Thixoforming of High Melting Point Alloys

Abstract

  Although thixoforming complex near net shaped products in aluminium alloys is now an established technology in many automotive applications, current developments are taking place in a number of fronts: dynamic applications for load bearing components; alloy development-expanding the existing portfolio of thixoformable alloys; in materials recycling and in thixoforming of high melting point alloys.
  In the automotive industry, the drive for materials' development stems from the necessity of weight reduction in cars (more use of light alloys, including aluminium, magnesium, and metal matrix composites) and reduced costs. Most of the major automotive producers are currently using or are experimenting with thixoformed products. In Europe, Italian, French, German, and Spanish manufactures all have invested in the technology. In the United States, Ford is the main investor.
  The current manufacturing routes involve either 1) the use of non-dendritic alloy feedstock, which is cut to slugs of appropriate size before re-heating the slugs to the semi-solid state by a carousel of induction heaters and once the slugs have reached the required liquid fraction, robots automatically transfer them to the shot chamber of the thixoforming press (an adapted form of die-casting machine) to inject them into a die, or 2) cooling from the liquid to the semi-solid state before directly injecting into the die (New Rheocasing Method NRC). The production route is very similar to diecasting, however, the resulting properties of the parts are of much higher quality.
  Thixoforming has seen commercialisation around existing alloy composition based on aluminium-silicon 356 and 357 materials. A number of players are undertaking research to expand the portfolio of existing alloys, both in low and high temperature alloys.
  Current research in the later field is concentrating on the development of high melting point alloys such as steels, iron-alloys, copper-alloys, superalloys and other exotic materials, to further exploit the potential benefits of this under-utilised metal forming technique. However, although thixoforming of high melting point alloys offers exciting possibilities and tremendous potential, and has already been part of the original work of over thirty years ago, it is currently still in the research stage of development.
  This paper will concentrate on the later strand and will review past and current developments and offer insights to as yet unforeseen possibilities.