Powder power

Technological improvements and the move to outsourcing in the automotive industry have catapulted powder metallurgy, a technique almost as old as the century, into a new and fast growing market. Even highly stressed parts such as car engine connecting rods are now being made by powder metallurgy, replacing forgings or castings. In recent years GKN […]

Technological improvements and the move to outsourcing in the automotive industry have catapulted powder metallurgy, a technique almost as old as the century, into a new and fast growing market.

Even highly stressed parts such as car engine connecting rods are now being made by powder metallurgy, replacing forgings or castings.

In recent years GKN has identified the area as a growth market and, partly through acquisitions, has risen to a dominant position in the world market.

‘Traditionally, automotive companies have had internal facilities for casting, forging and machining,’ says Seifi Ghasemi, chief executive of GKN Sinter Metals. ‘They kept making parts the same way until the early 1980s, when the industry came under pressure to reduce costs. Companies looked for the best technology and the cheapest way to make parts. That is when powder metallurgy (PM) really took off.’

The basic PM process involves combining a metal powder with a lubricant, compacting the resulting mixture into a die or mould and sintering it (heating at a temperature below the melting point of the main constituent until it fuses together).

The compaction process normally takes place at ambient temperature and results in the cold-welding of the powder grains so that the part keeps its shape and is strong enough to withstand ejection from the die. During sintering, atomic diffusion takes place and the particles bond to give the part its required strength.

Though most parts made this way are iron-based, a range of powders has been developed, including copper, brass, bronze, aluminium, beryllium and titanium.

The main advantage is that complex components can be made to net shape. With forging or casting the processes PM often replaces in automotive applications subsequent machining operations are required, which result in additional energy consumption and scrap material.

The disadvantage with PM is that the resultant part only has a density of about 90% because of the space taken up by the lubricant in the original mix. Designers seem to have trouble convincing themselves that this will not affect strength or durability, despite evidence to the contrary.

‘This is a psychological problem for design engineers,’ says Bernard Williams, executive director of the European Powder Metallurgy Association (EPMA). ‘They have not come to terms with designing parts with a 5 10% porosity.’

Where porosity is an issue, for example in parts such as gears subject to a high rate of wear, new techniques are becoming available to increase the density. The technology has taken great steps forward and the realisation that powder metallurgy can give big cost savings has started to dawn on industry, particularly the automotive sector.

Ghasemi estimates that the average weight of structural PM components in a car made in North America has risen from about 1kg in 1980 to about 15kg. He predicts that by 2003/4, the figure will be 25kg. The PM market worldwide has grown cumulatively at an average rate of 8% a year over the past 15 years to around $5bn (£3.1bn), Ghasemi adds, and is expected to stabilise at around 6% annual growth rate, at least for the next 10 years.

Following a number of acquisitions in recent years, GKN Sinter Metals is on target to achieve annual sales of more than £500m. It is already more than twice the size of its nearest competitor. Last year it set itself the target of doubling in size by 2002, since when it has acquired US company Interlake.

The automotive industry accounts for 70% of all PM applications, but the technology is also widely used for bearings and gears. The white goods sector shows great promise, as does garden machinery.

In a few applications the specific properties endowed on a part through the production process are a big factor in its selection. For example, the porosity of a powder metallurgy component is useful for self-lubricating bearings, as the pores can be impregnated with oil to give lubrication for life.

In most cases, however, the method is simply a cheaper production process. Cost savings depend on the volume required, but it is a technology that is well suited for high-volume production. Once a powder metallurgy tool has been designed and made, it can last for 500,000 parts or more, says Williams.

Automotive triumph

The automotive connecting rod is one of the latest triumphs. The cost saving is estimated at around 40% because machining operations are not needed. ‘A conventional con rod is cast or forged and then machined in a number of operations, the final one of which ensures that the two ends match,’ says Williams. ‘A PM con rod is fracture split, so the two ends match without any further operations.’ He adds that most car makers, including European companies such as Ford/Jaguar and BMW, specify PM con rods for new engines.

‘Ten or 15 years ago, most people would have said that a PM connecting rod was impossible. Today, 15% of all connecting rods in cars are PM ones,’ says Ghasemi.

Another recent use is in the main bearing cap, made from an iron/ copper alloy, which is being introduced into European engines.

The industry is focusing on ways to overcome porosity, the chief disadvantage of powder metallurgy. In the past, the density of the parts has been increased by a forging operation, after the powder compaction and sintering stages. But this adds considerable extra cost. Alternative approaches to high-density components are starting to produce results.

For an application such as transmission sprockets, porosity is only a problem for tooth flanks and journal surfaces, but not at the core. A solution has recently been developed by the Automotive Gear Division of Stackpole in Canada, which is producing PM sprockets for the General Motors Gemini 4T65 transmission at a rate of 1.8 million per year.

The company won an EPMA award for the technology last year. The secret is a process of mechanically working the outer layer of the part to increase its density.

For the high volumes found in the automotive industry, the extra cost of the mechanical working is negligible, says Williams. He adds that the process could be applied to almost any component, and is likely to have most impact for transmission gears.

For parts needing a consistently high density, because of the wear while in service, Swedish PM specialist Hoganas has developed a warm compaction process. Mixing and compaction are carried out at a temperature of 130 150 C instead of ambient. This results in higher density at little extra cost, the company claims. The niche for this technology is connecting rods, says Williams.

At Sintertech in France, the focus is on a new high-pressure compaction process which applies a pressure of 950 1,050 MN/m2 to achieve a density of 98%. The process is suitable for highly-loaded small gears, and is now in commercial production.

Efforts are under way to produce fully dense parts by eliminating the lubricant in the powder mix, lubricating the walls of the die instead.

However, Williams casts doubt on how critical high density is to the performance of some parts. ‘Opel recently conducted a research project with a number of component producers. It showed that con rods made by the basic PM process with 90% density were perfectly adequate, as long as they were designed appropriately,’ he says.

The psychological barriers to porous parts remain a big element, it seems, in advancing the penetration of powder metallurgy.