Integrated cast-extrusion of light alloys
Casting and forming operate separately in industrial practice. Forming processes such as forging and extrusion are normally carried out at elevated temperatures using billets prepared from casting ingots. Heat treatment, machining and reheating, etc., are required before forming and the overall manufacturing cost is high. Integrating casting and forming into a single process represents the possible shortest manufacturing route for metalworking, offering energy, material and cost savings.
As-cast ingots normally exhibit heterogeneous microstructures, macrosegregations and central shrinkage towards their ends. For these reasons, as-cast ingots are not suitable for direct forming before extensive homogenization treatment. Besides, conventional forming equipment is designed solely for shape change by plastic deformation in the solid state. However, the direct integration of casting and forming is possible for now. Technologies developed for grain refinement and process control are ready and available to obtain uniform casting structures that are essential for the integrated process. These technologies include electromagnetic processing, ultrasonic processing and mechanical convection, in addition to chemical inoculation by grain refiners. At BCAST, an advanced mechanical convection technology, i.e., high shear melt conditioning has been developed for grain refinement and microstructure control, together with a similarly advanced ultrasonic processing technology, which are all available to the proposed research. Moreover, with the assistance of modern computing
technologies, it is more readily nowadays to design and manufacture the apparatus and tools that are required for the integrated process, and to conduct experimentation in order to obtain the optimized process conditions. Additionally, adequate materials with required temperature resistance are now readily available, especially for light alloys, which are of relatively low processing temperatures. The proposed research will be the first trial of this process integration and hopefully open up a new research area, leading to a radical change in the casting and forming industry.
The project aims to integrate casting and extrusion into a single process for producing components of light alloys for structural applications or as feedstocks for further forming processes. In the proposed process, casting and extrusion are performed consecutively in a single apparatus, in which the casting mould is also used as the extrusion chamber. Extrusion starts when solidification is completed and the billet is cooled down to the predetermined extrusion temperature. Such novel integration has multiple benefits:
- Making best use of the solidification heat.
- Eliminating the multiple steps in conventional operations.
- Minimizing casting defects with rapid cooling such as excessive residual stresses.
- Increasing the range of extrusion temperature.
- Allowing manipulation of new heat treatment schemes.
Consequently, the consumed material, energy and overall manufacturing cost are at their minimum with uncompromised or improved product quality.
This project focuses on the establishment of a prototype setup for the integrated cast-extrusion process to demonstrate the benefits and potential of the process. Process optimization will also be conducted using aluminium magnesium alloys. The project till now has successfully completed a set of objectives including:
- The establishment of a prototype setup with a series of die structures for producing rods, wires, tubes and profiles with various dimensions from 8 mm to 20 mm in diameter (equivalent).
- Optimization of the thermal management system and tools including die structures.
- The production of Al and Mg samples.
- Mechanical and microstructural characterization of the produced samples.
The integrated cast-extrusion process is novel. This process integration is different from other approaches that use hot ingots as forming billets to save energy but casting and forming are performed on different equipment. It is also different from semisolid moulding, squeeze casting and high pressure die casting which involve limited or nil plastic deformation.
In an integrated cast-extrusion process, the as-cast billet is deformed immediately after solidification. Residual stresses, hot tears and porosities are removed and/or healed by plastic deformation during extrusion and chemical segregations can be substantially reduced during material flow and dynamic restoration of microstructures. Post-forming heat treatment will lead to a complete chemical homogenization in addition to strengthening and microstructural stabilization. The upper limit of forming temperature can be much higher as there are no overheating problems which can occur during normal heat treatment and pre-forming heating if the temperature is too high. Thus, the high formability of metals and alloys at high temperature can be fully exploited with largely reduced extrusion load. For castings as structural components, the application of plastic deformation will help improve material filling, in addition to healing defects, lowering the requirement for castability. It is our unique vision that the integration will eliminate the boundaries between casting and wrought alloys, change the paradigm of material and product design and ultimately lead to the merging of casting and forming into a single industrial sector.
The UK metalforming industry produces 1.3 million tonnes of metal products and consumes about 1,515GWh of energy, which represents CO2 emissions of 450,700 tonnes. In general, heat treatment and process heat accounts for ~65% energy consumptions, presses, hammers and other machineries account for ~20% and space heating
and lightening and others account for ~15%. An integrated cast-forming process has potential to reduce over 30% energy consumptions by making use of the solidification heat, eliminating heat treatment and machining, reducing scrap rate and adopting a closed loop operation, with an equivalent reduction of environment impact.
The integration also provides opportunities for the optimization of overall energy management for the industry. Considering the fact that our future manufacturing will face uncertainty in energy supply and reduction of availability in many key resources such as material, water and land, the integration offers a major opportunity for the UK industry to enhance its long term competitiveness. It is now the right time to address this important initiation and take a step forward in its technology development, which will allow the UK to retain its globally recognised casting and forming expertise that underpins many competitive, technology-based industries vital to keep the UK's manufacturing, in particular aerospace and automotive base ahead of the competition.