【正文】 cation pattern of the weld metal was found to vary with the travel speed and/ or frequency because of variations of the overlap factor. With a low overlapping factor, as shown in Fig. 2(a), from a solidi ?cation pattern point of view, two zones can be identi?ed Zone I is the part of the weld metal which is remelted by the next pulse before being cooled thoroughly. In this zone, the grains nucleate on the previous spot epitaxially and grow toward the center. Zone II refers to a single pulse microstructure which is not affected by the next pulse heat and is solidi?ed mainly from the base metal. In this part, the grain boundaries were relatively ?ner and more jagged. Fig. 1 Pseudo binary sec tion of FeCrNi system at 70% iron 4 Table 1 Chemical position (in wt.%) Element C Si Mn P S Cr Ni Mo Fe w t.% Bal. Between zones I and II, there exists a very narrow band of material which is affected by the heat of the next pulse welding, ., the HAZ of zone I in zone II. In Fig. 2(a), this region is marked as 3, and from a solidi?cation pattern point of view, it is a part of zone II. The development of the observed solidi?ca tion patterns is because in pulse laser welding, when the weld spots are not too close to each other, the previous weld spot is relatively cool when the next pulse strikes, and therefore, effectively two different peting routes exist for the extraction of heat from any point in the molten weld pool. The ?rst route is directly through the side walls (fusion line with the base metal), and the second route is through the previous weld spot (fusion line between consecutive weld spots). The temperature distribution ?eld of the weld pool is affected by both of these two heat sinks. Proximity of any point in the weld pool to each of these two routes of heat extraction is one of the factors determining the dominant cooling route and solidi?cation orientation. The preferential solidi?cation orien tation is also affected by the orientation of the grains on which the weld metal grows epitaxially. Zone 1 shown in Fig. 2(a) is mainly cooled through heat transfer to the previous weld spot (and then to the base metal), but zone 2 is cooled through transferring heat from side walls to the base metal. Also, when weld spots overlap each other extensively, zone 2 almost disappears and bees only limited to a narrow band just next to the two side walls. In such condition (high overlapping), zone 1 solidi?cation pattern dominates most of the weld metal central part, and they can effectively grow on each other epitaxially without being disturbed by zone 1 grains ing in between. Here, the grains in the consecutive zone 1 s form a clear preferred orientation, and the axial solidi?cation pattern is formed as shown in Fig. 2(b). The authors have experienced pulsed laser welding of various alloys including carbon steel, aluminum alloys, and titanium. However, it was in the case of DSS that such progress in understanding of the process of weld microstructures development became possible. It would be interesting to study the weld microstructures in other alloys in the light of the knowledge gained. As stated earlier, solidi? cation in zone 2 is dominated by heat extraction to the side walls, but solidi?cation in zone 1 is dominated by heat extraction to the previous weld spot. The larger grain sizes in zone 1 are due to a higher effective preheat temperature of the material the heat of which escapes to ., the metal which itself has been molten just a little earlier. However, the sidewalls are expected to have a lower temperature, as a steeper temperature gradient occurs for solidi?cation of zone 2. Thus, the cooling rate in zone 1 is expected to be paratively lower resulting in a coarser microstructure. Now, our attention turns to the post 5 Fig. 2 Mic rostruc tures of the w elds w ith various overlap factors as view ed from top direc tion at (a) 55% (sample B2) and (b) 73% (sample B4) SEM mic rostruc ture various parts of the w eld metal and the base metal. (a) Zone I in Fig. 2(a). (b) Zone II in Fig. 2(a). (c) The bulk of the w eld metal (aw ay from axial grains) in Fig. 2(b). (d) The wrought base metal 6 solidi?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. This lower hardness can be due to the presence of higher amount of austenite in the microstructure of 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 avera