Process Strengthens Die Cast Parts

Car components with doubled mechanical strength, higher fatigue resistance, and improved energy absorption are produced by a heat- treatment process for high-pressure die casting (HPDC) of aluminum, developed at the Commonwealth Scientific and Industrial Research Organization (CSIRO; clayton, Victoria, Australia). CSIRO is Australia's national science agency, and one of the largest research agencies in the world. A display of three test castings, which, from left to right, are: As cast; conventional heat treatment; and heat- treated using the new procedure.

"Our heat treatment methods offer major improvements in tensile mechanical properties, and enhancement of a range of other material properties for HPDC components," says metallurgist Roger Lumley of CSIRO's Light Metals Flagship. (The Light Metals Flagship is an organization that conducts research and scientific activities for the entire value chain, from resource development to metal production and new markets, for manufactured products. The Flagship's work is guided by an advisory council that includes leading representatives from the light metals sector.)

In comparison with conventional heat treatments, the process uses a much shorter solution-treatment stage at lower-than-normal temperatures to avoid blistering, and involves multiple microstructural changes that occur simultaneously.

Lumley explains that age hardening was previously not considered to be possible for HPDC's because traditionally much longer times and higher temperatures are used for solution-treatment of aluminum alloys-often only 10[degrees]C or so away from the solidus temperature. Aluminum alloy A357, for example, often may be solution- treated at 540[degrees]C, which is quite close to the solidus, for eight hours or more. This follows because dissolution and homogenization of the alloying elements required for precipitation is a function of solute diffusion, which is both time and temperature-factored.

A sufficient time at a high enough temperature is required to dissolve the alloying elements into solution, and also produce enough vacancies that are retained in the supersaturated solid solution when the alloy is quenched. Supersaturated solid solution then decomposes at lower temperatures producing very fine scale precipitates, which cause strengthening. Employing a conventional or traditional solution treatment (time plus temperature), as might be done for a different cast alloy product or a wrought alloy product, to heat-treat a HPDC causes blistering and distortion.

"We use a severely truncated solution treatment at much lower than traditional temperatures for heat treating HPDC's," says Lumley. "For example, temperatures in the range of 490[degrees]C down to as low as 440[degrees]C for times as short as 10-15 min is sufficient to produce a functional solid solution, and hence precipitation hardening is possible. There are also multiple microstructural changes happening simultaneously that work to prevent blistering or distortion."

Strengthening as a result of heat treatment occurs because the highpressure die-casting process generates a unique microstructure, and after heat treatment a partial solid solution of alloying elements occurs. HPDC alloys are very complex, typically containing 10 or more elements. All of these elements have different roles or effects for casting. Referring to heat treatment of alloy A380, however, silicon plates within the microstructure fragment and spheroidize during solution treatment, so the plates increase in number and decrease in size. This change creates a large number of pinning particles that prevent the blister-forming porosity from expanding.

After a critical time at temperature, silicon particles undergo growth via a process called Ostwald ripening, so their number decreases and their size increases. At this time, blisters may begin to form in the HPDC's because there are not enough pinning particles to restrict pore growth. "Therefore, during the solution treatment procedure we use," says Lumley," we have fragmentation of silicon, spheroidization of silicon, and competitive growth of silicon particles at the expense of others."

Copper, the second most prevalent element in alloy A380, dissolves into solid solution during the solution treatment step. This means that copper-containing particles tend to disappear from the microstructure as the copper dissolves. After solution treatment, when the alloy is quenched, copper is retained in the supersaturated solid solution, and is available to undergo precipitation of strengthening particles, generally around 2-100 nm in size.

Some of the silicon also dissolves into the aluminum during solution treatment, (around 0.6-1%) along with any magnesium or zinc present. All of these soluble elements may participate in the precipitation process, but in general zinc does not in the absence of higher levels of magnesium.

High-pressure diecasting produces a very fine-grained structure towards the surface, and a moderately coarser structure underneath. The casting quickly solidifies, as metal is injected into the mold. The microstructure is, in general, complex with a combination of aluminum grains, silicon plates, and Si-Fe-Mn sludge particles present.

Mechanical properties of as-cast and heat-treated HPDC castings.

As-cast, a HPDC alloy may develop around 170 MPa yield strength. Once it's solution-treated and the soluble elements dissolve, with no age hardening, such an alloy will have around 140 MPa yield strength. Once age hardening occurs by artificial aging to a T6 temper, it will have around 370 MPa yield strength.

"To illustrate the difference that the Al-Si matrix makes," says Lumley, "a simple binary Al-4Cu [wt%] alloy may develop a yield strength [0.2% offset] of around 240 MPa in a T6 temper. In a HPDC, with the same amount of Cu present in the aluminum grains [determined by atomic ratios], the HPDC alloy in a T6 temper will develop a yield strength of around 370 MPa. There are multiple levels of strengthening occurring, with precipitation within the aluminum grains being nanoscale, on the order of 2-100 nm. These aluminum grains are then present within the Al-Si matrix with the Si particles of the order of 2-5 pm. The combination of these factors provides a form of composite structure that gives the strengthening that we see."

Components treated with the new process do not show surface blistering or dimensional changes; they retain an as-cast appearance. Reportedly, fatigue resistance of aluminum HPDC components heat-treated with the new process can be as high as for some wrought aluminum products, tending towards limiting behavior usually observed in steel.

The new procedures may also substantially raise energy absorption during fracture, which has significant implications for crash- sensitive structural automotive components made by high-pressure die casting. It's claimed by the CSIRO group that one common secondary alloy almost doubles in energy absorption, when heat-treated specifically for this purpose. "We envisage that this will make it possible to use HPDC components more widely in load-carrying structural and safety applications," Lumley says.

Additionally, treated parts exhibit thermal conductivity about 20% above their as-cast status. Potentially, because heat extraction operates more effectively, heat-treated HPDC parts could operate with lower amounts of fluid in cooling and lubrication systems.

It's said that the heat-treatment process can be implemented in existing manufacturing facilities using conventional heat-treatment equipment such as continuous-belt furnaces, fluidized beds, or furnace systems designed specifically for rapid heat treatment.

"The HPDC process is more costeffective than other manufacturing mediods in mass production, and the net increase in design strength post-treatment may allow castings to be made using up to 30% less metal," asserts Lumley. "As a result, there is significant potential for cost reduction per part."

Researchers have also recently discovered a range of HPDC aluminum alloy compositions that they say display extraordinarily rapid strengthening behavior, which has major cost and energy-usage implications in manufacturing. These alloys can be heat-treated to high strength levels during a total cycle time of 30 min, and develop properties superior to conventional aluminum casting alloys requiring heat treatment in thermal cycles as long as .24 hr. The CSIRO-led Light Metals Flagship is now seeking partners for a published case study.

"Following our success with evaluations conducted on HPDC parts up to more than 30 kg, we would like to hear from OEM or Tier 1 suppliers who would be interested in submitting a component for heat treatment, and jointly publishing the results as a case study," Lumley remarks. Technical data sheets, providing test results after treatment with the new process for a range of aluminum alloys under various tempering conditions, are available at: http://www.bldiecasting.com

CSIRO initiated the National Research Flagships to provide sciencebased solutions in response to Australia's major research challenges and opportunities. There are nine Flagships that form multidisciplinary teams with industry and the research community.