Machine Design

Car designers are "hip" to aluminum castings

A new process that solves the porosity problem may be a boon for superlight suspension arms, turbo impellers, and other automotive components.

The aluminum blocks and heads for this Indy car engine are Densal II processed. Now, Densal II + T6 will let designers specify aluminum castings for commercial engines.
Comparison of the blade tip of a cast aluminum turbocharger wheel as cast (left) versus after processed using the Densal HIP process (right) shows that the HIP process virtually eliminates internal porosity, helping boost the blade's tensile strength, elongation, low and high-cycle fatigue, and stress-rupture properties.

Aluminum-casting processes are known for providing economical, near-net-shape parts. But the internal porosity of the resulting parts has generally brought mechanical properties inferior to their wrought machined or forged counterparts.

A special HIP (hot-isostatic pressing) process called Densal II + T6 from Bodycote Hot Isostatic Pressing, Andover, Mass. (, may solve the porosity problem. Bodycote claims it makes low-cost aluminum castings suitable for parts such as suspension arms, turbo impellers, bearing housings, and gearbox casings.

The new process places castings in a large pressure vessel. The vessel is pressurized with nitrogen and simultaneously subjected to high temperature and pressure. HIPing virtually eliminates internal porosity through metallurgical processes such as plastic yielding, creep, and diffusion bonding. The material "flows" to the center of the casting where pore walls touch and diffuse into a solid, intact metal structure. This removes defects of considerable size (30-mm diameter, for example). The process is also a cure for hydrogen-gas porosity (at elevated temperatures, hydrogen gas is soluble in aluminum) and shrinkage.

The removal of shrinkage and gas porosity leads to better tensile strength, elongation, low and high-cycle fatigue, and stress-rupture properties.

For high-volume automotive applications, the Densal II + T6 can be integrated into the foundry and especially into the heat-treatment process. The Densal II cycle time is 2 to 3 hr. By transferring the hot lots of parts from Densal II directly into the solution furnace, lengthy solution treatments are minimized and throughput can be synchronized with high-speed foundry operations. According to the company, the total cost of Densal II + T6 has been estimated at less than $0.50/lb for parts that pack well, such as brake calipers and steering knuckles.

Bodycote says HIP works well for most investment, sand, and permanent mold (gravity die) casting. High-pressure die and lost-foam castings can, however, have surface porosity and trapped air pockets that HIP may not be able to get out. Similarly, HIP will also not remove oxides or large gas bubbles. And it's recommended that melt chemistry be closely monitored to optimize grain size and control eutectic modification.

In addition, Bodycote offers similar cycles for nickel and cobalt superalloys, steel, and titanium.

Typical property comparisons of sand-cast A356 Al alloy
  Ultimate tensile
strength (ksi)
strength (ksi)
A356 Al alloy as-cast plut heat treatment 37.5 30.6 1.9 20 2 X 105
A356 Al alloy -- Densal II + T6 39.9 31.2 4 20 6 X 105
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