Why is tensile strength more important in joint strength analysis of Friction Stir Welding (FSW)? Why not other mechanical properties, such as yield strength, ductility, etc., are considered in the analysis? Is there any connection that TS is more important than other properties?
the yield strength is the minimum stress under which a material deforms permanently. in fsw as this is done on aluminium alloys or soft materials, where stress is very less and the material is ductile, therefore yield strength mean less and are not the properties which matter.
In all metal forming processes, what is more important is the UTS and not the YS. This is beacuse, UTS brings out the load that is required to plastically deform the material at a given temperature. this is quite gigh when compared to the load that is required for yielding. For ductile materias, typically the UTS to YS ratio will vary from 1.5 to 2.0. This essentially means that the load you require on the machine for forming is 1.5 to 2.0 times more than that is required for making a discernible plastic deformation.
Therefore, in forging, extrusion, FSW, LFW , RFWete., etc, we use UTS and not YS.
The answer could be found in the text books on design of machine elements. When a material reaches permanent deformation, it is said to have failed. This point i.e. yield point is more important to design machine structures. Factor of safety is decided taking into account yield point. Since brittle materials do not show up considerable plastic deformation, ultimate point is considered for design. Otherwise, yield point is taken into account for all design including welded joints. Ductility is also important. What if a boiler blasts instantaneously? The welded joint should possess adequate ductility.
Tensile strength gives the joint strength and joint efficiency. However, yield strength and ductility are also important for weld joint. Optimum properties are required for any joint at corresponding process parameters.
Tensile strength of weld joint gives you joint strength/joint efficiency of weld. However, yield strength and % elongation and other mechanical properties are also important of weld of engineering component. If your weld joint strength is not good then why you will determine other mechanical properties. In this sense tensile strength is more important of weld than other properties, otherwise every properties are important of weld.
Because as a result of increasing/decreasing the hardness of the metal (different crystal structure), with the presence of axial load, a separation of the tensile metal occurs either in the original metal or in the weld area.
If the breakdown occurs in the original metal, this indicates an increase in hardness in the welding area, but if the fracture occurs in the welding area, this indicates the presence of defects from welding defects using various tests being examined.
Therefore, we use the maximum tensile strength of the welded sample to determine the efficiency of the joint.
There is no standard for elongation and yield strength of the cross-weld joint for any welding method. This is due to the different material properties of the different weld zones, i.e., BM, HAZ, and weld zone. Each zone has a different stress state and deformation behaviour due to the different properties.
For example, if the HAZ is soft (has low hardness and strength), then a localised deformation will occur in the HAZ which causes a premature failure. The sample will failed while no remarkable deformation occured in the weld metal. Consequently, the measured yield strength and elongation percent are not reliable or accurate.
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