IMI: Exploitation Of Semi-Solid Processing

EPSRC research grant GR/K66611

Summary of the final report

Principal Investigators: Dr P J Ward, Dr A J Lowe*, Dr H V Atkinson (Principal Investigator), Prof K Ridgway*, S C
Hogg and Dr D H Kirkwood
Dept of Engineering Materials and *Dept. of Mechanical and Process Engineering, The
University of Sheffield, Mappin St, Sheffield S1 3JD
Grant Number: GR/K66611
Partners: University of Sheffield (Dept. of Engineering Materials and Dept. of Mechanical and
Process Engineering, Lucas PLC, Federal-Mogul PLC, Brook Hansen Ltd., Servotest Ltd. and
Rotary Electrical Ltd.

Introduction

In most metal alloys there is a temperature interval between the onset and completion of melting. This semi-solid range can be a few degrees up to several hundred. Usually, the alloy is stiff in the semi-solid state, its shape being maintained by interlocking dendrites of the solid phase. However, the metal can be treated so that the alloy consists of solid spheroids surrounded by liquid and is thixotropic. In this case, a cylinder of the semi-solid alloy with, say, 50% liquid, can stand up if left undisturbed but will flow like oil when sheared.

Semi-solid metal processing (or thixoforming) exploits this behaviour to make components, usually using adapted pressure die-casting machines. Thixoforming is commercialised in Europe, Japan and the USA, especially for aluminium car parts. The high viscosity and low heat content of semi-solid alloys mean that the products are stronger than pressure diecastings and can be made more quickly and conveniently than forgings. This project aimed to overcome the barriers to the exploitation of thixoforming in the UK by producing demonstrator components and developing a software package to allow companies to assess the business process implications of adopting thixoforming for particular applications.

Achievements

Writing of `Thixocost´ cost-benefit analysis Software: This is the first package in the world for cost-benefit analysis of thixoforming. It contains a unique approach to using the business process perspective for evaluating the cost implications of an innovative production technology. Three copies of the software have been sold at a cost of £1000 each. Several organisations have expressed an interest in generalising the software for other innovative technologies.

Thixoforming of an electric motor endplate for Brook Hansen: 25 successful repeats of this Al alloy component were thixoformed in a soft die to very high tolerances. This task showed that thixoforming can be used for short runs using `soft´ dies. These dies are cheaper and more readily machinable than conventional ones, affording greater responsiveness to the customer. The design of the die was optimised using computational fluid dynamics (CFD) modelling. This exercise led to an increased understanding of the flow behaviour of semi-sold metal slurries.

Thixoforming of a car component for Federal-Mogul: The University of Sheffield thixoformed 40 successful repeats of this challenging component using a hardened tool steel die. The product had an excellent surface finish. Less machining is required after forming and the shot weight is only ~60% of that used in fully liquid die casting. Die design optimisation with CFD led to a greater understanding of how to fill components with large variations in section and how to control solidification. Thixoforming is probably the best way to produce this highsilicon aluminium alloy component: such alloys are difficult to shape by liquid casting or forging. There is industrial interest in further work with this alloy feedstock, which is prepared by a process patented by Sheffield University.

Thixoforming of copper electric motor rotors for Brook Hansen: This demonstrator is important because of the potential for energy saving from using copper rather than Al. Eight components were made, using extruded Cu-0.11% O feedstock. This was a very challenging task due to the narrow melting range of the alloy and its susceptibility to oxidation. It is considered a major achievement to have filled the die. There is much interest in Cu rotors from the international copper industry, which is investigating their potential as an environmentally friendly technology.

Scientific study of spray-formed Al-high silicon alloys: This is leading to a greater understanding of spray-forming through the finding of Al inclusions in Si for the first time. Compressive tests have demonstrated that Mg retards the agglomeration of silicon particles in the semi-solid state. Such agglomerates interfere with slurry flow during thixoforming. A unique Environmental SEM study of ripening of Al/Si in semi-solid state has been done. Study of equal channel angular extrusion: It was shown that the non-dendritic, thixoforming microstructure can be produced in cylindrical bar by deformation but without reducing its cross section. The process extends the University of Sheffield patent on feedstock for thixoforming by cold and warm working. Modelling and empirical rules for die design: The modelling work in the project allowed a series of guidelines for die design to be established and enhanced the understanding of modelling thixotropic flow. This has led to the setting up of a further project on modelling.

Contact:

Department of Engineering Materials,
University of Sheffield,
Mappin Street, Sheffield S1 3JD
Telephone: 0114 222 5502
Fax: 0114 222 5943