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SSM: A Solid Option for Lightweight Aluminum Parts
Typically, aluminum alloys are processed either in the solid state by wrought processing methods such as rolling, forging, or extrusion, or in the liquid state by casting methods, such as die, sand, or permanent mold casting. There is a third option, however, called semi-solid metal (SSM) casting, in which the alloy is processed in a temperature range where it is part liquid and part solid, and this can provide some significant advantages for producing aluminum alloy parts. This article will provide some background on the SSM process and its advantages, typical applications, and challenges and opportunities for the future.
Semi-solid metal (SSM) casting of aluminum is a technology that has been practiced for over 25 years. It was discovered at MIT in the early 1970's and termed rheocasting. From a technical standpoint, this process takes advantage of the thixotropic behavior of a semi-solid aluminum alloy, meaning that it can be handled as a solid when at rest and flows like a liquid when subjected to shearing forces. Because of this behavior, the SSM alloy has adequate strength for robotic handling, facilitating transfer from the heating furnace to the shot sleeve. The unique properties of thixotropic aluminum alloys are often illustrated by the demonstration in which a semi-solid billet is cut with a butter knife. The high rates of shear imposed by the plunger during the filling process will cause the semi-solid alloy to flow like a very viscous liquid, filling the mold smoothly and without entrapping gases. In contrast, the high-pressure die casting process sprays molten metal into the die, which can lead to higher levels of porosity. In addition, because of the high solids content and the much lower temperatures involved with SSM casting, shrinkage porosity is also minimized. SSM forming processes have also other advantages, which include lower forming temperatures and elimination of molten metal handling. These result in longer mold life and lower energy consumption and reduction of environmental pollution, respectively.
Key to the success of the SSM casting process is a starting material with a non-dendritic, globular microstructure. Various technologies exist for producing so-called SSM billets, including a controlled billet casting process in which magnetic stirring of the solidifying billet is used to break up the typical dendritic cast structure. A competing billet casting process uses a highly grain refined alloy and a fast solidification rate to achieve an ultra-fine grain size in the billet. When reheated into the two-phase, liquid plus solid region prior to casting, the ultra-fine grain billet and the billet cast from the mushy semi-solid material, behave in a similar way. The eutectic phases melt first and the remaining solid, in the form of spherical aluminum particles, gives a semi-solid "mush" which can be shaped into the desired part. A new wrinkle is to make the SSM mushy feed just ahead of the die casting machine by a careful balancing of cooling and heating of the input liquid. This technique has been termed direct slurry forming or the DSF process.
SSM produced parts are of very high quality, being pressure tight and exhibiting excellent structural integrity. Quality-wise, SSM parts are often compared favorably to squeeze cast components. In general, the alloys most commonly used for SSM processing are 356 and 357. Recently, SSM billets in 319 and 390 alloys have been introduced as well, providing a wider range of material options. SSM processing of aluminum for automotive applications started in the early 1990's. The process is now being practiced around the world. In the automotive industry's quest for weight reduction, the ability to make thin wall castings (down to 2 to 3 mm) with good structural integrity offers a considerable advantage over other processes. Fuel rails are the largest volume automotive part in production today and, as critical components in fuel injection systems, must be pressure tight. Application of SSM in fuel rails is a prime example of the high quality capabilities of the process. In Europe, rear axle SSM aluminum alloy castings have replaced ductile iron parts saving weight and demonstrating the high structural integrity of this process. SSM engine brackets, hydraulic system and air conditioner components, and subframe assemblies have all been successfully produced using the SSM process. Primarily based on automotive requirements, shipments of SSM products are now approximately 30 million pounds per year, and are expected to grow at a rate of upwards of 10% per annum.
A novel process method that has been developed for producing parts from magnesium is called Thixomolding . It uses a modified plastic injection molder to make magnesium alloy parts that are being used in the electronics/communications, hand tool, and automotive industries. Metal alloy bits are feed into the heated barrel and a screw, which also provides the shearing forces, advances the material while it is heated into the semi-solid state. When sufficient mushy metal is in the accumulation zone at the front end of the barrel, which is inerted using argon gas, the screw is hydraulically advanced to shoot the semi-solid alloy into the die. A cycle time of 30 seconds or less, coupled with the ability to make a net-shape part, makes this process attractive economically. It also offers a user-friendly environment since the molten metal is wholly contained in the barrel.
Aluminum Thixomolding is currently under development on a commercial size thixomolder. As molten aluminum will react with conventional steel, the design of the barrel and screw must rely on materials of construction which are resistance to this corrosive environment and still be robust enough to handle the repeated thermal and impact shocks associated with operation of the unit. Considerable progress has been made to overcome these obstacles and aluminum trials on a commercial scale are underway.
A variety of technical challenges remain for SSM casting to continue to gain increased use in a broader range of applications. Raw material cost of the billet feedstock and yields in the range of only 50% contribute to the high manufacturing cost of SSM parts. However, these castings are high quality, net-shape parts that require little or no machining or finishing which can more than offset the higher manufacturing costs. The DSF process, which has recently been introduced, may significantly reduce the costs of producing the SSM feedstock that will favorably impact the economics of SSM castings.
There are a number of sources for further information regarding the SSM casting process and products. These include:
- The Advanced Casting Research Center/Consortium at Worcester Polytechnic
Institute has more than 45 organizations devoted to a cooperative effort to
technically advance semi-solid processing technology. http://www.wpi.edu/Academics/Research/MPI/
- The North American Die Casting Association (NADCA) is a trade association
committed to the advancement of die casting processes. This group supports
university and industry work on SSM technology. http://www.diecasting.org/research/
- Ormet Aluminum is a producer of SSM billet. Proceedings of the SSM World
Conference that they sponsor are available through their web page: http://www.ormet.com/
- Formcast is a SSM parts maker who is affiliated with Ormet. Visit their
web page to get a view of their capabilities. http://www.formcast.com/
- SPX Contech is a producer of SSM cast components via various different SSM
casting technologies. http://www.spxcontech.com/
- Northwest Aluminum is producing the highly grain refined billet used for
SSM parts. More information is available on their web site. http://www.nwaluminum.com/
- Hot Metal Molding is a SSM parts maker. Learn more about their capability
from visiting their web page. http://www.hotmetalmolding.com/
In the constant competition for the best combination of product properties and cost, the SSM casting process has an important niche to fill.
Article provided courtesy of The Aluminum Association - www.aluminum.org
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