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Double the strength of die cast parts

21st February 2013


Initiated in 2003 by the Commonwealth Scientific and Industrial Research Organisation (CSIRO), Australia’s premier research organisation, the National Research Flagships programme is one of the largest scientific endeavours ever undertaken in Australia. By 2010–11, total investment in the programme is expected to be close to AU$1.5bn. The nine critical flagship areas identified by CSIRO are: light metals, water, energy, climate, oceans, health, food, minerals exploration and manufacturing. 

The Light Metals Flagship was set up to meet the challenge of global demands for ultra-strong, ultra-light, recyclable materials as the world switches to low-emission vehicles, energy-saving devices and sustainable products. As a result of this, aluminium demand is forecast to climb by 30percent, magnesium by 200percent, while the sky is the limit for emerging industrial light metal titanium. 

As Austrialia already has a mature AU$8bn (E5bn) aluminium industry, a highly prospective magnesium industry, and one of the world’s richest titanium resources, the Australian government sees the country becoming a world leader in light metals. 

To do this, it aims to: double the revenue from light metals production to AU$20 (E12bn) per annum; develop technology for a new magnesium and titanium industry; and reduce energy use, greenhouse emissions and environmental impact. To achieve this, additional aims include increasing energy efficiency by 30percent, lowering manufacturing costs by 50percent, reducing life cycle impact by 50percent and increasing asset productivity by 30percent. 

The latest announcement from the Light Metals Flagship is a new heat treatment process that supplies stronger die cast parts – including doubled mechanical strength, higher fatigue resistance and improved energy absorption. 

“Our heat treatment methods offer major improvements in tensile mechanical properties and enhancement of a range of other material properties for high pressure die casting (HPDC) components,” says metallurgist Roger Lumley of the Light Metals Flagship. 

“Components treated with the new process do not show surface blistering or dimensional changes, they retain an as-cast appearance (Fig.1).”
Surprisingly, fatigue resistance of aluminium HPDC components heat-treated with the new process can be as high as for some wrought aluminium products, tending towards limiting behaviour usually observed in steel. 

The new procedures may also substantially raise energy absorption during fracture, which has significant implications for crash-sensitive structural components made by high-pressure die-casting (Fig.2). 

For example, 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 adds (see HPDC, below).
Additionally, treated parts exhibit thermal conductivity about 20percent above their as-cast status, meaning that for engine or transmission applications heat can be transferred or removed more efficiently and quickly. 

Potentially, since heat extraction operates more effectively, heat-treated HPDC parts could operate with lower amounts of fluid in cooling and lubrications systems.
The heat treatment process can easily be implemented in existing manufacturing facilities using conventional heat treatment equipment such as continuous belt furnaces, fluidised beds or furnace systems designed specifically for rapid heat treatment. 

The researchers have also recently discovered a range of HPDC aluminium alloy compositions that display extraordinarily rapid strengthening behaviour, 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 only 30minutes and develop properties superior to conventional aluminium casting alloys requiring heat treatment in thermal cycles of up to 24hours. 

The CSIRO-led Light Metals Flagship is now seeking partners for a published case study.

HPDC

HPDC is the most cost-effective process for making large quantities of complex aluminium components in near to final form. Typically, up to 20 small parts can be made every minute. Most are used in motor vehicles, but the process is also important for other industries. 

While other cast and wrought aluminium alloy parts can be heat treated to improve their mechanical properties (such as strength, hardness, toughness, fatigue and ductility), this has not been possible until now for high-pressure die-castings. 

High-pressure die-castings contain trapped gas pores (porosity) that expand, blister and distort the casting when heat treatment is applied. This makes the parts unusable. Previous attempts at making them heat-treatable have tried to remove porosity from the cast parts, but these proved costly and time-consuming. 

Hence why Flagship researchers are developing a heat treatment process for conventional high-pressure die-castings that doesn’t depend on reducing the porosity yet eliminates blistering and results in improved material properties. The aim is to develop an inline heat treatment system compatible with a die-casting cell. 

The new process may enable die-casters to make complex HPDC components, such as engine blocks and transmission housings, using up to 30percent less alloy to achieve the same performance. 

Apart from reducing manufacturing costs, the requirement for less metal will also reduce vehicle weight, leading to lower fuel consumption and lower greenhouse gas emissions.l







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