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Aluminum casting process finds new applications; developments in the Cosworth Process have spread its application to such fields as the aerospace and defense industries.

Aluminum Casting Process Finds New Applications

Developments in the Cosworth Process have spread its application to such fields as the aerospace and defense industries.

Since its inception in the early 1980s, the Cosworth aluminum casting process has been used in areas its inventors never originally considered. For example, castings for Rolls Royce helicopter engines, Dowty Rotol helicopter winch drums, high pressure refueling manifolds for tanker aircraft and Royal Ordinance PLC gun cradles all have been produced using this technique. Other applications include the manufacture of aluminum structural parts for airframes and marine engines.

At the same time, automotive castings, the products for which this method was first designed, continue to be manufactured with the Cosworth Process. The Mercedez Benz 2.5 liter, 16 valve cylinder head and the turbocharged Ford Sierra Sapphire cylinder head are two examples of automobile components made using this procedure.

In the field of auto racing, the 1989 Formula 1 Benetton race car is powered by a 3.5 liter Ford engine, many of the parts for which are cast with the Cosworth Process. It has also been reported that this casting method is used to produce approximately 150 Formula 3000 and CART engines annually for use in American automobile racing.

Process Origins

For at least the last decade, automotive manufacturers and racing groups have been moving toward lighter, higher compression engines. Because of its relatively low weight, aluminum was the logical material for these applications.

Unfortunately, aluminum and its alloys are highly reactive materials that suffer from impurities which can cause cylinder head cracking, distortion under load and even melting. The largest source of these impurities is the oxide films that instantly form when molten aluminum is exposed to air. Turbulence during transfer and pouring mixes these oxide impurities with the molten aluminum.

The process was designed to take advantage of aluminum's light weight while controlling problems with impurities. This process keeps the metal clean by holding it in a furnace under a blanket of inert gas.

Whatever impurities do form will either float to the top or sink. Because of this, a special electromagnetic pump was designed to draw molten aluminum from the center of the holding furnace, where the cleanest metal is found. The aluminum is then pumped into the bottom of the molds in a continual upward direction, thus reducing or eliminating turbulence. The metal is transferred under pressure to minimize porosity.

Technical knowledge about pumping aluminum alloys into sand and permanent molds has increased with the development of the process, and this pumping technique is now believed suitable for both high volume production of other metals and for adaptation to other molding processes.

Mold and core materials play an important part in the process (a detailed description of these materials was presented in modern casting, Mar 1987, pp 122-3). Sand with low thermal expansion is used with a sulfur monoxide and sulfur dioxide furan binder. This sand and binder system is said to have a volumetric thermal capacity approximately twice that of silica sand. This reduces mold expansion from 0.5% to 0.05%, which facilitates rapid cooling and creates a fine dendrite structure reportedly comparable to castings produced in permanent mold.

The same material also is used for cores, where its high heat capacity reduces linear expansion from roughly 1.5% to about 0.05%. This allows precise core printing and reduces the likelihood of core floating. The slow heating cores also lower gas evolution rates.

Because the Cosworth Process uses costly core and mold materials, a reclamation system has been introduced that cuts sand loss to an estimated 1.0%. Reclaimed sand can be returned to core- and moldmaking lines, thus reducing new sand costs and the problem of waste sand disposal.

Productivity Gains

Melting and holding losses with the process have been reported as low as 1.5-2.0%, with successful casting yields of 85% or higher. In most cases, fluxing is unnecessary, and the molten aluminum reportedly does not require additional grain refinement or modification.

Other benefits include accurate castings that require machining allowances of only 0.06-0.08 in. Such casting accuracy can lower costs by reducing the number of machining stages and the total time needed for machining (see modern casting, Mar 1987, p 123, for a description of productivity gains).

Although initially designed for low volume, specialty production such as the manufacture of Formula 1 racing engines, the process is now being used successfully in higher volume casting operations. Further developments are possible in this process, which may lead to even greater interest among design and manufacturing engineers.

PHOTO : Fig. 1. Shown are Formula 1 engine components produced using the Cosworth Process.

Russell Bray Birmingham Post & Mail Birmingham, England
COPYRIGHT 1989 American Foundry Society, Inc.
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 1989, Gale Group. All rights reserved. Gale Group is a Thomson Corporation Company.

Article Details
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Author:Bray, Russell
Publication:Modern Casting
Date:Dec 1, 1989
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