Hot Isostatic Pressing (HIP)
HIP combines very high temperature (up to 2,000°C) with isostatically applied gas under very high pressure (up to 30,000psi) – comparable to the Mariana Trench 11,000m deep in the Pacific Ocean. HIP is used to eliminate porosity in castings and consolidate powder metallurgy materials to give fully dense components. Further, dissimilar materials can be bonded together to manufacture unique, cost-effective parts.
HIP technology allows the engineer to optimise conventionally formed parts and also to design components unobtainable by other manufacturing routes, whilst simultaneously improving microstructural homogeneity and material properties.
Bodycote provides two major HIP routes for customers: HIP Product Fabrication, for the manufacture of components through powder metallurgy and diffusion bonding; and HIP Services, providing porosity removal through HIP densification.
Hot Isostatic Pressing (HIP) – Densification
Closed porosity and voids in cast engine components are potential initiators of failure; for parts that are subjected to high in-service stresses, the removal of porosity is essential to maximise the properties and working life of the component.
Casting is the ideal near-net shape process. The properties of cast components will, however, be limited by the unavoidable shrinkage of solidifying metal. Even the best castings may have a small amount of residual shrinkage or gas porosity; defects that are liable to impair their service performance and reliability. Impact resistance and fatigue properties, in particular, are extremely sensitive to small amounts of porosity.
HIP of castings increases fatigue strength, ductility, toughness and resistance to stress rupture, whilst improving machinability and surface finish. Significant savings may be achieved with an investment cast and HIP route in comparison to machining from solid.
Turbine blades and vanes from the high-temperature section of jet engines are routinely HIPed to ensure freedom from residual microporosity. HIP is used to optimise the properties of the latest generations of single crystal and directionally solidified investment cast blades.
Hot Isostatic Pressing (HIP) – Diffusion bonding/HIP cladding
It is possible to bond different materials together to give an optimum combination of material properties and to save on expensive materials.
The ability to diffusion bond dissimilar materials, each having specific properties, expands the manufacturing possibilities, enabling the protection of aerospace components to be addressed. For example, diffusion bonding and superplastic forming are used to make titanium airfoils in the fan section of large jet engines. Additionally, a thick cladding of wear and corrosion resistant material, such as the cobalt chrome alloys, may be applied by HIP to enhance the performance of actuators and other aircraft components.
HIP can produce multiple diffusion bonds in a single process cycle. Unlike other joining techniques there is minimal change in the properties of each material, for example good corrosion resistance may be combined with high thermal conductivity. HIP cladding is commonly used to coat premium materials with superior properties, such as corrosion and wear resistance, onto more economical substrates, so that a part may be designed cost effectively. Bi-metallic compounds can be produced by binding powder – powder, powder – solid, solid – solid.
Bodycote is working closely with aerospace OEMs to explore and develop opportunities for the wider use of HIP powder metallurgy in this sector.
Hot Isostatic Pressing (HIP) – Product Fabrication
Imagine a component with no weaknesses, no cracks due to welding, optimised weight and material properties, and a superior lifetime – all achievable without expensive machining and deliverable in weeks rather than months.
In addition to the densification of castings and bulk powders, HIP provides a different means to produce components which would be unobtainable using traditional methods (such as casting and forging) or the ability to consolidate multiple components into one. Using Bodycote’s Powdermet® process, net and near-net shape techniques allow the design engineer additional options as they produce value-added, fully dense components with isotropic material properties.
Working together with customers, Bodycote can provide cost-effective development of exotic and novel materials using HIP technology. New classes of raw materials, such as metal matrix composites (MMCs), were developed using the HIP process. For example, an aluminium alloy matrix with a high proportion of silicon carbide ceramic particles may be compacted to full density by the HIP process to give a very light and stiff material. Many precision airframe castings from alloys such as titanium, aluminium and steel are HIPed to ensure integrity, optimise mechanical properties and improve fatigue life.
Bodycote’s Powdermet® process provides:
- Design flexibility
- Short delivery times
- Reduced welding and machining
- Isotropic mechanical properties
- Mechanical properties equivalent to wrought
- No hydrogen induced stress corrosion
- Ultrasonic inspectability
- Bonding of dissimilar materials without the need for temperature-limiting adhesives
- Weight optimisation