Hot tearing in aluminium alloys

David Viano (2011). Hot tearing in aluminium alloys PhD Thesis, School of Mechanical and Mining Engineering, The University of Queensland.

       
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Author David Viano
Thesis Title Hot tearing in aluminium alloys
School, Centre or Institute School of Mechanical and Mining Engineering
Institution The University of Queensland
Publication date 2011-10
Thesis type PhD Thesis
Supervisor Prof. David StJohn
Assoc. Prof. Carlos Caceres
Dr. John Grandfield
Total pages 216
Total colour pages 66
Total black and white pages 150
Language eng
Subjects 09 Engineering
Abstract/Summary Hot tearing continues to be a major problem in both Direct Chill (DC) and shape casting of aluminium alloys. It can limit productivity and influence the viability of casting some alloy compositions. While the underlying cause of hot tearing is understood i.e. the inability of liquid to feed imposed strain on the mushy material (two-phase region where developing solid and liquid coexist), work continues on improving the understanding of the mechanisms at play. There have been numerous hot tear experiments developed using different configurations and various levels of complexity. There have also been many models developed to predict hot tearing. Some are empirical in nature or based upon the solidification characteristics of a given alloy while others are more detailed and take into consideration parameters such as solidification shrinkage, thermal contraction and permeability. The objective of this thesis was to develop a better understanding of the hot tear process and to determine the fundamental mechanisms that lead to this phenomenon. Hot tear experiments were conducted using the CAST Hot Tear experimental rig at the University of Queensland on a range of Al-Cu and Al-Zn alloys. Hot tear severity was quantified and compared with published work. Data from the experiments was also used to validate some established models. The experimental rig was also modified to allow visual observation of hot tear initiation and propagation. Hot tensile testing was also carried out on selected Al-Cu alloys to assess the suitability of using the reheating method to study hot tearing. However, comparison of results with hot tear testing was difficult as the slow reheating rate in the hot tensile tests allowed solid-state diffusion to occur. With hot tear testing of both Al-Cu and Al-Zn alloys, it was found that as the solute content increased, tensile coherency (fraction solid at which load development began) occurred at lower fraction solids. In the Al-Cu alloys, it was found that the type of tearing varied with solute content. At low solute levels there was generally a single large primary tear in both grain refined and non-grain refined alloys. As the solute level increased, the large primary tear gradually diminished and was replaced by smaller secondary tears distributed over a wider area. In the grain refined alloys, the smaller secondary tears transitioned into distributed porosity as the solute content increased further. Hot tear susceptibility results from the Al-Cu alloys correlated well with other published studies, particularly with data from an experimental rig with a similar configuration (where feeding is provided to the central hot spot). The results also correlated well with established hot tear models. In the Al-Zn alloys, hot tear susceptibility results did not correlate with previous studies. This variation was attributed to differences in the design of the experimental rig. The CAST Hot Tear rig provides liquid feeding to the central hot spot while the experimental rigs used in previous studies did not. However, susceptibility results correlated well with hot tearing models that were based on permeability of the mushy material. Visual observation of hot tearing showed that intergranular tears first appeared at very low loads in the centre of the cast bar and later linked with interdendritic tears through a sidewall grain. With a given test geometry and cooling conditions, it is argued that hot tearing susceptibility is determined by the tensile coherency point and the permeability of the mushy material at that point. Therefore, it is proposed that there are two mechanisms that can lead to hot tearing. The first occurs when feeding becomes insufficient due to low permeability before tensile coherency occurs. This leaves a void between opposing solidification fronts and is characterised by the presence of just one primary tear. Alternatively, the classic tear mechanism occurs when feeding becomes insufficient after tensile coherency. The extent and localisation of tearing is dependant upon how early tensile coherency occurs and how long afterwards feeding becomes insufficient. In addition, a hot tear susceptibility map has been developed to visually represent the combined influence of tensile coherency and permeability on hot tearing susceptibility.
Keyword aluminium copper alloys
aluminium zinc alloys
Casting
Hot Tearing
Additional Notes Pages to be printed in colour: 38, 48, 58, 59, 63, 68, 71, 78-80, 82-84, 90-97, 99-107, 111, 112, 116, 117, 119, 121, 125, 131, 134, 138-141, 145, 151-155, 157, 159, 162, 163, 175-187.

 
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Created: Tue, 20 Dec 2011, 14:54:28 EST by David Viano on behalf of Library - Information Access Service