The effect of cold spray coating parameters on the residual stress and fatigue performance of magnesium alloys

Global warming, dwindling fossil fuels, and the low efficiency of alternative energy sources such as batteries and solar panels compared to traditional carbon-based energy sources, have shifted the industrial design paradigm towards minimizing structural weight. In this scenario, employing low density grade metals is inevitable. Magnesium (Mg) alloys, such as AZ31B, are one of the lightest structural materials, and have been able to gain strong foothold in the industrial world due to their high “strength to weight ratio”. However, the long-term performance of parts manufactured from Mg alloys are required for reliable structural applications. Hence, research on the methods to enhance the fatigue properties of Mg alloys is crucial. Surface treatment, particularly surface coating of manufactured parts with a thin layer of high fatigue strength material is considered a versatile approach to address fatigue shortcomings of Mg alloys. Cold gas dynamic spray is an emerging technology that deposit metal powder with a supersonic velocity on substrate in a solid-state. The nature of this technology (peening) promotes compressive residual stress and results in a refined grain structure, which reinforces a material’s surface and prolong crack initiation, thus enhancing the fatigue performance of materials like AZ31B alloy. Among all potential candidates for coating Mg parts, aluminum alloys are one of the best, due to their low density and strong fatigue performance. Hence, in this research, cold spraying is employed to deposit a thin layer of Al7075 on AZ31B-H24 Mg substrates in order to improve their fatigue performance. The complexity of the cold spray process requires some in-depth study through in situ measurements. For this, fiber Bragg grating (FBG) sensors is employed for monitoring and assessing the thermo-mechanical behaviour of Mg alloy during cold-spray coating. Furthermore, the hole drilling, X-ray diffraction and in situ FBG sensors residual stress measurement techniques are used to explore the effect of processing parameters and thermal mismatch on the residual stress distribution of coated Mg samples. In addition, SEM, TEM, CT scanning tomography, micro indentation hardness, surface topography and roughness measurements are utilized to identify the physical and microstructural characteristics of the coating, interface and substrate. Based on the findings of this research, the common understanding that cold spray induced residual stresses are compressive is questioned. It is shown that the heat associated with the spraying, although much lower than melting temperature, can wash out the peening effect and result in tensile residual stress in the substrate. Hence, to customize the residual stress and for inducing compressive residual in the Mg substrate, the cold spray coating parameters were tuned to decrease the heat input to the substrate. Moreover, a cooling system was designed that increased heat transfer from the substrate during the coating process. Another major factor affecting the state of the residual stress is the thermal mismatch of the coating and the substrate materials. The detrimental effect of thermal mismatch between the Al7075 coating and AZ31B substrate on the residual stress of substrate and coating was addressed by adding an interlayer of zinc, which has a higher thermal expansion coefficient than the substrate and coating material. Zinc is successfully deposited, and its coating parameters were selected in a way that resulted in inducing compressive residual stress in the substrate, coating and the zinc interlayer. However, detail characterization of the zinc and AZ31B substrate reveals that intermetallic phases have formed at the interface. Therefore, despite the induced compressive residual stress, cracking at the zinc magnesium interface restricts the application of this interlayer. Two extreme coating conditions that lead to induced tensile and compressive residual stress in the Mg substrate have been selected for the rest of this research. The quality of the coating is examined by CT-scan, which demonstrates that the compressive samples have less porosity than the tensile samples, although the densities of both samples are above 99%. The physical and mechanical properties of the compressive samples, including hardness and surface roughness, have also been significantly improved compared to tensile samples. Finally, the fatigue performance of the two types of coated samples (tensile and compressive) is investigated revealing that the compressive samples demonstrate exceptional fatigue life in high cycle regime compared to the bare AZ31B samples with 130% life enhancement