Liquidation cracking is induced by the re-melting of low melting alloy compounds and formation of brittle films on grain boundaries in the heat affected zone (HAZ). By comparison, solidification cracking occurs in the fusion zone when the molten material cools down just below the solidus temperature. In this paper, only solidification crack will be investigated since liquation cracks have rarely been observed due to the low heat input and narrow HAZ of laser welds [19]. As discussed by [PERSON] et al. [20], solidification cracks in aluminium alloys are induced due to the presence of high thermal strain which exceeds the critical strength of semisolid material in the mushy zone and propagate along the continuous grain boundaries. [PERSON] [21] reported that the control of solidification cracking is challenging in the case of 6 xxx aluminium alloys because the addition of Si content extends the critical solidification temperature range during rapid cooling period after welding. As reported in our previous study [22], solidification crack can be divided into centreline macro crack and transverse micro crack according to spatial distribution and scale. Centreline macro crack generally propagates along the centreline of weld seam at millimetre-scale and is visible on the external surface. In comparison, transverse micro crack is defined as internal or sub-surface crack at micron-scale and is only detectable by non-destructive techniques or observation of cross-section after metallographic preparation. In general, transverse micro cracks are found near the boundary between fusion zone and HAZ where local grains exhibit columnar structures. This is attributed to: (i) higher level of localised strains near the boundary of fusion zone [23]; and, (ii) higher crack susceptibility of columnar grains compared to equiaxed grains in the centre of fusion zone due to the increased grain size and reduced number of grain boundaries [24]. Several technological and metallurgical approaches have been proposed to prevent solidification cracks in laser welding of 6 xxx aluminium alloys. These include: (i) modification of chemical composition through the use of filler wires [25; 26]; (ii) refinement of grain structure by means of magnetic stirring, laser beam oscillation and addition of alloying elements [19; 24]; and, (iii) temporal modulation of thermal history provided by beam shaping and beam oscillation [27]. The effect and characterisation of transverse micro crack on weld performance during RLW process has been rarely reported. [PERSON] et al. [28] reported the presence of transverse micro cracks in RLW of AA6082 with fillet lap joints and observed that cracksformed initially at the rear area beside the molten pool and propagated subsequently to the middle and the side of weld seam. Furthermore, they concluded that the molten pool geometry has a strong influence on crack formation and beam oscillation can therefore, be an effective solution for crack mitigation which generates an elliptical molten pool with a high curvature at the tip of the fusion zone. [PERSON] et al. [29] further discussed the formation of transverse micro crack by thermomechanical analysis and related the hot cracking susceptibility to the local maximum strain rate in the vicinity of weld pool. As proposed in our previous research [22], fillet lap joint configuration is a preferred joint design not only because of the effective control of centreline macro crack but also due to the pronounced capability of optical seam tracking and consequently, the opportunity for in-process measurement of the part-to-part gap [12; 14]. [PERSON] et al. [12] developed a closed-loop part-to-part bonding architecture to adaptively select welding process parameters as a function of the part-to-part gap and achieve the desired level of bonding between parts being welded.