Thixoforming of a high performance HP9/4/30 steel
Introduction
Thixoforming is a forming process that shapes metal components in their semi-solid state. Prior to forming, the microstructure of the alloy consists preferably of solid metal spheroids in a liquid matrix, which can be difficult to achieve with hot-worked, high alloy steels prone to strong segregation bands. Alloy slurry with a non-dendritic microstructure is said to be in a thixotropic state (from the Greek words of thixis which means ‘the act of handling’ and trope meaning ‘change’), whereby the internal structure of the material is changed by an external load. If an alloy in this state is sheared, this will result in a fall in its viscosity and it will flow like a liquid, but if allowed to stand it will thicken again. The behaviour of this type of slurry, which is acting like a non-Newtonian fluid, was first discovered by Spencer et al. [1] and has since led to extensive work on the thixotropy of alloy slurries [2].
On thixoforming of high temperature materials, one major attraction of thixoforming such materials, e.g. steels, is the low forging force involved during thixoforming as compared to that in conventional forgings [3]. This means that more intricate and complex shapes can be formed faster with some reduction in forming steps and with near net shaping capabilities [3], [4], [5], [6]. Other major advantages include prolonged die life due to less thermal shock (forging below liquidus as against castings), weight savings in components with less porosity than conventionally, plus improved usage of feedstock materials because of improved designs [7], [8].
The work described in this paper is the result of development in the semi-solid processing of a high melting point alloy: a high performance HP9/4/30 steel having a typically banded starting microstructure. The development of its microstructure in the semi-solid state is discussed. The thixoforming induction heating trials and the resulting load–displacement signals of the thixoformed products are explained.
Section snippets
Material
The high performance HP9/4/30 steel used was produced by a vacuum arc remelting (VAR) process and rolled at 1250–1310 °C. The high temperature of rolling is to achieve homogenisation in the ingot. It was subsequently normalised at 899–927 °C (air cooling to room temperature), hardened by heating to 843 °C (oil or water quenched) and refrigerated at −73 °C (warming in air to room temperature), double tempered at 538 °C and lastly double annealed at 677 °C (air cooling) and 621 °C (air cooling),
Starting material
An optical micrograph of the as-received high performance HP9/4/30 steel is shown in Fig. 5. The ingot had been subjected to rolling at 1250–1310 °C. The high temperature was to assist in achieving some homogenisation in the ingot. A complex heat treatment process had also been applied, that is, normalising, hardening, refrigerating, double tempering and annealing at temperatures and conditions as mentioned in Section 2.1. The complex microstructure shown by the material is related to this
Conclusions
The work carried out has demonstrated the feasibility of thixoforming a high performance, high alloy HP9/4/30 steel with typically microsegregated starting structures. The material is successfully thixoformed from its semi-finished ingot at processing temperatures in the range of 1470–1480 °C at 1 and 2 min holding. This is despite the lack of a ‘conventional’ thixoformable microstructure (i.e. spheroidal solid grains in a liquid matrix). Here, shearing may cause liquid pools to link up resulting
Acknowledgements
The authors wish to thank Dr. Ian Todd for helpful discussions, Corus Engineering Steels, Rotherham, United Kingdom, for supplying the materials, and the National University of Malaysia (UKM), Bangi, Malaysia, for partly providing the financial support.
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2019, Materials Science and Engineering: ACitation Excerpt :The results showed that there was an improvement in the wettability between the reinforcement and matrix, better uniform distribution, lower porosity and enhanced mechanical properties when the particles were introduced into the semi-solid slurry temperature [9–14]. SSP has many advantages, such as complex shapes can be formed with some reduction in forming steps and with near net shaping capabilities, less energy consumption, less solidification shrinkage, prolonged die life, good filling of the die and improved mechanical properties [15–18]. As processing is required in the semi-solid state, a middle state in the physical rheological sense, process conditions fall within a narrow band such that environmental temperature conditions must be considered.