【正文】 ostructures of the welds with various overlap factors as viewed from top direction at (a) 55% (sampleB2) and (b) 73% (sample B4) SEM microstructure various parts of the weld metal and the base metal. (a) Zone I in Fig. 2(a). (b) Zone II in Fig. 2(a). (c) The bulk of the weld metal (away from axial grains) in Fig. 2(b). (d) The wrought base metalsolidi?cation transformations and the corresponding micro structural features as shown in Fig. 3. From the phase diagram in Fig. 1, the microstructure of the initial solidifying weld metal is expected to be fully ferritic and austenite, formed during solid state transformations. One might expect to ?nd a higher percentage of austenite in zone 1 because of its lower cooling rate. However, the percentage of austenite in zones 1 and 2 at 55% overlap was measured as and %, respectively. A closer look at Fig. 3(a) and (b) indicates that formation of austenite is limited to the grain boundaries. It seems that at lower overlaps, the cooling rate is so high that formation of austenite has been mainly limited to higher free energy sites at grain boundaries. In this sense, zone 2 material which has a ?ner solidi?cation structure has provided relatively more preferential sites to form austenite rather than ferrite during the cooling process. However, on increasing the overlap factor to 73%, the cooling rate has decreased so much to form austenite both at the grain boundaries and within the grains, resulting in an austenite content of %, see Fig. 3(c).　　The microhardness in these regions was measured using a 500g load. At 55% overlap, Zone 1 was found to have a higher hardness of 385 177。 10 HV, in parison with zone 2 a hardness of 328 177。 10 HV. This lower hardness can be due to the presence of higher amount of austenite in the microstructureof zone 2. The result of Vickers microhardness survey in B4 specimen with a high overlap factor of 73% showed mostly homogenous distribution of microhardness with an average value of 313 HV.Fig. 4 Microhardness pro?le along the transverse cross section of the weld in different travel speedsThe microhardness measurement was carried out on a number of transverse cross sections of weld specimens with various overlap factors. The result of hardness surveys is shown in Fig. 4. Basically, the hardness at the center of the weld pool is higher than that of the spots near the fusion line. Higher hardness at the weld center can be due to the higher volume fraction of ferrite phase in this region.Conclusion　　The results of pulsed Nd:YAG laser welding DSS are summarized as following:　　In pulsed laser welding, an interesting range of microstruc tures is formed. DSS provided an opportunity to enhance the understanding of how the solidi?cation patterns and the resulting microstructures can develop. When the weld spots do not overlap each other much, two distinct zones of solidi?cation can be identi?ed. One zone is formed under the in?uence of heat extraction directly to the base metal, while the second one is formed under the in?uence of heat extraction to the previous weld spot. Proximity to each of these two heat transfer channels and the presence of any strong preferred growth direction of the grains inherited through epitaxial nucleation determines the solidi?cation pattern at any point. At low overlaps, these two zones appear consecutively in the middle part of the weld. By increasing the overlapping, the solidi?cation pattern governed by heat extraction to the previous weld spot can form continuously without disruptionleading to an array of axial grains in the middle. 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