Hot Isostatic Pressing will have an important international role in the
materials arena, especially in medical ceramics and in aerospace and defense
industries, says Hebeisin.
Design News: What exactly is Hot Isostatic Pressing (HIP)?
Hebeisin: HIP is the somewhat unfortunate terminology for a process that subjects a material simultaneously to high temperature and high gas pressure. Typically, the temperature is selected to permit limited plastic deformation of the material being processed in the solid state at an argon gas pressure of 15,000, 30,000, or at times 45,000 psi (1,000 to 3,000 atmospheres). The combination of high gas pressure at high temperature is used to modify the properties of a material in a predetermined way. For example, HIP can close internal porosity in a material without distorting external geometries, consolidate powder materials to 100% of theoretical density, or form perfect diffusion bonds between similar or dissimilar materials.
Q: How has HIP developed?
A: HIP was developed at Battelle Memorial Institute in about 1955 as a way to bond metal claddings onto nuclear fuel rods. At high temperature and gas pressure, the metal cladding shrunk into intimate contact with the fuel rod and formed a metallurgical bond providing optimum heat removal from the core.
Researchers quickly found that this new tool was ideal for densifying many powder materials to 100% of theoretic density, if they could form an external, pressure-tight container that would survive the HIP cycle without rupturing. Complex shapes could be formed, only limited by their ability to fabricate the starting pressure-tight container.
It was the early 1970s before researchers found that they could use the process to heal internal porosity in cast or sintered materials. The only restriction was that the porosity not be surface connected. This generally happens in a sintered part at above about 95% of theoretical density. It has since been found that the process is very versatile, having been used to densify ceramics, glasses, plastics, and other materials.
Q: What should a design engineer know about HIP and its ability to maximize material performance?
A: Many design engineers will avoid the use of castings in applications that require freedom from porosity, for example for high-vacuum surfaces, for surfaces that need to be antiseptically cleanable, or for fatigue-critical applications. They need to know that HIP densified casting is a cost-effective alternative to machining parts from a wrought chunk.
They also need to know that HIP densified powder metal (P/M) material is very different from conventional pressed and sintered P/M parts. The HIP material is 100% dense with properties competitive with wrought material.
Q: Can you give a few examples of where HIP is being used successfully?
A: HIP casting densification is a $30 to $50 million service business in the U.S. It is used extensively for fatigue-critical applications like nickel and titanium alloy structural castings for jet engines, turbine blades and vanes, and cast cobalt-chromium or titanium alloy orthopedic implants, such as hip joints. It is also becoming much more popular for densification of steels, stainless steels, and aluminum alloys for pump and valve components.
Q: How does HIP cost and delivery compare against other approaches?
A: In the past five years, HIP cost and delivery have improved significantly with the proliferation of larger, more cost-effective HIP units in regional settings. The added cost for casting densification is in the range of $1.50 to $2 per lb for steel castings that pack fairly well. The industry has developed "Coach" pricing concepts similar to that used by the heat-treating community where small lots run with other customers' material in common cycles.
Q: What future developments can engineers expect?
A: HIP will continue to become more user friendly and more cost effective as larger, faster units get deployed into regional settings. Special cycles have already been designed for cost-sensitive materials like aluminum. Our Densal process has been specifically designed for densification of the most common Al-Si at a significantly lower price than the standard HIP cycle. Much more rapid HIP units are being designed and built.