The Effect of Cold Spray Coating on Fatigue Life of Magnesium Alloy, AZ31B
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Wrought magnesium alloys are considered attractive candidates for structural members in automotive and aerospace industries due to their high specific strength. Although new processes have helped to produce high purity magnesium alloys with higher resistance to corrosion, these alloys still need protection against corrosion when they are used in aggressive environments. Cold spray coating is one of the protective methods that are employed for this purpose. The similarity between cold spray coating and shot peening process poses the question whether cold spray coating can improve the fatigue strength in addition to providing corrosion protection. The objective of this research is to answer this question for the specific case of the coating of wrought magnesium alloy AZ31B with aluminum powder. This study comprises two parts. The first part characterises the residual stress induced by cold spray coating. This investigation employs both numerical and experimental methods. For the numerical study, the cold spray coating process has been simulated via ANSYS software classic package. The numerical results have been compared to experimental results from X-Ray Diffraction (XRD) stress measurement of a coated sample. For the second part of this research, the fatigue strength of as received, stress relieved, and stress relieved/coated specimens have been compared. Three groups of AZ31B specimens have been prepared and tested by rotating bending machine and their S-N curves have been prepared. Comparison of the results reveals that there is a considerable loss in fatigue strength of as received specimens after stress relief. This is due to the removal of compressive residual stress in the raw material induced by the extrusion process. Also, comparison of S-N curves of stress relieved and stress relieved/coated specimens shows fatigue life improvement after cold spray coating. The maximum improvement is 49 percent in the load of 120 MPa and the endurance limit has improved 9 percent.