7.1 Summary
In this thesis, numerical analysis and experimental investigations of Friction Stir Welding on both similar and dissimilar alloys have been carried out. Numerical modelling considering all the physical phenomena is an extremely tough task. However, with the help of suitable assumptions and few other methodologies developed by the previous researchers have made this process simulation possible. The developed FE model has an ability to address issues like temperature distributions in similar and dissimilar FSW. Experimental data related to temperature distributions are important to verify the results obtained from the modelling.
The present simulation study was performed based on the finite element method (FEM) in which distributed moving heat source based three-dimensional modelling has been adopted.
Numerical modelling of FSW process benefits by eliminating the failures occurred in the experimental work. In this present investigation an attempt was made to demonstrate that the numerical analysis and experimental investigations can be a complementary to each other and at the same time numerical analysis can be implemented independently depending on the physical and mechanical properties of the materials to be joined. Although a good amount of published literature available on experimental study of FSW process, but there is a lack of modelling approaches with experimental validations which is applicable for both similar and dissimilar material combinations.
This thesis also focused on the experimental investigations on effect of tool geometries and process parameters on the weld quality of similar and dissimilar joints. FSW of similar materials was conducted on 6mm thick alluminium alloys to find out the optimal tool pin geometry for producing defect free welds. This study was further extended to investigate the same tool geometries is feasible enough to weld thicker alluminium plates. Then the work shifted to FSW of dissimilar materials which consists of joining dissimilar material combinations (i.e. Cu-Al and AA1100 and AA5083). The conclusions made from the above work have been given below.
Transient thermal analysis and experimental investigations of FSW on similar and dissimilar materials
7.2 Conclusions
The important conclusions from the present work can be précised as:
The developed 3-Dimensional FE model for transient thermal analysis on FSW was successfully conducted and compared with the experimental analysis and validated with the published literature. This model stands flexible to analyze FSW for any kind of material.
The 3-Dimensional FE model for friction stir welding of similar alluminium alloys have been developed based on the following assumptions:
a) The material is isotropic
b) All the material properties are temperature dependent
c) Linear Newtonian convection cooling was considered on all the surfaces.
The most influencing operating parameters of FSW (i.e. tool geometries, tool rotational speed, traverse speed, and plunge depth) were rigorously analyzed by using FE simulation, experimental investigation, and multi objective optimization approaches.
The optimum tool shoulder size and pin geometries for obtaining the required temperature of FSW on AA1100 was performed and the obtained numerical results were fairly evaluated and they were matched with the experimental results with in the variation of 8%.
In concern of several practical hurdles in the FSW process (i.e. tool wear, tool breakage and tool manufacturing difficulty), the best tool pin geometries were investigated both numerically and experimentally to achieve the better weld quality and it was observed that the trapezoidal pin tool and tapered pin tool performs well compared to other standard tool geometries.
The overall mechanical response depends on the ratio of tool rpm to tool traverse speed.
A correlation was done between tool rpm and traverse speed for obtaining the maximum ductility as well as load bearing capcity of 6mm thick alluminium alloys. It was found that to achieve friction stir welded joints having maximum ductility as well as load bearing capacity a suitable ratio of tool rpm to tool traverse speed between around 95 to 100 rad/mm should be considered to decide on the weld parameters.
For welding of thick alluminium plates the most significant parameter which decides the weld quality was found to be the dwell time. An optimum dwell time was analyzed by experimental study for achieving the essential temperature on FSW weld quality. The
effect of dwell time was checked by mechanical and microstructural properties and found that at very low and high dwell time the ductility of welded joints significantly reduced.
It was also seen that the tools with trapezoidal pin and tapered cylindrical pin profiles produces the acceptable welds for welding thick alluminium alloys, the same result was observed for lesser thick plates also.
For welding of dissimilar material particularly high strength to low strength alloys, the above said tools doesn’t gives good welds. It was found that for welding of Cu-Al dissimilar combination, the right helix threaded tools with clockwise tool rotations generates sound welds. The material mixing was homogeneous and this was revealed by the mechanical properties study, microstructural investigations and EDX analysis of welded joints.
It was noticed that for welding of dissimilar material combinations, (i.e. AA1100- AA5083 and Cu-Al) clamping the harder material at advancing side with slight tool offset towards softer material side leads to homogeneous material mixing and produced good welds.
The results of the present investigations are very encouraging. The numerical and experimental methodology developed in this work can be gainfully applied in the similar and dissimilar FSW in practical application.
7.3 Scope for future work
The computational procedure developed in the present work enables to acquire the thermal history of friction stir welding process. The numerical methodology of present work can assist to investigate the heat losses during the FSW through both the backing plates and work piece holding devices.
It is also important to understand the material flow phenomena during the tool stirring and traversing actions. Hence, involvement on this research area may help to gain advancement to this process.
The present numerical and experimental analysis can be extended for friction stir welding of high melting point alloys (viz. ferrous based alloys).
Transient thermal analysis and experimental investigations of FSW on similar and dissimilar materials
This process can be extended to hybrid FSW process like plasma arc assisted FSW or Laser assisted welding especially for high melting point materials (e.g. Low carbon steel, stainless steel).