Study on fracture behavior of AA5083–H111 friction stir welded joint

Mohammed El Sayed El Sayed Allam

Abstract

The fracture behavior of friction stir welded joints of aluminium alloy 5083-H111 was studied in this investigation. Different samples of stir welded joints at variable tool rotational speeds (1660 rpm, 850 rpm and 675 rpm), different traveling speeds (24, 42, 55, 74 and 98 mm/min) and different tool pin shapes (cylindrical, tapered and threaded) were studied to investigate the effect of process parameters on the joint strength/load bearing capacity. A series of tensile tests using MTS machine, scanning electron microscope (SEM) analysis and optical microscope analysis were conducted to study the effect of varying the rotational speed, traveling speed (v) and tool shape on the strength and microstructure of the welded joint. The tensile tests were used to study the mechanical behavior of the AA5083 base metal and the welded samples. The mechanical properties of AA5083 (yield strength, ultimate strength/fracture strength, modulus of elasticity and percent elongation) were measured. This was followed by measuring the fracture strength of the welded joint. Scanning electron and optical microscopy were conducted to study the effect of friction stir welding (FSW) process on the grain size in the different welding zones (parent, HAZ, TMAZ and nugget). A thermal camera was used during the welding process to measure the temperature of the welding zone.

Thermo-mechanical model has been developed in this work to predict some of the FSW parameters. The model was based on the energy balance. The energy generated due to friction between the tool surfaces and the metal and plastic deformation is balanced with heat energy due to rise in metal temperature.

In general, the results illustrated that tool profile has great effect on the strength of the welded joint. Threaded pin tool resulted in the highest strength among the used tools. A strength of 85% of the base metal strength was achieved. The tensile tests performed indicate that the tool rotation speed and traveling speed have also influenced the strength of the joint. The highest strength was achieved at tool rotational speed of 675 rpm if compared to the other rotational speeds regardless of the traveling speeds. As the tool rotational speed increases, the strength of the joint reduces. Tool traveling speed was also observed to be a major factor influencing the strength of the joint. Attempts were done to obtain the suitable traveling speed that should be implemented to achieve the desired strength. It has been noticed that the joint strength increases with traveling speed up to a certain speed (depending on the rotational speed and pin profile) and then reduces with increasing the traveling speed.

Scanning electron microscopy and optical microscopy investigations showed that the grains inside the welding zone were refined and equiaxed resulting in higher hardness inside the nugget.

The results of the developed thermo-mechanical model overestimate the welding temperature. This was expected as the heat loss (due to conduction, convection and radiation) was not considered in the model. Further, the effect of the generated temperature on the plastic deformation energy while welding was also not considered. Therefore, a scaling factor was introduced in the model to consider the reduction in the plastic energy due to the increase in temperature. A scaling factor of 0.22 was found to predict the measured temperature.