Chilled sand casting process delivers higher strength components. (Technology in Progress).

Volvo's "Future Process for Castings" uses chemically bonded sand molds with a water-cooled steel chill to increase casting strength and dimensional stability while reducing environmental concerns.

Since 1995, the Swedish automobile and heavy truck manufacturer Volvo has had its Powertrain foundry in Skovde working to develop new materials and processes that can meet the increasing demands of its diesel engine customers. Castings for this market are being pushed to the limits of their abilities in terms of structural strength as designers dramatically increase the combustion pressures in today's engines.

In response, the foundry developed and brought online a new casting process, the Future Process for Castings (FPC), in late 1999 as a means to increase casting quality while reducing the environmental affects of sand casting. The new process required an $11.1 million investment. With actual production begun in the Fall 2001, the foundry currently is using FPC to produce gray iron cylinder heads for some of Volvo's 9 and 12 liter engines. Total production for the process is currently 6000 tons/yr.

"The long-range objective is to cast a greater number of components with this process, which will undoubtedly live up to its name," said Sven-Erik Dahlberg, head of development at Volvo Powertrain's foundry in Skovde, and the leader in the development of the patented process.

FPC Development

When initially developing this process, Volvo's Powertrain foundry focused on creating a new casting process that provided an increased cooling rate for the casting with a more stable mold than green sand casting can provide.

For gray iron castings, according to the foundry, a high cooling rate results in:

* quick solidification with refined solidification structure;

* higher carbon content in the austentite matrix, resulting in a high volume percent of carbide in the pearlite structure;

* quick eutectoid phase transformation, resulting in a refined pearlite structure.

The effect of cooling rate on gray iron castings is shown in the relationship between casting cross-section thickness and its tensile strength (Fig. 1). Thin-wall gray iron castings are strong while thick-walled parts are weak, even in the same casting. This is due to differing chill rates.

According to the foundry, the stable mold is critical for gray iron casting because of the constant expansion and reduction in volume gray iron undergoes during casting solidification, cooling and solid phase transformations. The ability of the mold to withstand the pressures exerted during these phases leads directly to the amount of dimensional variation observed in the castings.

Addressing these issues, the FPC method developed uses chemically bonded sand molds surrounded by a water-cooled steel container called a "chill mold" (Fig. 2). During casting, constant pressure is exerted on the chill mold and the sand mold. The clamping force on the chill walls controls the pressure level of the melt in the mold and is a control tool for lowering the risk of penetration or shrinkage porosity.

The high cooling rate desired by the process is achieved via a thin-walled sand mold that is chilled by a water-cooled metal chill that pulls away the heat faster than air. The stable mold is achieved through the chemically bonded sand mold supported by constant pressure. In addition, the process is designed to allow the following variables to be adjusted depending upon the casting job:

* molding material;

* cross-section thickness of the mold;

* chill metal material;

* cooling water temperature and flow rate;

* supporting pressure to the chill.

The casting benefits Volvo's Powertrain foundry will receive from FPC are:

* increased casting tensile strengths through higher cooling rates;

* the possibility to cast iron with lower carbon equivalents;

* reduced opportunity for shrinkage porosity or mold penetration through higher cooling rates and supporting pressure through casting;

* greater casting design freedom for reduced weights and increased efficiencies in application due to chemically bonded sand molds;

* increased dimensional accuracy due to chemically bonded sand molds and constant pressure;

* reduced casting cleaning costs.

Using the FPC method makes it possible to raise the quality of the castings while reducing cast component weight.

"This, in turn, results in lower fuel consumption and reduced emissions, which naturally strengthens Volvo's competitiveness," added Dahlberg.

Environmental Benefits

From the environmental perspective, the three main benefits derived from FPC are:

Increased Environmental Performance of Engines--As demanded by the governments of the U.S. and European Union, emissions from diesel engines must be reduced. One method being used by engine designers to achieve this is higher peak combustion pressure in the engines. The problem is that the engine blocks and heads must be able to withstand these higher thermal and mechanical loads. The FPC process increases the castings' properties to meet these demands.

Reduced Virgin Sand Usage--"The Future Process for Castings method enables us to save molding sand, in some cases up to 70% depending upon the casting," said Dahlberg. "The new method also gives us the technical capacity to eventually reduce the risk of odors in the area around the foundry." In traditional casting, approximately 0.5 kg of raw sand is consumed per kilogram of castings produced. In the FPC method, 0.15 kg of raw sand is consumed per casting kilogram. One of the environmental priorities in Sweden is a reduction in the use of virgin sand; this is achieved with FPC.

Energy Recycling--During casting solidification and cooling, 50% of the theoretical heat energy from the casting is adsorbed by the cooling water, according to the foundry. The foundry is able to recycle this energy.

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RELATED ARTICLE: Volvo's Future Process Receives 2002 Environmental Award

At last October's World Foundrymen Organization (WFO) World Foundry Congress, the WFO presented Volvo's foundry in Skovde, Sweden, with the Environmental Award 2002 for the firm's development of the Future Process for Casting. The award was given for the process' contributions "to a reduced environmental impact from production as well as from the completed truck engine."

"It is very gratifying that our environmental efforts, with extensive investments in advanced technology, have been recognized in such a major forum," said Leif Hultman, manager for Volvo Powertrain in Skovde.

A Look at Volvo's Powertrain Foundry

Volvo's Powertrain Foundry in Skovde, Sweden casts 50% of the gray iron components produced in Sweden. The foundry's primary products are cylinder blocks and heads, brake drums and disks, and flywheels. Its production is 84,000 tons/year.

While green sand casting still dominates its production, the foundry believes a good portion of its future production will rest in FPC. Beyond casting, the foundry also serves as the competence center for the Volvo Group's engine developers.

Beyond the development of the FPC process to improve casting quality, the foundry also is focusing on material development as a means to improve castings' capabilities. According to the foundry, it is developing casting designs and production processes for compacted graphite iron to take advantage of the material's unique properties that make it a cross between iron and steel. The stability of the FPC is expected to provide manufacturing advantages in the production of compacted graphite iron.