NHML Resources - Application of Hot Isostatic Pressing in Manufacturing Design

It may sound like some new, exotic dry cleaning process and though many have heard of "HIP", Hot Isostatic Pressing, few of us understand the many benefits of this materials process. Since it's largely misunderstood, many conservative engineers are reluctant to adopt HIPping as an element in their manufacturing designs, thus missing a valuable process tool.

HIP is a process that subjects a material simultaneously to both high temperature and high gas pressure, usually Argon, in vessels equipped with sophisticated control systems and telemetry.

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) is isostatically exerted on the heated parts for a period of time. The chamber is then slowly cooled, depressurized and the parts removed.

Typical hot isostatic pressing schematic

(Figure 1. Typical HIP schematic.)

Since modern HIP chambers tend to be very large, huge multiples of small parts can be accommodated in a single HIPping run, thus rendering the unit cost of the process to a small number.

HIP can close internal porosity in a material without distorting external geometry, consolidate powder materials to 100% of theoretical density or form perfect diffusion bonds between similar or dissimilar materials.

All raw materials contain microscopic voids and "bubbles" of gas, from that standpoint, all raw materials can be considered porous. The advantages of HIP for such porous materials include the elimination of all internal porosity in simple or complex shapes with resulting improvement of mechanical properties such as ductility and fatigue life.

The HIP process falls into three categories: Densification, Powder Metallurgy and Composites, diffusing two like, or unlike metals together.

The benefits for powder materials include the ability to produce 100% dense billets and powdered metal near-net shapes at relatively low cost because of the reduction in machining costs. Near net shapes can be pressed to 100% density, Shape control is obtained by cans and mandrels designed using both CAD and FEA programs. Small variations in packing density lead to small distortions in the final shape. Mating surfaces, for instance, will almost always have to be machined. HIP can also achieve 100% diffusion bonds in clad composite materials.

Advantages

The decision to employ hard chromium plating would be dictated by the following needs and requirements:

  1. Control of microstructure
  2. Higher content of alloying elements
  3. High material purity
  4. Near net shape components up to 12,000 kg
  5. Complex capsule and material design

Typical HIP products now include automotive parts, pump bodies, valves, vacuum chambers, bearings, sterile enclosures, etc. anywhere residual porosity causes high rejection rates, unacceptable property levels and surface finishing problems after machining. Commercial alloy applications include steel, stainless steel and aluminum castings. Not limited to metals, the process is very versatile, having been used to densify ceramics, plastics, glasses and many other materials.

Many design engineers will avoid the use of low-cost castings in applications that require the freedom from porosity, for example for high-vacuum surfaces or containers, for surfaces that need to be antiseptically cleanable, or for fatigue-critical applications. They need to know that a HIP densified casting is a very cost-effective alternative to machining parts from a wrought blank.

Material densification via the HIP process, densifies to the material's theoretical limit. This has several advantages including, improvement of mechanical properties, enhancement of physical assets and ease of manufacturing. Typical applications have been 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.

The HIP process allows for the manufacture of composite or dissimilar materials, thus avoiding welds or costly, gasketed assemblies. The same high temperature and pressures may be applied to achieve the diffusion bonding of encapsulated metal powder to a solid material or the diffusion bonding of different solid materials. Thereby providing for the combination of properties such as a stainless steel body diffusion bonded to a titanium nozzle that is nearly free from residual stresses.

Photograph of a diesel engien valve lifter

(Figure 1. Diesel engine valve lifter.)

For some material combinations, an interlayer is required to prevent brittle phase transformation or to minimize thermal expansion differences between materials for bonding.

The diesel engine valve lifter shown in figure two is a good example of a clad product. This product replaced a troublesome design that used furnace brazing to apply a tungsten carbide wafer to a steel lifter body. The HIP bonded valve lifter proved to be significantly more reliable and saved a substantial amount of money in scrap and repairs. In total, in the past five years Bodycote-IMT of Andover, MA reports that they have produced well over 3 million lifters without a single field failure.

Hot isostatic pressing can provide many benefits by stabilizing a material, removing residual stresses, densifying and eliminating voids and occlusions. The process "homogenizes" an alloy and in most cases, the properties of the material are enhanced, providing greater stability and wear characteristics.

With cast materials, parts can be cast to near net shape and HIP'd, thus eliminating costly machining and additional machining stresses. A good example would be aluminum castings, notoriously thermally unstable to the point where producing a cast aluminum mirror was once considered virtually impossible.

However, a large 12" flat stabilization mirror is being used in the fire control system of the FVS Bradley tracked vehicle. Originally, a thick, 6061-aluminum alloy blank was heavily machined and ribbed in order to provide light weighting of the backside of the mirror. More material ended up in chips than remained in the mirror structure. In an effort to reduce costs, a cast aluminum mirror of A-201 alloy was produced. The mirror blank was HIP'd and the only post machining required was for the two mirror trunnions. The mirror face was subsequently diamond fly-cut to an optical surface. Being theoretically dense, there were no voids or occlusions to mar the optical surface.

The final cast and HIP'd mirror proved to be far more thermally stable than it's machined counterpart. In addition, this material processing reduced the final production mirror cost by more than 30%.

Beryllium and titanium are both sintered materials and both are extremely difficult to machine.

Tensile properties table sowing the difference between treated and untreated and HIP tensile bars from 7 x 7 3/4 in permanent mold plates of several different aluminum alloys.

(Table 1. These tensile properties show the difference between treated and untreated and HIP tensile bars from 7 x 7 3/4 in permanent mold plates of several different aluminum alloys.)

Beryllium is not only a costly material but it is also prone to micro-cracking or "twinning" during the machining process. Final dimensions require extremely careful machining with very small cuts of .0005". This is followed by an acid etch, hopefully, to final dimensions. By HIP'ing beryllium powders to near-net shape, high machining costs, are thus eliminated and the process radically enhances the properties of the material.

The highly and oft misstated toxic effects of beryllium aside, HIP'ing Be to near net shape greatly reduces the final cost of a part made from this remarkably strong and lightweight, material.

Cast Titanium aircraft engine parts are routinely HIP'd for greater reliability and performance. Today's jet engines could not function without the HIP process to densify and improve this difficult cast material.

The HIP process is recognized as a means of providing enhanced soundness or integrity, increased density and improved properties to w wide rane of materials. The process significantly improves the mechanical properties an fatigue strength aluminum alloy, sand and permanent mold castings. It has proved capable of eliminating microporosity resulting from the precipitation of hydrogen and the formation of internal shrinkage during solidification. The HIP advantage is of importance in the manufacture of castings subject to radiographic inspection when required levels of soundness are not achieved in the casting process.

At elevated temperatures and pressures voids formed by hydrogen precipitation in the castings are collapsed and healed as are shrinkage voids uncontaminated by hydrogen.

Mechanical densification such as forging did not provide the same results, even at elevated temperatures. Typical tensile improvements in Aluminum castings are illustrated in table one.

 

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