The use of High Pressure Heat Treatment for Aerospace Applications

Hot isostatic pressing (HIP) is commonly used for both castings and additively manufactured parts for high demand and critical aerospace applications. This process has been used to increase density and eliminate defects, with any modification or optimization of the microstructure performed in separate heat treat equipment. Advancements with hot isostatic pressing design and equipment now enables the ability to perform both HIP and heat treatment in one process step. This combined HIP and heat treatment approach is referred to as high pressure heat treatment (HPHT), and is made possible by utilizing forced convection cooling of the highly pressurized argon gas. The main purpose for such a development was to eliminate an additional process step, offering a more time and cost-effective heat treatment method. However, recent developments have proven the benefits of HPHT extend beyond just time and cost savings. Various material systems processed with HPHT have shown improved quality and capability. It has been shown that applying high pressure gas through the entire heat treatment steps yield a reduction or complete elimination of defects over conventional approaches. Also, the rapid gas cooling or quenching may achieve a more desired microstructure for optimized mechanical properties.

This article will cover the fundamentals and technology of HPHT, which is now being offered in modern commercial HIP equipment. Then a number of case studies will be reviewed showcasing the benefits that HPHT may offer for aerospace applications for both new make and repair. These cases range from porosity elimination and refinement of the microstructure for cast alloys to that of additively manufacturing alloys. To conclude future developments of HPHT will be discussed.