emi-solid metal (SSM) processing also known as 'Semi-solid metallurgy' (SSM) is a forming technique that was devised at The Massachusetts Institute of Technology in the 1970s [1'5].Today, semisolid metal processing has been posed as a modern and advanced technology in the manufacturing of engineering components. Semisolid metal processing enables us to manufacture complicated shape components with near-net wall, good mechanical properties and high-dimension tolerance accuracy . Semisolid processing (SSP) occurs through mushy zone. Solid'liquid slurries have been formed under external forces between dies, named thixoforming, or injected into die cavity, named thixocasting process . Thixoforming uses semisolid slurries with globular solid particles uniformly suspended in a liquid matrix, which can be handled as a solid when at rest and flow like a liquid when sheared during the forming operation . Thixoforming is a technology of producing semisolid metal components . The components that can meet the stringent requirements for the automotive industry, by combining the near-net-shape capabilities of die casting with the mechanical properties of forging [10-13]. Besides, thixoformed parts are reported to be substantially higher in quality than die castings but lower in cost than forgings . While thixoforming offers significant advantages compared with traditional metal forming methods, the process comes with the requirement for a special feedstock [15'19]. During the last 40 years of research efforts, a lot of different materials have been developed and used with this process. For successful SSM processing, the microstructure should have solid near-globular grains with a wide transition area from solidus to liquidus, as shown in Figure 1.1 . These microstructures impart thixotropic properties in slurries; that is, they have shear and time dependent flow properties. A unique property of the flow behavior in SSM is related to the non-Newtonian behavior of an alloy where, in this case, when the material state is 50% solid and is sheared, the coalescence of the material will break up, its viscosity will fall, and it will flow like a liquid, but if it is allowed to stand for a certain time, globular coalescence will increase the viscosity of the material, which leads to it being able to support its own weight and be handled in the same way as if it was solid [21, 22]. As described above, a globular microstructure with an appropriate amount of liquid fraction is required in order for SSM forming to be successful. When shear forces are applied, the near-globular particles move easily past one another, causing a decrease in viscosity and making the material behave like a liquid. In contrast, when shear forces are applied on dendrite microstructures, typical of conventional castings, the liquid is trapped between dendrite arms and prevents them from moving freely, thus increasing the viscosity of the material [23, 24].
Different methods have been developed to produce a suitable feedstock with non-dendritic (thixotropic) microstructure for thixoforming. These include mechanical stirring, magneto hydrodynamic stirring, spray casting, strain induced melt activation (SIMA), recrystallization and partial melting (RAP), grain refinement and the cooling slope (CS) method [25,26,27]. Inspite of many methods developed, thixoforming has not enjoyed the wide spread application due to high production costs and limited choice of size and non-uniform microstructure of ingots from these technologies . The cooling slope (CS) casting process is the oncoming semi-solid process which employs simple equipment and involves low running costs. CS casting is a simple process which involves pouring molten metal with a suitable superheat through a cooling slope plate and subsequent solidification in a mould [29, 30]. In the CS casting process, there are several process parameters such as mould material, mould temperature, cooling slope length, cooling slope angle, pouring temperature, water cooling etc. which directly influence the final microstructure of the solidified slurry .
1.2 AIM OF WORK
It's required to investigate the effect of pouring temperature and the material of mould/slope plate combination on the microstructure of A356 aluminum alloy. Also to know what is the most effective parameter on the microstructure. Most of researchers focus on some parameters and ignore others, less of them don't take the material of mould/slope plate and study its effect so in this thesis, and the material of mould/slope plate combination was studied to show its effect. Carbon Steel, Stainless steel, and pure copper were chosen to make the study. The Taguchi's and analysis of variance statistical (ANOVA) approaches were employed to find out the optimum parameters settings of CSC process.
1.3 OUTLINES OF THESIS
Thesis: Effect of cooling slope casting process parameters on the thixotropic microstructure of semisolid aluminum alloys, it is organized in four chapters as follows:
' Chapter 1 is an introduction. Its presents an overview of the semi-solid metal processing and its applications. The aims of the thesis are also stated in this chapter.
' 'Chapter 2 is the literature survey. This chapter describes the different semi- solid processing (SSP) techniques and the importance of the obtaining the thixotropic microstructure on the characteristics of parts produced by SSP.
' 'Chapter 3 is the experimental work. This chapter describes the experimental procedures conducted in the present investigation.
' Chapter 4 presents the results obtained from the experimental work. The analysis and discussion of the results are described in this chapter.
' Conclusions of the investigation is listed in the final part of the thesis.
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