## 3 Results and Discussion
### Crack Distribution  
Figure 7 shows the representative morphology of micro cracks from weld cross-sections. Micro cracks are distributed along grain boundaries of columnar grains near the fusion lines rather than equiaxed grains in the centre of fusion zone. This can be explained as follows: first, a higher localized thermal gradient and resultant thermal strain are generally found in the region adjacent to the boundary of weld zone where columnar grains distribute [23; 37] and this is more obvious at the root of fusion zone because the entire lower part can work as \"heat sink\" for the molten droplet and cools down the fusion zone faster. Next, as the formation of micro crack is associated with the tearing of low melting eutectics along grain boundaries during final stage of solidification [38], equiaxed grains allow a distribution of thermal strain across more grain boundaries and also a reduced thickness of low melting eutectics on grain boundaries [39].  
Figure 8 reports the evolution of cumulative total length of micro cracks and averaged length of individual micro crack on transverse cross-section in each processing window, measured by SEM over three cross-sections of each weld. In addition, the grey symbol represents the averaged level of micro cracks in each process window. Overall, it is evident that micro crack in RLW of AA6063 filler lap joints cannot be eliminated by the optimisation of welding speed. One-way ANOVA with the significance level of 0.05 has showed \(p\)-value of \(\sim\)0.68 and \(\sim\)0.65 for test of welding speed on the total length and individual length. This result confirms that increasing welding speed in the sample will not induce additional risk of transverse micro cracks, in terms of total length and averaged individual length of cracks. Although less equiaxed grains and greater scale of columnar grains have been observed in welds with higher welding speed, which is favourable for the formation of crack, it is likely that the lower residual stress near the fusion zone due to a reduced line  
Figure 6: Distributions of (**a**) averaged grain size (length) of columnar grains and (**b**) total area of equiaxed grains in the weld zone as function of welding parameters, obtained from EBSD grain morphology maps. Note that welds with insufficient part-to-part bonding are not included.  
Figure 7: (**a**) SEM images taken in the weld zone of S2-F50 at \(P_{B}\) = 3.25 kW showing representative view of micro cracks. (**b,c**) show detailed distribution of micro cracks along the grain boundaries of columnar grains.  
energy [40] compensates the possibility of crack formations. In addition, for the process window of S6-F150 and S4-F100, there tends to be a sudden increase in the total/averaged length of crack at the line energy of 47 J/mm and 55 J/mm respectively. This is possibly due to the promotion of part-to-part bonding as illustrated in Figure 3, and resultant heat transfer between two parts that first narrows the region with undercooling, facilitating the formation of columnar grains (Figure 6) and secondly accelerates the cooling stages and development of thermal strain [37], which are favorable for the formation of micro crack.