Addaero is committed to producing additively manufactured components that can withstand the harshest of environments. To meet these demands, we employ various post-processing procedures that improve the quality and functionality of our components. For aerospace applications, the most common post-process is to HIP (hot isostatic press) the parts. In general, the HIPing process is a furnace heat treatment with added pressure. The components are placed inside a high-pressure containment vessel where they are subjected to high temperatures and isostatic pressure from an inert gas. The process became popular in the aerospace industry to close-up porosity in castings and has been carried over into the processing of additive components since their induction into the industry.
Effect of HIPing on EBM Produced Ti6Al4V
To explain the when and the why of implementing HIPing in additive component production, we’ll focus on a material that Addaero has extensive experience with, Ti6Al4V. In production, an as-printed Ti6Al4V component will go directly from support removal to the HIP furnace, where the recommended parameters are roughly a 120 minute heat treatment, at 920° C and a pressure of 100 MPa. This treatment will have the following effects on the component:
Reduction of Residual Porosity
In the as-printed state, EBM Ti6Al4V components are 99.70% dense. The lack of density is due primarily to the residual gas porosity originally present in the powder that survives the in-process electron beam melting. The main reason for using HIPing is that the process reduces the amount of porosity in the component, increasing the density to roughly 99.97% and thus enhancing the overall properties of the material. HIPing can close porosity (primarily residual gas) that is ~100μm in size and below. Other porosity types such as those resulting from lack of fusion or delamination will be removed if they meet the size requirement.
As with any heat treatment, the effect over prolonged heat treatment times will be the coarsening of the grain structure. In the image reproduced below from (Tong et al. 2017), the images from c) and d) offer a good example of this coarsening effect. As can be seen, at the same magnification, the microstructure has grown substantially from c) to d) following a HIP process at the following parameters: 900°C, 102 MPa, for a dwelling period of 2 hours.
The influence that HIPing has on reducing the percent of porosity while coarsening the microstructure has a positive impact in the mechanical properties of the material. While the yield and ultimate strengths of the material are slightly decreased, there is a subsequent increase in ductility due to the increased size of the alpha laths in the microstructure. Hence, HIPing enables a tradeoff between maintaining good levels of strength and ductility in the finished components.
In an aerospace industry laden with high-quality standards and safety factors, it becomes clear why HIPing is commonplace for additively manufactured components. The process provides an extra level of insurance that the end-use parts can brave the most demanding of climates by reducing defects and creating a baseline for the mechanical properties of the component.