Powder metallurgy (PM) has made a major contribution to the technology of automotive manufacture over the past 70 years. It has been and is still the fastest-growing metalworking technology. Today, on average, it provides about 10kg of the total components of a car — more in the US, where cars are larger and use automatic transmissions in nearly all units sold and less in Europe and Japan, where cars need to be lighter and petrol is expensive.
Today, the use of these highly precise components per automobile is increasing by between 6% and 7% per year. One of the reasons for this formidable growth is the combination of favourable cost and the properties of the components, both with respect to complex shape, precision and desirable physical properties. Low cost is provided by the ease with which the PM process is automated. Complex shape is provided by the internal contour of the die and the profile of the other tool members. Cutting, drilling and turning and other costly forming operations can in most cases be completely eliminated. This not only serves to lower cost, but also results in a practically total elimination of waste.
Automotive PM Components
This article examines some typical components made using PM. The process of making sintered components will be briefly described. The most important raw material — the iron powder — will be treated in some detail and, in particular, will demonstrate how high-performance iron powders — and press-ready premixes — are produced in the iron powder plant of Hoeganaes Europe in Buzau, Romania.
Figure 1 depicts a relatively simple part: an exhaust flange for an automobile. The production cost is quite low for this kind of relatively uncomplicated shape. The part is made by Gt.B Components Ltd in St Helens, UK, using powder from Hoeganaes Europe and other additives.
The PM process offers great flexibility also in providing tailor-made material properties. The fact that the material is made up of an assembly of small particles of iron and other metals or compounds, providing an extra degree of freedom for the materials engineer. A degree of freedom that metal made by conventional melting methods cannot provide.
Figure 2 depicts an example of such a product — a camshaft made from a tube with cam lobes made from highly abrasion-resistant PM steel. The lobes are simply shrunk on to the shaft. In the background is a preform of a shaft made using conventional methods. It can easily be seen that the conventional camshaft requires much costly machining, giving rise to a substantial amount of waste.
Another unique feature that can be built into the PM material is porosity. The interstices between particles in the raw pressed component leads to pores after sintering, which can permeate the component like a system of canals. Usually, the powder metallurgist tries to eliminate the porosity, but sometimes it can be used at an advantage. This is the case with porous bearings — simple porous bushings — in which the porosity is filled with lubricating oil. When the shaft rotates in the bearing, oil is circulated to the interface between shaft and bearing, thus providing lubrication to exactly the location, where it is needed most. The so-called self-lubricating bearing made from powder is the component that provides life-long service in many small components such as windscreen-wiper mechanisms.
A multitude of examples of PM components can be found virtually in all parts of the car. The exhaust system, the shock absorbers, the steering mechanism, the transmission (both manual and automatic) and the engine.
The PM Process
The process of making sintered components starts with the metal powder, usually iron or steel powder. Plain iron powder is usually mixed with alloy powders such as copper powder and other additives such as graphite powder and a solid lubricant powder, usually zinc stearate or amide wax. Such premixes are the most common raw materials to be put into the press.
Figure 3 illustrates the basic PM process and a number of post-treatments, which may or may not be used for imparting special features on the parts.
It is essential that the mixing operation results in a homogeneous mix, as any variation in composition will be reflected in variations in dimensions and physical properties of the components. The handling of the powder after mixing is also crucial. Since the constituents of a mix are quite different with respect to particle characteristics, de-mixing easily takes place, and the result is again variations between the parts.
One way of counteracting segregation is to bond the alloying element particles to the iron particles or to make the particles entirely homogeneous, by placing the alloying elements in the molten metal prior to pulverisation (atomising). Both of these methods are used.
The presses used for powder forming are complex machines, as they have to provide utmost precision combined with high compacting pressure and complex, well-controlled movements of the various punches. Each type of component requires a separate tool. These are made with great precision and are quite costly. A consequence of this is that many identical parts need to be produced before the process becomes profitable. Typically, at least 100,000 and often many millions of identical parts are made.
Compaction does not provide a bond between particles and, therefore, the parts are quite weak and brittle directly after pressing. Full strength of the material is achieved by sintering under a protective atmosphere. The parts are heated to a temperature well below the melting point of the iron, usually between 1,100ºC and 1250ºC in continuous furnaces.
A density of 90% to 95% of theoretical is quite normal, leaving between 5% and 10% porosity in the part. This has some influence on the properties of the component, but still the strength and hardness that can be achieved ranges from those of cast iron to those of hardened and tempered tool steel.
Iron- and Steel-powder Manufacture
Almost a million tons of iron and steel powder are used worldwide each year. Around 90% of this quantity is used in the production of sintered components and 75% to 80% of the sintered components are used in the automotive industry. The rest can be found in appliances, business machines, bicycles, hand tools and other implements made in large numbers.
Hoeganaes Europe is one of the smallest producers of iron powder in Europe, making about 10,000 tons of high-quality iron powders and premixes per annum. The plant was built in the early 1990s with technology from Mannesmann of Germany (see Figure 4). From the beginning, great emphasis was placed on satisfying the growing need, especially in the car industry, for dependable, consistent quality. International Organization for Standardization (ISO) 9002 certification was obtained after only one year of operation and, in 2002, the certificate for ISO 9001/2000 was achieved. Due to this quality consciousness, the products from Hoeganaes Europe have rapidly been accepted by producers of sintered parts all over the world. The largest share of the PM powders from Hoeganaes Europe are used in Western Europe.
The process at Hoeganaes Europe is illustrated Figure 7. Selected steel scrap is melted in an electric arc furnace, the molten steel is refined, the composition determined by spectroscopy, adjusted if necessary and then poured into a tundish from which it emerges as an even stream, which is immediately struck by powerful high-pressure water jets, thus being split into small droplets. These solidify extremely rapidly, so that they do not have time to become spherical. The particles formed are thus irregular in shape — this is important for the properties of the powder.
The mixture of iron powder and water thus created is pumped to a hydrocyclone and subsequently passes a centrifuge, where most of the water is separated from the powder. Finally the wet powder is passed through a rotary dryer. The dry powder is then sieved to eliminate oversize particles, it is passed through a magnetic separator to rid it from nonmagnetic particles and finally passed through a continuous annealing furnace, in which it is treated at about 1,000ºC with hydrogen. This treatment removes most of the oxygen and carbon from the powder, it becomes grey instead of black and, above all, it becomes soft, which makes it compressible and thus suitable for die compaction.
During annealing, the powder forms a quite hard cake, which has to be disintegrated. Finally, the powder is sieved to check that all particles are within the specified size fractions. Now it remains to homogenise the powder in five-ton lots. This is performed in a double-cone mixer. The powder is checked for conformity with specification, packed in one-ton bags and shipped to the customer.
A mixing station has recently been installed in the plant. It is equipped with a 10-ton Munson mixer. This state-of-the-art machine ensures utmost homogeneity throughout the whole lot. The mixing plant gives Hoeganaes Europe access to the rapidly growing market for press-ready mixes and prealloyed powders. The customer is thereby ensured a consistent stable raw material for an entire production campaign — enabling him/her to better serve the ultimate customer — the automobile maker.