چكيده به لاتين
Ferrite/martensite dual-phase (DP) steels are a group of advanced high strength steels that are being used in many industries, e.g., automobile industry, due to their good formability as well as a high ratio of strength to weight. Friction stir welding (FSW) is a solid-state joining technique having advantages over fusion welding techniques such as lack of liquid phase and its related pollution problems as well as low heat input. These advantages have made FSW as an applicable joining method for DP steel parts. In the present study, DP700 steel was jointed using FSW method. Welding experiments were conducted using a tungsten carbide tool, and the parameters of rotational speed, transverse speed, shoulder diameter and the material of backing plate were investigated. Microstructural investigation of the welded joints was studied using optical microscopy, scanning electron microscopy equipped with EBSD, transmission electron microscopy, and scanning transmission electron microscopy. Transverse tensile tests and microhardness measurements were used to characterize the mechanical properties of the obtained welds.
It was found that while using a steel backing plate, reduction of heat input rate can be considered as an effective solution to decrease the softening of the heat-affected zone and, in turn, enhance the mechanical properties of the weld. Decreasing rotation speed, increasing transverse speed, and reduction of shoulder diameter, as they lead to a sound weld, can decrease the heat input rate. Under the condition of using steel backing plate, in the best condition ultimate strength of 698 MPa and total elongation of 16.2% was achieved which is equivalent to 96.5% of joint efficiency. In this condition, middle and lower parts of the stir zone experience temperatures between Ac1 and Ac3, while the temperature of upper part of the stir zone exceeds Ac3. Thermomechanically affected zone (TMAZ) was not observed under conditions of steel backing plate. In fact, the occurrence of allotropic phase transformation destroys the signs of this region.
It also was found that using copper as the backing plate decreases the temperature of the stir zone and it also develops a TMAZ in the macrostructure of the weld. Development of a TMAZ, as well as an increased cooling cycle, compensate for the hardness reduction of the heat-affected zone. In fact, the grain refinement of the tempered region, with a grain size of about 500 nm, increase the hardness to those higher than the base metal. In this condition the ultimate strength of 711 MPa and total elongation of 19% were achieved, which led to a joint efficiency of 98.3%.
