Strength control of welded joints is used in many industries. X-ray microtomography as a method of non-destructive testing allows not only to detect the presence of defects, but also to classify them and assess the size of cracks and non-crack-like defects (pores). This article discusses the non-destructive testing of welded joints of aluminum alloy 6061 T6 using computer micrototomography and a mathematical model of the thermal welding process implemented in ANSYS Workbench. Experimental results of the X-ray microtomograph are presented, and the size of defects in this sample is estimated. The mathematical model allowed us to obtain the thermal histories at which the faults were formed.