Document Type dissertation Author Name Saha, Deepak URN etd-041405-150300 Title Novel Processing Methods and Mechanisms to Control the Cast Microstructure in Al Based Alloys - 390 and Wrought Alloys Degree PhD Department Materials Science & Engineering Advisors Richard D. Sisson, Department Head Makhlouf M. Makhlouf, Committee Member Rathindra DasGupta, Committee Member Ray Donahue, Committee Member John Jorstad, Committee Member Diran Apelian, Advisor Keywords liquid mixing CDS casting wrought Al-Si Al alloys Al Aluminum Date of Presentation/Defense 2005-03-02 Availability unrestricted
The enactement of the Energy Policy and conservation Act of 1975, led to a paradigm shift in material selection and design in the automotive industry. The net effect was an increased focus by the automotive industry toward the use of light metals leading for the reduction of weight and hence, the dependence of imported oil. Increasing use of aluminum was a transition in that direction. However, raw aluminum on an average is 1.5 – 2 times as expensive as steel. Near net shape manufacturing processes (Die casting, Thixo-forging, etc) provided the much needed competitive advantage vis-à-vis steel / iron parts by permitting the manufacturing of Al components.
Semi solid processing involves the net shape manufacturing of alloys in a two phase region (liquid + solid). The reduced turbulence (during casting), less entrapped gases and lower operating temperatures (processes below the liquidus) make semi solid processing ideal for the manufacturing of high integrity Al parts. Traditionally, semi solid processing involved the heating of billets to a two phase region (called Thixcasting). Rheocasting is a new semi solid processing technique wherein the alloy is cooled from a liquid state (a combination of controlled heat / nucleation and growth phenomena) to yield structures similar to the Thixocasting process. Rheocasting or Slurry-On-Demand is in its early stages of development (the first industrial prototype of rheocasting was invented in the late 1990’s) and forms the central point of interest in this work. Much research is underway around the globe to understand the controlling mechanism as well as the structure – property relationships in rheocast parts, primarily limited to the hypoeutectic Al-Si alloys (less than 12.6% Si). This work is dedicated in the development of novel methods for the rheocasting of hypereutectic Al-Si alloys (greater than 12.6% Si) and Al based wrought alloys (alloys with Cu, Zn, Mg and Si as alloying elements).
The thesis presents the problems associated with microstructure control of hypereutectic Al-Si (primary Si coarsening and accelerated growth) and Al based wrought alloys (dendritic structures and hot tearing) with currently available technologies. Novel processing techniques are presented for the casting of hypereutectic Al-Si alloys and Al based wrought alloys with a combination of industrial trials and a through analysis of the underlying mechanisms.
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