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Casting process gaining ground.

A hybrid of plastic injection molding and metal diecasting, Thixomolding makes near-net shape parts with minimal porosity--and without molten metal processing.

Near-net shape manufacturing processes, ranging from powder metallurgy to advanced casting techniques, continue to increase in number and variety. One of the near-net casting processes, called Thixomolding, combines advances in equipment design and alloy processing and is being commercialized by Thixomat Inc, an Ann Arbor, MI, joint venture of Amptech Corp, Comalco Aluminum Ltd, Dow Chemical, Lindberg Corp, George Spalding, and USP Inc.

Based largely on patents held by Dow, Thixomolding uses equipment similar to a plastic injection molding machine and a semi-solid form of standard magnesium diecasting alloys. The one-step process produces near-net shape castings with no investment in molten metal processing and handling equipment. Currently limited to producing magnesium components, the process also should be applicable to aluminum, zinc, and metal-matrix composite materials.

HPM Corp, Mt Gilead, OH, has been licensed to build Thixomolding machines in the US. In Japan, Japan Steel Works has been licensed to build machines and is supplying one to a Japanese automaker for use in its research facility.

The material

The key to Thixomolding is a unique material--or rather, a unique physical form of standard magnesium diecasting alloys. The semi-solid material eliminates the safety hazards associated with handling molten magnesium, while providing parts with less porosity than conventional magnesium die casting.

In casting, low molten metal pouring temperatures result in components with relatively fine, equiaxed grains. Higher pouring temperatures, on the other hand, result in a structure consisting of larger, columnar grains.

It first came to light about 20 years ago that stirring of molten metal during the early stages of solidification could produce unique structures consisting of rounded solid particles in a matrix of semi-solid material. During continuous cooling, the viscosity of the stirred metal is inversely proportional to the rate at which the metal is stirred. That is, faster stirring results in denser, more rounded solid particles. The same is true of decreasing cooling rate.

The viscous flow (thixotropy) of semi-solid magnesium alloys in Thixomolding is achieved at temperatures 80 C to 120 C lower than the molten metal temperature used in magnesium diecasting. The process takes advantage of the semi-solid nature of the metal--it's roughly the consistency of soft butter--to fill the molding die as more of a solid front, rather than the stream or spray characteristic of liquid metals. The result is parts with less gas porosity and solidification shrinkage, higher density, and better strength than conventional die castings.

The machine

Built by HPM Corp, the first Thixomolder has a 400-ton die clamp and was designed to process approximately 270 lb (123 kg) of magnesium alloy in one hour. The machine can produce parts weighing up to 3.5 lb (1.5 kg), and permits a 20-second shot cycle time for parts weighing up to 1.5 lb (0.7 kg).

The Thixomolder uses a mold clamp mechanism similar to that used on diecasting machines. The injection system, which is similar to plastic injection molding machines, consists of a high-temperature screw and barrel. These components are coupled to a high-speed shot system that drives the reciprocating screw. A volumetric feeder supplies the alloy feedstock to the Thixomolder. The material is thermomechanically processed by the rotating screw, transferred to the accumulation zone, and then injected into a die cavity. The metal injection temperature is set to achieve a 30% to 50% volume fraction of primary solids. Nominal slurry injection temperature for processing AZ91D, a commonly used magnesium diecasting alloy, is 580 C.

An induction coil and resistance band heaters heat the alloy to processing temperature, with barrel temperature profile dependent on the alloy system being processed. The induction coil is located near the feed throat on the barrel and has a maximum output of 50 kW. The resistance band heaters provide closely controlled heat at the discharge end of the barrel and have a combined output of 28 kW. The temperature profile develops the appropriate solid to liquid ratio in the alloy being processed. An argon atmosphere in the barrel minimizes oxidation of the magnesium alloy.

Shot size--the distance the reciprocating screw retracts during a shot cycle--is determined primarily from the weight of the part being produced, and to a lesser degree by the screw retract rate and rotational speed. Screw retraction deposits magnesium alloy into the accumulation chamber, and the injection cycle activates when the system reaches the predetermined shot size.

