The melting point of pure aluminum is about 40% of the melting point of titanium. Thus, if one want to butt joint these two metals by FSW, the tool axis must be given an offset towards the low melting point metal (in this case aluminum) from the joint interface. If offset is not given, the extremely high amount of heat generation will melt off the aluminum metal. On the other hand, if FSW parameters are controlled to reduce heat generation then tool pin failure will be the result.
When tool offset is given, the stirring will be mostly in the low temperature material and the tool pin only scrapes off the faying surface of the high temperature material (which is at the advancing side) to form a type of diffusion bonding.
The melting point of pure aluminum is about 40% of the melting point of titanium. Thus, if one want to butt joint these two metals by FSW, the tool axis must be given an offset towards the low melting point metal (in this case aluminum) from the joint interface. If offset is not given, the extremely high amount of heat generation will melt off the aluminum metal. On the other hand, if FSW parameters are controlled to reduce heat generation then tool pin failure will be the result.
When tool offset is given, the stirring will be mostly in the low temperature material and the tool pin only scrapes off the faying surface of the high temperature material (which is at the advancing side) to form a type of diffusion bonding.
when dissimilar metal welding performed with FSW,the tool offset ( along with direction of rotation)plays a vital role in determining advancing retarding side heat generation and distribution and hence on mechanical properties of joint.
The tool supplies heat and the pin apart from supplying heat also stirs the material At a given temperature materials with different strength, melting point and thermal conductivity requires different flow stress in order to move and be mixed. At zero offset materials on both side of the faying line are mixed in equal quantity. During dissimilar materials welding Al-Ti (e.g. Al- 660oC temperature relatively soft and highly thermally conductive Vs Ti- 1670oC very strong and poor in thermal conductivity in comparison to Al) when appropriate off is chosen in the Al side more of the Al is mixed with less of Ti. Thus, the inequality of flow stress is compensated by the difference in relative mass of materials being mixed.
Please refer to Chapter 8 of the text book "Friction Stir Welding and Processing: Science and Engineering" by Mishra et al. for a detailed discussion on dissimilar metal welding using FSW technique. It has answers to questions such as the role of offset and position of the workpieces with respect to each other.
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