Evolution of Residual Stresses in Micro-arc Oxidation Ceramic Coatings on 6061 Al Alloy

Most researches on micro-arc oxidation mainly focus on the application rather than discovering the evolution of residual stresses.However,residual stresses in the surface coatings of structural components have adverse effects on their properties,such as fatigue life,dimensional stability and corrosion resistance,etc.The micro-arc oxidation ceramic coatings are produced on the surfaces of6061 aluminum alloy by a homemade asymmetric AC type of micro-arc oxidation equipment of 20 kW.A constant current density of4.4 0.1 A/dm2and a self-regulated composite electrolyte are used.The micro-arc oxidation treatment period ranges from 10 min to 40min,and the thickness of the ceramic coatings is more than 20μm.Residual stresses attributed toγ-Al2O3constituent in the coatings at different micro-arc oxidation periods are analyzed by an X-ray diffractometer using the sin2ψmethod.The analysis results show that the residual stress in the ceramic coatings is compressive in nature,and it increases first and then decreases with micro-arc oxidation time increase.The maximum stress value is 1667±20 MPa for period of 20 min.Through analyzing the coating thickness,surface morphology and phase composition,it is found that the residual stress in the ceramic coatings is linked closely with the coating growth,the phase composition and the micro cracks formed.It is also found that both the heat treatment and the ultrasonic action release remarkably the residual compressive stress.The heat treatment makes the residual compressive stress value decrease 1 378 MPa.The ultrasonic action even alters the nature of the residual stress,making the residual compressive stress change into a residual tensile stress.