Hot Isostatic Pressing of Castings

14 Sep 08

Hot Isostatic Pressing (HIP) is an innovative thermal treatment carried out in a pressure vessel under high
isostatic pressure and temperature, in order to:

• eliminate porosity, particularly in castings prior to finish machining;
• densify metal and ceramic powders;
• consolidate powder-metallurgy parts.

Applications of HIP include:

• Castings : Titanium - Alloy Steel – CoCrMo- Aluminium- Magnesium;
• Ceramics;
• Diamond tools- Gallium arsenide mirrors;
• Glass;
• Medical implants
• Sputtering targets;
• Infra-red Windows.

The Process

The process of Hot Isostatic Pressure treatment, which is often combined with Solution Heat Treatment of
materials, involves applying heat, very high gas pressures up to 200 MPa and the control of the cooling rates.

A hot isostatic press applies both pressure and temperature to form and uniformly densify a particular material. Pressure is applied inside a vessel by compressing a gas medium, such as argon or nitrogen, while the heat is supplied through a resistance-heated furnace, also located inside the pressure vessel.
Temperature and pressure are monitored and controlled within a specified tolerance throughout the process, and a computer provides man/machine interfacing. The main advantage of the HIP process is its ability to densify a powder or preformed material to 100% of theoretical density.

Additional advantages include the creation of a more homogeneous microstructure, the ability to bond dissimilar materials, or the ability to process to net or near net shapes. The process treatment effectively removes porosity and other micro-defects, refines the grain structure and carbide distribution to obtain optimal strength, wear and fatigue properties.

The microstructures in samples show the elimination of the elongated dendritic structure prior to treatment and equiaxed microstructure after treatment giving better fatigue strength.

Processing methods for advanced ceramic materials include Hot Isostatic Pressing (HIPing), Sintering, Hot
Pressing, Cold Isostatic Pressing, Injection Moulding, Extrusion, Slip Casting which are often used in
combination to form ceramic products for various high-performance applications. When uniform densities and/or a variety of sizes and shapes are needed, HIPing is often the forming method of choice. With HIPing,
components can be directly formed from powder, or they can involve a combination of techniques with a preforming step, such as CIPing, sintering or injection moulding. The attainable densities and basically limitless size capability of the HIP operation are not found in any other forming process.

Capability

Only a handful of facilities around the world have the equipment and expertise for Hot Isostatic Pressing. The Australian Nuclear Science & Technology Organisation (ANSTO) is unique in Australia in ceramic research
facilities and capabilities, including:

• Powder preparation and characterisation including; spray drying, milling, particle sizing and shape analysis, measurements of flow characteristics and others;
• Powder Pressing/fabrication facilities that include Uniaxial Die Pressing, Cold Isostatic Pressing facilities, gel casting, injection moulding, tape casting, screen printing and green machining;
• Densification facilities including Cold Crucible Melting, Hot Uniaxial Pressing, Sintering and Hot Isostatic Pressing.
• Finishing of ceramic components via CNC machining, Ultrasonic milling and diamond polishing.

ANSTO uses its technology to support and jointly develop better processes to improve the performance of
ceramics and metals. Currently ANSTO is working with a number of medical device manufacturers in providing
a HIPing and heat solution heat treatment of Titanium and CoCrMo orthopaedic implants.

Acknowledgement

Article produced by WTIA. WTIA wishes to acknowledge the contribution of the WTIA Medical Devices and Sensors Industry Specific Group.

Click here to locate suppliers of hot isostatic pressing or sintering.

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