Technology Overview
The US transportation industry is the biggest user of aluminum (Al) castings. Considering that a typical full-size sedan weighs anywhere between 2,000 – 3,000 lbs., greater use of low-cost and high-performance Al castings can contribute significantly to cost-effective vehicle weight reduction. Further, metals and metallic alloys like Al castings are often heat treated to achieve desired properties, such as strength, ductility, toughness, fatigue life, or corrosion resistance. Unfortunately, long heat treatments can be expensive and energy intensive and, in some cases, cause blistering. A more efficient heat-treatment method is needed that compensates for local microstructural variations and can be performed for shorter times, at lower temperatures, and with less potential for residual stress and distortion.
Pacific Northwest National Laboratory has developed methods to lower the cost of Al castings and maximize the strengthening potential of heat treatments. PNNL’s proprietary heat-treatment technique leads to similar mechanical properties as conventional heat treatment but with a shorter duration and at lower solution temperatures.
Metallic Alloys Heated by Electrical Joule Heating
In conventional heating, the metallic sample is placed in a furnace and heated up by a combination of radiation, convection, and conduction. In PNNL’s energy-efficient method, metallic alloys are heated by electrical Joule heating which directly passes a current through the metallic sample. The electrical current heats the metallic materials for the desired temperature-time profile. Experiments at PNNL have shown that electrical current heating can accelerate microstructural changes, produce better mechanical properties, and/or lower the temperature needed to achieve the desired microstructural changes.
Joule heating provides advantages to precipitation-strengthening heat treatments. It provides an energy-efficient method to heat treat metallic alloys to increase throughput and reduce energy consumption and costs. Lower temperature and/or shorter time enabled by Joule heating can also help in avoiding blister formation in high-pressure die castings. By controlling the current path and the resulting Joule heat distribution, PNNL’s method can also compensate for the variability in the initial microstructure in different sections of a casting.
Low-Temperature, Low-Energy, Faster Heat Treatment
Compared to heat treatment in a conventional furnace or oven, PNNL’s method has resulted in similar hardness and microstructure in a cast Al alloy—but in about six times shorter duration and at a slightly lower temperature. In the same cast Al alloy for the same solution times and temperatures, PNNL’s Joule heat-treatment method resulted in approximately four times higher hardness relative to furnace heating. Additionally, initial results with magnesium a showed PNNL’s method to be more effective. In the case of a magnesium alloy, PNNL’s method was approximately three times more effective in producing the desired microstructural change than compared to oven/furnace heating to the same temperature time.
Advantages
- Revolutionizes the heat-treating process with energy, time, and cost savings not previously achievable by traditional methods.
- Increases the throughput of the heat-treatment process.
- Heat treats objects of complex shapes, allowing different sections to be subjected to different temperatures and temperature-time histories.
- Can be performed at lower temperatures and for shorter durations to reduce the potential for residual stresses and subsequent distortion.
State of Development
PNNL’s method has applications in automotive, aerospace, and other industrial settings where metallic materials are often heat treated using high-temperature and long-duration approaches to achieve desired properties.
In particular, the cost and mechanical performance of Al castings are two key long-standing industry challenges that need to be overcome to enable greater lightweight materials for mass-market vehicles. If the cost of Al castings could be reduced and their mechanical performance enhanced, even greater market penetration is possible. Using PNNL’s method, approximately a 20 percent reduction in energy requirement was estimated for heat treating a specific Al-casting alloy used for making engine cylinder heads provided by an automotive original equipment manufacturer. If such impressive energy reductions can be achieved in commercial production across multiple metal alloys, this PNNL method could reduce the US annual energy use for metal heat treating and reheating by ~40 TBtu.