 2. Standard pour temperatures, flask temperatures and firing curves were used.  The flasks were cast using a centrifugal casting machine with induction heating and each tree contained two test geometries for each alloy.  One casting was retained for as-cast sampling and the second casting was HIPed according to parameters specific for platinum alloys but proprietary to the HIP vendor.

Another important finding is that grain size is not negatively affected by the HIP process (Figure 7). A simple heat treatment to the same thermal parameters as the HIP processing (without the use of pressure) is neither capable of fully closing the pores from the as-cast condition, nor maintaining grain size (Figure 8). While the amount and the size of pores are clearly reduced, grains are growing substantially during heat treatment. Thus, any beneficial effect of porosity reduction is compromised by grain growth.  Based on this result, it would appear that the pressure used in the HIP process has the added benefit of retarding grain growth. Figure 9 demonstrates the comparative grain sizes of 950PtRu in the as-cast, HIPed and heat-treated conditions.

The casting sample of 95Pt5Co shows no visible macroscopic or microscopic porosity in the as-cast condition (Figure10). However, compared to 95Pt5Ru the grains are extremely large. Their size and shape indicates a relatively slow solidification process where a few grains were nucleated at the surface of the part, which then grew into the center. As a consequence, there was sufficient time for feeding and the microstructure is free of pores. Therefore, during HIPing few changes of the microstructure occurred in the 95Pt5Co (Figure 11).  This is not to say that HIP does not provide any benefit to 95Pt5Co castings.  The previous research on larger samples demonstrated a tendency of this alloy to form large gas pores and centerline shrinkage porosity that were either eliminated or reduced in size by the HIP process.

Certain defect types either do not respond to HIP or have a lower densification response. A key limitation of the process is that only porosity that is fully subsurface will collapse; if pores are open to the surface of the casting in any way, they will not respond to HIP. This effect can be seen in the shrinkage porosity (Figure 12a). Another limitation of HIP is seen in gas pores.  Pores created by gas are less responsive to HIP than shrinkage pores due to the pressure they contain. Rather than being eliminated, the pores are typically reduced in size by HIP as can be seen in the cross section below (Figure 12b).

Before launching our tensile tests we thought it prudent to compare the porosity levels found in our as-cast test bar geometry versus those from the test design used in the porosity analysis reported in the 2011 research. In Figure 15a is a 50X photomicrograph of a cross-section taken from the gauge area of a 95Pt5Ru test bar.  In comparing this with Figure 15b, a 95Pt5Ru cross section at 50X derived from our earlier research, we note a somewhat different distribution in the porosity. The test bar has a centerline orientation to the microshrinkage, whereas the ring geometry has more scattered microshrinkage.  Notwithstanding the notable differences seen in the samples below, the important point here is that both of these geometries produce what can be considered a near-optimal condition for 95Pt5Ru in the as-cast condition.  Conversely, if we refer to Figure 16 depicting a typical jewelry design, we can easily surmise that solidification of this geometry will be more challenging due to the thick-thin orientation and lack of ability to directionally solidify.  Thus, tensile results reflect an optimal condition in comparison with much of the real world cast product.

At strains above UTS, which marks the maximum of the stress-strain curve (Figure17), the cross-section is locally reduced through necking of the sample. Further deformation is localized in the necking region and as a consequence the required stress for further deformation is continually decreasing until failure occurs. The total elongation () indicates how much plastic deformation the material can withstand. Pores in the material will significantly reduce the elongation because they act as stress concentration sites. The effect of pores is even more pronounced on the reduction of area, which indicates how much necking occurs until the sample finally fails.

While UTS or hardness are clearly important properties to measure, they are not necessarily the most critical properties to predict failure in a broad number of applications.  When it comes to fatigue life, elongation and reduction of area are generally viewed as being more important. Specifically, in cases where subsequent cold working of the material is involved, an increased ability to bend before cracking is of paramount importance.  

The HIP treatment affects the mechanical properties of the four alloys in a different way. For all alloys the effect on YS and UTS is rather low. For 90Pt10Ir and 95Pt5Co there is little effect on elongation and ROA. However, for 95Pt5Ru and 90Pt10Rh a significant increase in elongation and ROA is found through HIPing. These results correlate well with the porosity levels of the different alloys, and are a clear indication that reduction of porosity increases ductility in the alloys.

Another interesting observation came from our analysis of UTS scatter in the sample population. The graphs below demonstrate the difference in spread between the as-cast and HIPed groups (Figure 17). The HIPed bars exhibit a very tight distribution, whereas the as-cast bars are more scattered.  This result correlates well with our observations of lower porosity levels together with a more homogeneous grain size and structure in the HIPed samples.

As stated above, reduction of area values posted the most impressive gains in the HIPed product. This property is of particular interest in the jewelry industry given the substantial amount of bending and forming that is inherent in stone setting, engraving, sizing, and myriad bench operations. Reduction of area indicates a material’s ductility and is crucial to successful performance in many of these operations.  Figure 18a and 18b demonstrate a profound visual difference in ductility between the test bar fractures in the as-cast and HIPed 95Pt5Ru. 

With increasing loads the Vickers hardness number and the scatter between the individual measurements decreases. At loads of 1000 grams or above the values become constant. In the case of lower loads, this may be at the expense of the accuracy of the result for several reasons. With lower loads, the hardness indent becomes smaller and the hardness information originates from a smaller sample volume. Therefore, pores, inclusions or other material inhomogeneities affect the obtained results strongly due to their relative concentration in the indent itself. Conversely, if there are pores outside of the indent and the indent itself was too small to capture these, the sample would yield a higher result than the bulk material would warrant. The measurement principle of Vickers hardness testing requires measuring both diagonals of the hardness indent. With small indents the accuracy of such measurements becomes lower and thereby the scatter among the individual hardness measurements increases. Therefore, 1000 grams would appear to be the more suitable load for testing of macroscopic platinum alloy samples.  Detailed recommendations and standardizations of Vickers hardness testing can be found in national and international standards, such as DIN EN ISO 6507-1:2006-03.

Having said that, it must be acknowledged that the jewelry industry in North America is not standardized in its approach to measuring and reporting Vickers hardness.  For example, the Hv values in this paper will appear very low in comparison to the typical reported hardness levels for these well-known alloys simply because of the high load we chose to apply.  Further work should be pursued towards the use of a more standardized approach with respect to micro-hardness testing and reporting across the jewelry industry.  This would improve understanding of material performance expectations by designers and manufacturers and help predict wear resistance in a more standardized manner.

Table 6 reports Vickers hardness values for three of the tested alloys.  With the possible exception of 950 PtRu, all alloys report values that are so close in the before and after HIP conditions that any difference is seen as essentially inconsequential.  Even the 950 PtRu that shows a 12-point spread is not considered enough to be characterized as an appreciably harder material by performance.  Thus we can conclude that hardness is not significantly impacted by HIP on the platinum-based alloys we tested.