Current state of semi-solid net-shape die casting

Semi-solid processing of alloys is nearing the end of its fifth decade in existence. This promising manufacturing route has undergone a number of changes over the past five decades and apart from certain successful industrial applications, it appeared that it had become a niche technology with limited applications, notably in the automotive and consumer product markets. Nevertheless, despite the strong competition of traditional casting and in particular die-casting processes, and the emergence of Additive Manufacturing (AM), there seems to be a resurgence in demand for Net-Shape Die Casting of Semi-solid alloys as testified by the high numbers of delegates at the 15th SSM Conference at Shenzhen, China in September 2018. What follows is a brief review of the historical development of the process as well as more recent developments with an eye to future prospects

An experimental evaluation of electron beamwelded thixoformed 7075 aluminum alloy plate material

Two plates of thixoformed 7075 aluminum alloy were joined using Electron Beam Welding (EBW). A post-welding-heat treatment (PWHT) was performed within the semi-solid temperature range of this alloy at three temperatures, 610, 617 and 628 °C, for 3 min. The microstructural evolution and mechanical properties of EB welded plates, as well as the heat-treated specimens, were investigated in the Base Metal (BM), Heat Affected Zone (HAZ), and Fusion Zone (FZ), using optical microscopy, Scanning Electron Microscopy (SEM), EDX (Energy Dispersive X-ray Analysis), and Vickers hardness test. Results indicated that after EBW, the grain size substantially decreased from 67 µm in both BM and HAZ to 7 µm in the FZ, and a hardness increment was observed in the FZ as compared to the BM and HAZ. Furthermore, the PWHT led to grain coarsening throughout the material, along with a further increase in hardness in the FZ

A microstructural evaluation of friction stir welded 7075 aluminum rolled plate heat treated to the semi-solid state

Two rolled plates of 7075 aluminum alloy were used as starting material. The plates were welded using a simultaneous double-sided friction stir welding (FSW) process. One way of obtaining feedstock materials for Semi-solid processing or thixoforming is via deformation routes followed by partial melting in the semi-solid state. As both the base plate materials and the friction weld area have undergone extensive deformation specimens were subjected to a post welding heat-treatment in the semi-solid range at a temperature of 628 °C, for 3 min in order to observe the induced microstructural changes. A comparison between the microstructural evolution and mechanical properties of friction stir welded plates was performed before and after the heat-treatment in the Base Metal (BM), the Heat Affected Zone (HAZ), the Thermomechanically Affected Zone (TMAZ) and the Nugget Zone (NZ) using optical microscopy, Scanning Electron microscopy (SEM) and Vickers hardness tests. The results revealed that an extremely fine-grained structure, obtained in the NZ after FSW, resulted in a rise of hardness from the BM to the NZ. Furthermore, post welding heat-treatment in the semi-solid state gave rise to a consistent morphology throughout the material which was similar to microstructures obtained by the thixoforming process. Moreover, a drop of hardness was observed after heat treatment in all regions as compared to that in the welded microstructure

Evolution of Globular Microstructure and Rheological Properties of Stellite™ 21 Alloy after Heating to Semisolid State

Metal alloys can be successfully thixoformed in the partially liquid state if they display non-dendritic nearglobular microstructures. The article presents the development of feedstock with such non-dendritic microstructure produced through the solid-state route of strain-induced melt-activated (SIMA) method, for a Stellite 21 alloy. Stellite alloys are a range of cobalt-chromium alloys designed for wear and corrosion resistance, currently shaped by casting, powder metallurgy or forging processes, but semisolid-state processing offers the possibility of a near-net-shaping method for these alloys. In this work, sprayformed followed by extrusion samples were heated to the temperature range at which the liquid and solid phases coexist in the material and spheroidal shape solid particles in a liquid matrix were obtained as required for semisolid processing. Microstructural investigations were carried out using scanning electron microscopy (SEM) in combination with energy-dispersive spectroscopy (EDS), with a further objective of analyzing the rheological properties of Stellite 21 alloy in the semisolid state, providing results to be used for identi- fication of a processing window of temperature and viscosity ranges for thixoforming this alloy

Thixoforming A201 aluminium alloy: is there a future in aerospace applications?

The alloy investigated in this study is the aluminium alloy A201; an important commercial alloy because of itshigh mechanical properties, excellent machinability and good formability, shown to be suitable for semisolidapplications due to its wide solidification range.Although this alloy is difficult to cast, it has a particularly high response to age-hardening and as a resultoffers good mechanical properties, especially the near doubling of elongation values when thixoformed in theT6 & T7 heat treated conditions, comparable with the values of wrought alloy 2014.In addition, initial fatigue tests of thixoformed A201 specimens, using non-dendritic feedstock generated by therheocasting process, have yielded very promising results.The paper looks into the microstructures of A201 feedstock derived from two different routes Rheocasting andMHD) and compares and contrasts their suitability and potential for future thixoforming applications

Developments on the delivery of Non-technical modules to Engineering Materials & Bio-engineering students

Like everything else in life, in business we have to deal with people. A successful manager will have to ‘Plan, Organize, Lead and Control’. Each of these four aspects of management requests dealing with people. In the practice of management it will always be ‘You’ and ‘Them’. To be a successful entrepreneur one requires a number of skills but success will not be forthcoming without ‘networking’. Successful networking needs communication skills. Teaching management to engineering students requires the development of their ‘soft’ skills. They need to be made aware of not only who they are as persons, what makes them tick and what their moral stand point is but also how to interact and communicate with others.This paper will give you a flavour of a module created to impart these kinds of skills to students of Engineering Materials and Bio-Engineers

Thixoforming M2 tool steel: a study of different feedstock routes

Different aspects of thixoforming M2 tool steel feedstock produced through deformation recrystallizationand partial melting (RAP) and through Sprayforming routes. The spheroidal microstructures obtainedare compared as are the resulting properties after thixoforming. Experiences on die materials are describedas are the various challenges of thixoforming high melting point alloys

Semisolid Metal Processing Techniques for Nondendritic Feedstock Production

Semisolid metal (SSM) processing or thixoforming is widely known as a technology that involves the formation of metal alloys between solidus and liquidus temperatures. For the procedure to operate successfully, the microstructure of the starting material must consist of solid near-globular grains surrounded by a liquid matrix and a wide solidus-to-liquidus transition area. Currently, this process is industrially successful, generating a variety of products with high quality parts in various industrial sectors. Throughout the years since its inception, a number of technologies to produce the appropriate globular microstructure have been developed and applied worldwide. The main aim of this paper is to classify the presently available SSM technologies and present a comprehensive review of the potential mechanisms that lead to microstructural alterations during the preparation of feedstock materials for SSM processing