Melting moments - rheoformed melt processing technologies
Rheoforming is the generic name for a new group of melt processing technologies that have been under active development for the past few years at the UK-based Brunel Centre for Advanced Solidification Technology (BCAST) on the Brunel University campus. The family includes the rheo-diecasting (RDC) process for production of near net shape components, the rheo-extrusion (RE) process for direct production of extruded profiles from liquid melts, direct chill rheocasting (DCRC) for developing billets and slabs, and twin roll rheocasting (TRRC) for the creation of sheet products.
The common feature of these technologies is controlled, high shear, melt processing, which promotes the transformation of liquid alloys into a solid product that has a fine and uniform microstructure with no macro-segregation and significantly reduced casting defects. As a direct consequence, rheoformed components have an improved mechanical performance, requiring less solid-state deformation processing to manufacture high performance products.
Twice as nice
The key component for rheoforming is the twin-screw melt-conditioning device (TSMCD) (see image). The TSMCD utilises a pair of co-rotating, fully intermeshing and self-wiping screws inside a temperature-controlled barrel that allows rapid heat extraction. The screw profile is designed so that the melt is simultaneously subjected to conditions of high shear (orders of magnitude greater than electromagnetic stirring) and high turbulence.
Under such intensive forced convection, the melt always has an extremely uniform temperature and chemical composition with well dispersed heterogeneous nucleation sites – recalescence is suppressed. This provides unique conditions for subsequent solidification. Processing times are of the order of a few seconds, and conditioned melts can be delivered at a range of rates to feed both discreet and semi-continuous casting processes.
Nucleation in the conditioned melt becomes continuous, with an enhanced rate throughout the entire liquid volume and coarsening kinetics that become extremely slow. The result is a fine and uniform solidified microstructure with reduced cast defects.
Rheoformed materials exhibit both high strength and high elongation in the as-cast condition. Compared to high pressure diecasting, the RDC process offers an AZ91D alloy with a 15% increase in strength and up to 300% increase in elongation. Similar improvement in mechanical properties has been achieved for a range of magnesium and aluminium casting alloys.
Conventional twin roll casting of aluminium and magnesium alloys results in multi-layer segregated microstructures, while the range of alloys that can be successfully cast is limited. The TRRC process offers increased production rates and eliminates segregation both at the cast surface and mid section and extends the range of castable compositions.
Finding materials for screw and barrel manufacture to use with aluminium melts was a difficult and demanding task. However, a suitable material was eventually found and has proved to be reliable over extended casting campaigns. Intellectual property issues are presently being addressed for this important material in a wide range of applications.
The potential impact of the rheoforming processes is considerable. Theoretically, rheoforming pushes the boundary of solidification from the current static condition to a controllable dynamic one.
Solidification under intensive forced convection promises a number of interesting and useful phenomena, such as volume and continuous nucleation, an increased effective nucleation rate, spherical growth and slow coarsening kinetics. This has triggered a new approach to investigating the atomic structures of liquid metals and alloys and will lead to a better understanding of the nucleation process. It has already confirmed that crystal growth under intensive forced convection is spherical. This is a far simpler process than the complex growth patterns that occur under static conditions. The research has also identified increased undercooling, formation of divorced eutectic microstructures and elimination of constitutional undercooling.
Technologically, the rheoforming processes represent a disruptive step change in the field of melt processing and casting of metallic materials. They not only offer improved mechanical performance for existing alloys, but also the opportunity to develop new metallic materials. This is due to the unique ability of rheoforming to process materials that are usually difficult or impossible to make using conventional technologies.
With rheoforming, most, if not all, conventional wrought alloys can be cast routinely into near net shape components with high integrity. Immiscible alloys, such as aluminium-lead and aluminium-indium, can be rheocast into solid shapes with fine and uniform microstructures, and aluminium-tin alloys with a freezing range over 400°C can be directly cast into parts for bearing applications. High quality metal matrix composites can be cast into near net shape components for applications requiring wear resistance, high thermal stability and high stiffness. In addition, the lack of porosity in the as-cast condition enables rheoformed components to be heat treated to improve mechanical performance.
|Mechanical properties of rheoformed light alloys|
|Alloy||Processing conditions||Yield strength (MPa)||UTS (MPa)||Elongation (%)|
|AZ31||DCRC, direct extrusion||263||308||20|
There is no longer a need to have an artificial distinction between cast and wrought alloys. In the future, metallic alloys could have a much wider composition range, offering greater property coverage compared to current alloy specifications. Potentially, a range of hybrid light alloys could be created based on aluminium and magnesium with more equal proportions.
A degree of tolerance
A further feature of the rheoforming technologies is the ability to tolerate impurities in melts of both magnesium and aluminium alloys. Dependence on expensive primary metal can therefore be reduced and melts can be made either entirely with secondary metal, or the use of primary metal as a melt sweetener can be minimised. For aluminium, it is tolerance of iron and the modification of as-cast intermetallic phases that is important. With magnesium, tolerance and modification of oxides are vital for successful use of casting scrap without expensive refinement.
Casting the net
The successful development of these methods could result in a major reshape of the metallurgical industry. Currently, near net shape casting plays a limited role in providing metallic materials for engineering applications. Most of the demanding applications are achieved through thermo-mechanical processing, such as extrusion, rolling and forging. Such processes are characterised by high levels of capital investment and energy consumption, low materials yield, and inevitably high cost. It is possible that, in the future, exploiting the rheoforming technologies will result in materials that meet the majority of engineering demands with dramatically reduced levels of thermo-mechanical processing. The requirement for solid state deformation processing will be minimised.
As a direct consequence the metallurgical industry as a whole will be more energy efficient, cost effective and environmentally friendly.
Professor Zhongyun Fan, tel: +44 (0)1895 266406, email: firstname.lastname@example.org.
The understanding of the rheoforming process has been supported by successive grants from EPSRC and the industrial development of specific rheoforming processes has been assisted by the DTI Innovations Programme.