A method of quality assurance of an additive manufacturing build process. An additive manufacturing system is operated to perform a build process by building a part on a build platform, the part being built by forming a series of layers of metallic material on the build platform. The metallic material melts and solidifies during the build process thereby bonding the part to the build platform and creating thermally induced stress in the part which tends to distort the build platform. During the build process, a parameter is measured which is related to the thermally induced stress in the part to generate measurement data. The measurement data is stored and analysed to determine whether a defect has formed during the build process. A warning is generated if the analysis of the stored measurement data concludes that a defect has formed during the build process. The warning includes an indication of a position in the part.