## 4 Conclusions  
This paper investigated the impact of transverse micro solidification cracks in remote laser welding of AA6063-T6 fillet lap joint at different welding speeds with consideration to part-to-part bonding, laser beam oscillation and power modulation. Results show that:  
1. Transverse micro cracks occur along the columnar grain boundaries near fusion lines. Increasing welding speed from 2 m/min to 6 m/min, simultaneously with an increasing laser power for the purpose of good part-to-part gap bridging, did not show any risk of introducing additional transverse micro cracks.
2. Equiaxed grains were observed in the centre of fusion zones while columnar grains were determined near fusion lines. A reduced welding speed facilitates the formation of equiaxed grains due to the greater deposition of beam energy at two turning points of the beam oscillation path, which results in a more uniform temperature distribution within the fusion zone during subsequent solidification.
3. Although transverse micro crack remains at similar level for welding speed between 2 m/min and 6 m/min, joint tensile strength can be improved by 30% by welding at 2 m/min compared to 6 m/min. This is attributed to expansion of fusion zone, measured by throat thickness and refinement of columnar grains near fusion lines.
4. Weld fatigue durability is higher when fatigue failure occurs in weld root instead of fusion zone due to the avoidance of transverse micro cracks. Weld fatigue failure position in this study is greatly influenced by weld shape, represented by weld root angle. A higher weld root angle leads to the transition of fatigue failure from the centre of fusion zone (fusion zone failure mode) to the interface between fusion zone and HAZ (veld root failure mode). The highest weld fatigue durability can be achieved by increasing weld root angle with optimum weld fatigue durability at weld root angle of around 55\({}^{\circ}\).  
In order to harness the full benefits of remote laser welding at higher speeds (>2 m/min) compared to tactile laser welding and achieve significant reduction in number of transverse micro cracks, future research will explore RLW with dual laser beam to achieve a localised  
Figure 15: Representative fatigue fractography of (**a**) the fusion zone failure and (**c**) the weld root failure, with detailed views in (**b**) and (**d**).  
control of the thermal field (both heating and cooling stage) and grain structure refinement in the fusion zone.  
Conceptualization, T.S. and P.F.; methodology, T.S. and P.F.; investigation, T.S., C.L., F.P. and P.F.; writing--original draft preparation, T.S.; writing--review and editing, all authors; supervision, P.F. and D.C.; funding acquisition, P.F. and D.C. All authors have read and agreed to the published version of the manuscript.  
This study was partially supported by the WMG HVM Catapult, APC UK project: Chamaeleon--New lightweight Materials and Processing Technologies for Common Lightweight Architecture of Electric and Hybrid Powertrain Systems, and the Innovate UK IDP15 Project LIBERATE: Lightweight Innovative Battery Enclosures using Recycled Aluminium Technologies.  
Not applicable.  
The data supporting this research article are available upon request to the corresponding authors.  
Tianzhu Sun would like to thank [PERSON] for the enthusiastic support at WMG Materials testing laboratory.  
The authors declare no conflict of interest.