Semi-solid processing of alloys and composites

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.

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Semisolid Metal Processing Techniques for Nondendritic Feedstock Production

Semi-solid processing of the CoCrCuFeNi high entropy alloy

A suitable microstructure for thixoforming consists of spheroidal grains in a liquid matrix. In order to achieve this, some processing routes have been established. In this study, microstructural evolution of XW-42 and AISI M2 steels via direct partial remelting from the as-annealed condition was examined. Both steels were found to possess certain types of carbides that dissolved when subjected to heating inside the semi-solid zones. The types of carbides were MC-M6C for AISI M2 steel (fully dissolved between 1280 and 1300 °C) and M7C3 for XW-42 steel (fully dissolved between 1250 and 1270 °C). The dissolution of these grain boundary carbides during partial remelting helps with the grain spheroidisation and also with providing grain lubrication for the forming process through solid–liquid particle contact. In addition, solid–solid contacts were also observed (in 2D as well as 3D microstructural observation) and considered useful to provide structural integrity prior to forming. However, these contacts also must also be weak enough to be sheared easily. Compression test results for AISI M2 steel showed that thixotropic behaviour and complete mould filling were observed starting from 35 liquid percent. The same behaviour is also expected to occur for XW-42 steel due to its similarity in microstructural evolution with AISI M2 steel.

Microstructural investigations of XW-42 and M2 tool steels in semi-solid zones via direct partial remelting route

https://hal.archives-ouvertes.fr/hal-01090556/document

Impact of experimental conditions on material response during forming of steel in semi-solid state

The semi solid forming of steel has the potential of an innovative technology, which could open a new possibility of metal components production. Earlier research works in the 1990s have already shown the principal feasibility of the process but also revealed the various kinds of technical problems. Since then, research works on steel processing have been continuously made in several locations in Japan, USA and Europe. Currently, the focus of these research activities is the development of suitable tool material and the achievement of high part quality, which is the key point for the success of the process. This paper gives an overview of the previous research works and the current state of the research with an outlook on future tasks for this challenge.

Semi Solid Casting and Forging of Steel

This work describes the processing of different aluminum cast- and wrought alloys in the Cooling-Channel-Process. The investigations take the castability, the microstructural evolution for the SSM-processing and the resulting properties in the test specimen into account. Beside the alloys AlMg5Si2Mn (M59), AlCu4TiMgAg (AF48), AlSi12CuNiMg (SF70), AlSi17Cu4Mg (SF90), AlZnMgCu1,5 (AA7075) the optimization of the alloy AlSi7Mg0,3 (A356) by the addition of magnesium and copper were a main task of the work.

Potential of the Rheocasting Process, Demonstrated on Different Aluminum Based Alloy Systems

Some excellent projects have been finished successfully since the last conference on the pre-industrial stage showing the great potential of thixoforming technology for steels [1][2]. Experiments presented in this paper on induction heating, process window and mould filling of three widely-used steel alloys offer an even greater field of application for industrial companies. In the past thin walled cutting tools and complex impellers made of cold working steel X210CrW12 were produced at the Foundry Institute [3][4]. Constitutive on this knowledge the field of applications is enlarged by research on two further steel alloys. Processing of 100Cr6 (roller bearing steel) and 42Cr4 (annealing steel) is challenging due to a decreasing window at a higher temperature level with decreasing carbon content. An exact procedure has been worked out to create new control programs for the induction heating unit. It is based on thermo-chemical calculations and delivers billets with feasible temperature distribution and well defined content of liquid phase. Material characterization is performed in a step-die (seven steps between 25 and 0.5mm), especially developed for demands of semi-solid casting. Mould filling capacity, micro structural evolution and mechanical properties are determined on each of the seven steps. For any reproducible process the knowledge of suitable system parameters is essential. To find the limits of the process window sensitive experimental parameters were changed systematically for each of the three steel alloys. In addition to tool temperature the process windows consist principally of content of liquid phase fs, piston velocity vP and pressure during freezing pfr. Characteristic differences appeared between the different steel grades. The thermal simulation was used to find functional dimensions for the gating system and to shorten process times. Simulation of tools system was used to estimate the additional thermal load induced by higher working temperatures. The abrasive wear at the offsets to the next steps was in the same order of magnitude for all examined steels. These results provide the opportunity for commercial steel grades to cast complex steel parts in high pressure die casting.

Steels with different carbon content for high pressure die casting in semisolid state

Semi-solid metal casting is an innovative technology for the production of near-net-shape parts with demanding mechanical properties. The paper describes different processing routes and materials for semi-solid-metal casting (SSM), which have been investigated and also partially developed at the Foundry-Institute of Aachen University.

The standard thixocasting process for aluminium, highly reactive magnesium alloys and steel alloys with high melting points was investigated under variation of a wide range of process parameters. Specially adapted pre-material production and reheating methods were developed for different materials and their application and future potential is pointed out. The thixocasting experiments were executed on a modified high pressure die-casting machine with a specially designed “step-die” providing wall thicknesses from 0.5 to 25 mm. The mechanical properties were tested in dependence of the wall thickness and the metal velocity. The results of these examination show high tensile strength values in combination with very good elongations.

The rheocasting process is a new SSM-forming method with liquid melt as feed-stock and a high recycling potential. The research results of RCP-technology (Rheo-Container-Process) invented at the Foundry-Institute and of the Cooling-Channel-Process for aluminium and magnesium alloys are promising and are presented in this paper.

Studies on semi-solid processing of magnesium alloys and mixtures of them were conducted by ThixomoldingTM. To establish the most adequate process parameters, the temperature and the mixture relations were varied. Using a mould for tensile test specimens, the mechanical properties and the microstructure evolution could be evaluated. The chemical composition of the different phases was determined using SEM and EDX technologies. Evaluations of the flowing properties were conducted using a spiral mould with a total length of 2m and a cross section of 20mm x 1.5mm.

Semi‐Solid Processing of Metal Alloys

In order to broaden the field of application for the innovative thixocasting process, much research is dedicated to the thixocasting of high melting point alloys. The wide property range of modern high alloy steels combined with the productive semi-solid die casting process opens up new fields of application. The Foundry Institute of the Aachen University has therefore been concentrating on the research of the possibilities and limits of high pressure die casting of high alloy steels in the semi-solid state. This paper gives an overview of the current work dedicated to thixocasting of steel alloys by a high pressure die casting machine at the Foundry-Institute of the Aachen University of Technology. In order to understand and describe the material properties in the semi-solid state, basic test specimens have been investigated. Weak points of tool preheating as well as directional solidification of the produced parts can be controlled by numerical simulation of the temperature distribution inside the dies. In consideration of the outstanding flow properties of semi-solid steels more complex geometries with accurately defined applications are now being investigated. Extensive metallographical analyses of the pre-material, the reheated billets and the produced parts have been done to evaluate the viability of the process. The mechanical properties of the specimens outline the outstanding potential of the thixocasting process.

Carsten Afrath

Papers

Semi Solid Casting and Forging of Steel

Potential of the Rheocasting Process, Demonstrated on Different Aluminum Based Alloy Systems

Steels with different carbon content for high pressure die casting in semisolid state

Semi‐Solid Processing of Metal Alloys

Casting of high alloy steels in the mushy state