This paper presents studies on the microstructure and mechanical properties of AISI 316 L stainless steel and AISI 4340 low-alloy steel joints formed by the Nd:YAG laser welding process.The weld microstructures and heat affected zones(HAZs) were investigated.Austenitic microstructures were observed in all of the samples.The sizes of the HAZs changed when the heat input was varied,and the 316 L sides exhibited a larger HAZ.The cooling rates were calculated by measuring the solidification dendrite arm spacing.It is shown that high cooling rates lead to an austenitic microstructure.Tensile tests were carried out,and the results revealed the tensile properties of both the base metals and the weldments.The hardness test results agreed well with the tensile test results. This paper presents studies on the microstructure and mechanical properties of AISI 316 L stainless steel and AISI 4340 low-alloy steel joints formed by the Nd: YAG laser welding process. Weld microstructures and heat affected zones (HAZs) were investigated. Austenitic microstructures were observed in all of the samples. sizes of the HAZs changed when the heat input was varied, and the 316 L sides exhibited a larger HAZ. cooling rates were calculated by measuring the solidification dendrite arm spacing. It is shown that high cooling rates lead to an austenitic microstructure.Tensile tests were carried out, and the results revealed the tensile properties of both the base metals and the weldments. hardness test results agreed well with the tensile test results.