Investment Casting History
Investment casting is a versatile process, used to manufacture parts ranging from turbocharger wheels to golf club heads, from electronic boxes to hip replacement implants. The industry, though heavily dependent on aerospace and defence outlets, has expanded to meet a widening range of applications. Modern investment casting has its roots in the demands of the Second World War, but it was the adoption of jet propulsion for military and then for civilian aircraft that stimulated the transformation of the ancient craft of lost wax casting into one of the foremost techniques of modern industry.Investment casting expanded greatly worldwide during the 1980s, in particular to meet growing demands for aircraft engine and airframe parts. Today, investment casting is a leading part of the foundry industry, with investment castings now accounting for 15% by value of all cast metal production.
Modernising of an Ancient Art
Lost wax casting has been used for at least six millennia for sculpture and jewelery. About 100 years ago, dental inlays and, later, surgical implants were made using the technique. World War II and then the introduction of gas turbines for military aircraft propulsion transformed the craft into a modern metal-forming process. Turbine blades and vanes had to withstand higher temperatures as designers increased engine efficiency by raising inlet gas temperatures. Initially, forged alloy steels were used but soon more heat-resistant alloys were needed, leading to the development of special nickel-base and to a lesser extent cobalt-base alloys, which became known as ‘superalloys’. Further alloy developments allowed turbine operation at even higher temperatures and greater efficiencies, but the materials became more refractory and less forgeable by traditional methods although becoming more costly to machine (particularly to the demanding airfoil configurations).
Modern Demands of a Casting Process
Against this background, attention turned to lost wax casting to produce accurately shaped blades. In meeting this challenge, four key problems had to be solved:
- Castings had to be reproducible within close dimensional limits.
- Castings had to be produced in high melting point alloys.
- There had to be high standards of metallurgical quality.
- Costs had to be lower than for alternative techniques.
It was the successful solution of these problems that laid the foundation for the modern investment casting industry.One buoyant niche market relies on special techniques (including hot isostatic pressing after casting) to produce components with fatigue strengths equal to forgings. Castings can now be made for applications with oscillating stress.
What Materials are Used in Investment Casting
Investment casting is used for a wide range of applications. Nickel and cobalt-base superalloys account for 50% of total output by value, steels (of all types) account for 35%, aluminium accounts for about 10%, and copper and titanium alloys make up a large part of the remaining 5%.
Gas turbine engine blades and nozzle guide vanes are exposed to very demanding conditions. Some of the most advanced materials and processes have been developed for such applications, which are now a major outlet for vacuum cast nickel-base superalloys. Complex coring technology to promote internal cooling is used routinely and effectively in blade applications. The metallurgical achievements of the directional solidification and single crystal techniques are used cost-effectively in volume production, but most precision-cast airfoil components downstream of the highest temperature/pressure stages are still based on equiaxed castings.
Large, complex thin-walled engine carcass parts such as diffuser housings and combustors are made by investment casting, producing parts up to 1500mm across and 600mm deep. These are replacing sheet metal fabrications, not only because they are more cost effective but because they also provide a high rigidity monolithic component with superior service characteristics. Investment cast integrally-bladed turbine wheels for smaller turbine engines, offering large cost savings over mechanical fabrications, have been widely adopted.
Recently, demand has been growing for investment cast blades and fans to be used in land-based turbines for power generation. Components are larger and operate at lower temperatures, but the growing demand parallels that for aero engine parts in the 1980s. This sector of the market has in fact sustained airfoil investment casters over the past few years when aircraft demand dropped.
In a different field altogether, there is a long-established market for hip replacement prostheses made from cobalt-base superalloys formed by investment casting.
The variety of steels used and of parts cast has increased dramatically as designers and engineers have realized the potential of investment castings. The aerospace, armament, automotive, food, petrochemical, nuclear, textile, valve and pump, and other general engineering industries all use the technique. From golf club heads to gearbox parts for automotive applications, from bicycle cam forks to various gears and cam components, investment casting with wear resistant steels covers them all.
Investment casting markets increased steadily over 40 years to the mid 1980s, when demands for new aircraft boosted both the sales of investment castings and industry capacity. The market continued expanding up to 1990, when annual worldwide turnover reached about 5.2 billion. Over the next few years, aerospace and defense demands dropped and output decreased, in some countries by as much as 15-20%. Since 1994, however, general commercial demand has picked up and with recent aircraft orders turnover is approaching its previous high.
The Outlook for Investment Casting
Investment casting is recovering lost ground. Commercial business is up, land-based power generation demand is buoyant and the aircraft market is increasingly active. Growth can be expected until the end of the century and beyond if the industry can promote itself well enough.
The Automotive Market
European investment foundries should look to opening up the automotive market to investment castings. Realistic pricing of standard parts, however, can only come with sufficient automation to deal cost-effectively with demand. But foundries will only invest in such equipment when orders are secured. Industry competition – particularly price competition – is intense and likely to get tougher. The process serves an international market in which there are growing imports from one region to another.