Fatigue of magnesium (Mg) spot welds

Material competition in auto industry has been traditionally intensive. Steel has been the dominant material used in automobiles since 1920s; however, requirements for cleaner vehicles are forcing automakers to reduce exhaust emissions. As a result, interest in magnesium, as the lightest engineering metal, has been increased during the last decade. Current Mg automotive parts, however, are only manufactured by casting and extrusion, and it has not yet been used as sheet in body parts. This research is being done as a part of feasibility study of Mg alloys application in automotive body components.
Resistance spot welding (RSW), on the other hand, is already the most common joining process in auto bodies. A typical vehicle in North America contains about 4000 to 5000 spot welds. However, applicability of this joining process for wrought Mg alloys has not yet been studied.
As spot welds are points of stress concentration, they are more likely for fatigue failure. Therefore, verified fatigue models are needed to make sure that spot welded components will perform satisfactorily throughout the total life of vehicles.

This research was conducted as part of the Magnesium Front End Research and Development (MFERD) project, a joint research collaboration involving institutions from Canada, China and the United States.

3. Research Objectives and Plan

• Characterization

This phase involves extracting the microstructural and mechanical properties of the magnesium alloy of interest, i.e. AZ31B-H24 in rolling and transverse directions. Tensile-shear test on the lap joint spot welded specimens is required to check if the spot welds meet the minimum qualification criteria of standards. The geometry of spot welded specimens is shown below.

 

Tensile-shear test results are also used for choosing proper loads for performing fatigue tests. Fatigue properties of the base material and spot welded specimens are of interest, as they are needed for developing a fatigue damage parameter. Studying mechanisms of failure is required to develop a successful and comprehensive fatigue damage parameter.

• Fatigue model development

The main objective of this research is developing a fatigue model for spot welded magnesium sheets. A number of fatigue models have been developed and verified for steel spot welds during the past 30 years. Some of these models, proposed by Sheppard, Rupp, Kang and Swellam, are evaluated for magnesium spot weld. A fatigue damage parameter will be developed based on the sample test results and considering different modes of failure.

• Model verification

The model verification step aims to confirm that the model is capable to predict the fatigue life of spot welds in any structure and under any type of loading, considered in the fatigue model development phase. The component should satisfy the generality, in terms of applying loads on nuggets.

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