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Research Programs

Hot Tearing in Aluminum Based Casting Alloys: mechanisms and control (measures)

Research Team:

Shimin Li
Diran Apelian

Introduction

With increasing use of light metals in automotive and aerospace applications, control of castability indices in aluminum alloys is becoming more and more critical. For casting alloys, castability refers to a set of characteristics that determines the suitability of an alloy for that specific casting process and ultimately for producing sound castings. The two main castability indices of an alloy are: fluidity, and the tendency to hot tear.

Hot tears are common defects encountered in castings. The defect is identified as cracks, either on the surface or inside the casting. Hot tears are usually large and visible to the naked eye. Sometimes, they can be very small and only visible under magnetic particle inspection. The main tear and its numerous minor offshoots generally follow intergranular paths and the failure surface usually reveals a dendritic morphology. Variables that influence hot tearing include alloy composition, mold properties, and casting design. A fine grain structure and a controlled casting process mitigate and limit hot tearing. Since the 1950s, much work has been devoted to understanding the mechanism of hot tearing; however, the mechanism is not totally clear, and how hot tearing is measured (or controlled) is not standardized. In general, it is well accepted that hot tearing is caused by solidification shrinkage and thermal deformation developed during solidification. Whether thermal stress or thermal strain, or strain rate is the controlling factor is not clear.

Objectives

The objectives of the project are to:

• Establish the mechanism that controls the formation of hot tears in aluminum alloys;
• Establish a criterion to characterize hot-tearing tendency of aluminum alloys;
• Develop a reliable experimental methodology and apparatus to measure hot-tearing tendency of aluminum alloys.

Methodology

The principal goal of this work is to fully understand the mechanism that governs the formation of hot tears in aluminum based alloys, and to develop a reliable methodology that can be employed to quantitatively characterize the hot tearing tendency of aluminum alloys. In order to achieve this goal, the following methodology and strategies will be pursued:

1. Design experimental apparatus and conduct experiments to identify the major factors that control the formation of hot tears in aluminum based alloys
2. Perform systematic experiments to fully characterize the effect of various variables (including alloy chemistry) on the formation of hot tears in aluminum based alloys. Specifically, the effect of alloy chemistry on the hot tearing tendency will be evaluated through thermodynamic simulations
3. Based on the experimental results, a mechanistic model (criterion) that can take into account all major factors will be established and validated through experiments
4. Develop/standardize a methodology/apparatus that can be utilized to quantitatively characterize the hot tearing tendency of aluminum alloys. The testing results should be reliable, and have a good correlation with those generated from actual casting processes

Expected Outcomes / Deliverables

1. A fundamental understanding of the mechanism that governs the formation of hot tears in aluminum based alloys
2. A reliable criterion that can be employed to characterize hot tearing tendency (susceptibility) of aluminum based alloys
3. A robust methodology/apparatus that can be utilized to quantitatively characterize hot tearing tendency of aluminum based alloys

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Last modified: September 07, 2007 16:44:07