Activation drives the screw forward hydraulically at the desired velocity, filling the die cavity with semi-solid alloy. The rotating screw then begins retracting, filling the accumulation chamber in preparation for the next shot. Shot velocities of more than 100 inches/sec (250 cm/sec) with metal pressures in the range of 4400 to 7800 psi (31 to 55 MPa) are common.

The process

The Thixomolding process begins when solid particles of conventionally solidified magnesium diecasting alloy are fed into the machine barrel. The feed material used is a machined chip with a geometry approximately 1 mm square by 2 to 3 mm in length. The high-temperature screw and barrel, coupled to a rotary drive system, advance and shear the feed stock while heating it to a semi-solid temperature.

The nominal slurry injection temperature, 580 C, produces a semi-solid material containing about 30% volume fraction solids. With the temperature profile selected, the machine is started cold (room temperature) and brought up to operating temperatures in about ninety minutes. During this time the die heater, if used, is activated to preheat both the ejector and stationary halves of the die.

The volumetric feed system used to provide the magnesium alloy to the Thixomolder is initially set at 25% of the steady-state rate, then ramped up to 100% as the machine progresses from making partial shots to complete shots. This is generally accomplished in 15 to 20 shot cycles.

Once steady-state operation is achieved, shots are automatically activated as soon as the predetermined amount of semi-solid magnesium has entered the accumulation chamber. The reciprocating screw moves forward and injects slurry into the die cavities. The rotating screw then retracts to fill the accumulation chamber for the next shot.

Operating delays are accommodated simply by turning the feeder off and dialing down the temperature. The system can be restarted by reactivating the feeder and allowing the barrel temperature to reach its original setpoint. Steady-state conditions will be reached within a few shots. For sustained shutdowns, the barrel is purged of semi-solid material by cycling the machine a few times with the feeder off. An inert (argon) atmosphere is maintained in the barrel during shutdowns to prevent oxidation of any residual magnesium alloy.

Die design is being evaluated, but the thixotropic behavior of the semi-solid alloy favors direct injection over the long runner systems sometimes used in standard diecasting processes. Conventional magnesium alloy die design provides a good starting point, and gating systems used in plastic injection molding appear to be more accommodating because of the semi-solid nature of the alloy. Extreme gate velocities, however, can result in shear separation where the liquid phase moves out in front of the solid phase. The objective is to fill the die as quickly as possible without creating critical shear separation velocities.

The results

Thixomolded AZ91D magnesium alloy components had significantly lower porosity levels than high-pressure die-cast parts made in identical molds. Porosity in Thixomolded parts was 1.4% to 1.7%--considerably lower than the 3.2% to 3.4% seen in identical components made using high-pressure die casting. Thixomolded exercycle clutch plate housings, for example, routinely had porosity measurements less than 1%. The reduced porosity in Thixomolded components should permit heat treatment of finished parts to optimize physical properties. As-cast part to die shrinkage is about 0.006 in/in. Thixomolded components have been easily ejected from zero-draft dies.

The lower operating temperatures of Thixomolding also reduce thermal stresses in as-cast parts. Thixomolded parts show no distortion or warpage on relatively thin-walled, large-diameter plates. The reduced operating temperature should also extend die life.

If scrap can be recycled, even greater economies will be realized. Feasibility studies of both mechanical and thermal processing methods are being conducted to determine an effective method to reuse gates, runners, and rejected castings.

So far, 15 prototype runs have been completed at Thixomat's Advanced Development Center. All have resulted in production of good parts with minimal scrap. Customers have included automakers, automotive suppliers, appliance manufacturers, and electronics suppliers.
COPYRIGHT 1992 Nelson Publishing
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 1992 Gale, Cengage Learning. All rights reserved.

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Title Annotation:thixomolding process from Thixomat Inc.
Publication:Tooling & Production
Date:Dec 1, 1992
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