Vertical squeeze casting of aluminum components.

Vertical Squeeze Casting of Aluminum Components

Developed in 1976 for the manufacture of cast aluminum wheels, vertical squeeze casting is finding new applications where high density, low porosity and good surface finish are important.

Vertical squeeze casting was introduced by Ube Industries  in 1976. This vertical shot system was developed to resolve a specific customer problem. Since that time, it has become clear that use of this technology is suitable -- even preferable -- for a wide range of products previously produced by other metal forming processes.

The process, as depicted in Fig. 1, is finding its niche by bridging the three major forming methods so that the flexibility of design combined with the part density achievable and the quality that it offers will present a more than suitable alternate casting method.

Toyota Motors presented their problem to Ube in 1975 because the car-maker was experiencing leakage in its aluminum wheels. Regardless of the casting process used, the wheels were subject to slow leaking over time because of porosity in the aluminum.

Toyota asked Ube for help in developing, jointly, an economical casting method by which aluminum wheels could be cast to a density that would prevent leaking and in alloys that were heat treatable in order to maximize the alloy's physical properties.

The vertical squeeze casting machine was the result. Those first machines are still producing wheels in Japan.

In 1987 Ube established a subsidiary company, U-Mold, in Japan for the production of aluminum wheels. Using a three-station rotary table, the company produces wheels for Cadillac and Pontiac by using the vertical shot system.

With property levels in 356 alloy approaching forging values and surpassing other casting methods (Fig. 2 & 3), wheels produced with this system are a minimum of 20% lighter in weight and exhibit an extremely fine grain microstructure (Fig. 4).

Vertical Squeeze Casting

Several unique characteristic distinguish vertical squeeze casting from other processes. These features, which come from the patented tilting-docking mechanism of the shot unit, include: bottom-to-top pouring and injection; slow fill by controlled injection; and sustained pressure during solidification.

Each of these features, as described below, contributes to the production of a dense, fine grained casting with reduced air content and excellent response to thermal treatment.

The proprietary shot system, which has four stages in the injection cycle, is illustrated in Fig. 5. The complete cycle takes four seconds. The major effect of this short cycle time is to prevent a drop of the metal temperature in the sleeve just before injection. To date, experience indicates that this drop could be from 120-210 [degrees] F. Of course, any increase of the time that the metal is at temperature results in a proportionate increase in die filling time.

One of the reasons that the cycle time is only four seconds is that the die opening and closing, including spraying, are done independently from and at the same time that the sleeve is receiving metal from the ladler. Experience also indicates that automation of the various operations (ladling, die spraying, sleeve spraying, etc) is extremely helpful to the efficiency and timing of the docking-injection cycle.

In addition, the swiveling unit uses mechanical linkage for minimum maintenance and fast, smooth motion. The docking is accomplished hydraulically and, in the final docked position, the injection unit operates within 0.003-0.006 in. of clearance.

Experience has shown that die life with the Ube process compares favorably with that of conventional diecasting. However, it is expected that with more experience die life could be substantially improved.

The first of the features resulting from this unique system is the bottom-to-top handling of the metal. Since the transfer of molten metal to the sleeve is done outside the machine and the sleeve is held at an angle, the pour from the ladler is without severe turbulence and the air in the sleeve is gradually replaced by the metal.

Likewise, during the injection cycle the metal is transferred under pressure in a bottom-to-top filling of the cavity. Because of the higher metal temperature and the ability to slow fill, again there is no turbulence or splashing as the air is gradually replaced by metal.

The air has the opportunity to find the vents without being trapped in the metal. Of importance in the die filling is the lubricant used as a die spray. Experience indicates that water-based graphite lubricant performs best.

One of the most important factors in reducing air in the castings produced by this system is the ability to control shot or rate of fill (Fig. 6). Gate velocity for the vertical shot is about 16 in./sec, compared to conventional high-pressure diecasting velocity that would be about 2300 in./sec. This requires large gates with areas perhaps three to five times larger than conventional diecasting.

For overall effectiveness, the shot should be programmable and repeatable. To assist in this regard, a control system called Direct Digital Valve (DDV) has been developed. This system allows up to six speed selections and the storage of up to 15 programs per cassette.

Not all products can be made with slow fill only. Fast shot capability for filling thin sections may be required. In these cases, gate velocity can be the same as the conventional high-pressure process. However, fast shot speed causes air problems in relationship to the amount of fill required. For this reason, a patented vacuum system, called Gas Free, is used to assist air removal from the cavity just before final die filling.

Efforts to reduce air porosity would be wasted on some castings unless shrinkage porosity also is prevented. To accomplish this, the system's slow fill is designed to maintain a constant pressure during solidification. This means that the cavity pressure on the metal of 10,000-14,000 psi is maintained through the end of solidification.

In addition, the pressure on the metal allows for quick heat transfer from metal to die surface, which results in extremely fine grained parts. This fine grain structure is needed to obtain maximum properties such as yield strength and elongation.

The vertical shot system is available in two basic configurations, both of which are produced in weights ranging from 300-880 tons. The original machine is a vertical-vertical system (VSC) in which a vertical clamping unit and a vertical shot unit are used.

This machine, which originally was designed to make wheels, is now used to make pistons, disc drives, connecting rods, compressor housings and other parts. This unit has only the slow shot capability and is preferred for thick wall symmetrical and multi-cavity applications.

The other configuration, horizontal-vertical (HVSC), a horizontal clamping unit with a vertical shot, was developed more recently to allow for the casting of thin wall sections. This machine has the accumulator required for fast shot and has been used to make master-brake cylinders, brake calipers, manifolds, fuel rails, valve bodies, shift forks and other components.

It should be noted that where castings must be leak-proof and surface finish requirements are stringent, this system is most applicable. The venting of air with this unit is accomplished as the pressure flow is directed straight from the sleeve to the top of the die.

The parting line alignment is in a straight line. Because of the sleeve, dock and cavity alignment, it is possible to contain the solidification layer, which was formed in the sleeve, to the biscuit area preventing it from entering the cavity.

Market Applications

The immediate application of this vertical shot system technology has been largely automotive, although there are business machine and bicycle applications. This may be because of the initial development for wheels and the comparative heat treat properties that were developed. However, applications for the process have grown because of this system's ability to cast a wide range of aluminum alloys, including 319,356, 390, 2000 series, 7000 series and others.

In some instances, the magnesium alloy AZ91 has been used interchangeably with aluminum. In alloy series where heat treat capabilities are present, those capabilities have been able to be fully utilized. Thus far, this has occurred without regard for configuration constraints. In some cases, where a vertically cast product has replaced a forging, the comparable density and physical properties of the casting as well as its near net shape provided the economic rationale.

In both machines, the cycle time depends on the configuration of the part.

There is no direct limitation on number of cavities and the guidelines for high-pressure diecasting generally apply. Actual cycle times may vary from 40-80 seconds or from 90-42 shots an hour. No special heating or cooling technology is required for the die. The spraying of die lubricant, mentioned previously, must be done after each shot.

In conclusion, there appears to be a market niche for vertical squeeze casting technology. The full potential for the process has not yet been realized, although in the past year more than 100 parts have been designed for or converted to this system.

This technology's specific place as a casting system and its relationship to forging and other casting methods have yet to be determined. [Figure 1 to 6 Omitted]

Shu Suzuki Heavy Machinery Div Ube Industries, Ltd Ube City, Japan