【正文】 1 英文原文 Development of Weld Metal Microstructures in Pulsed Laser Welding of Duplex Stainless Steel F. Mirakhorli, F. Malek Ghaini, and . Torkamany (Submitted October 30, 2021) The microstructure of the weld metal of a duplex stainless steel made with Nd:YAG pulsed laser is investigated at different travel speeds and pulse frequencies. In terms of the solidi?cation pattern, the weld microstructure is shown to be posed of two distinct zones. The presence of two peting heat transfer channels to the relatively cooler base metal and the relatively hotter previous weld spot is proposed to develop two zones. At high overlapping factors, an array of continuous axial grains at the weld centerline is formed. At low overlapping factors, in the zone of higher cooling rate, a higher percentage of ferrite is transformed to austenite. This is shown to be because with extreme cooling rates involved in pulsed laser welding with low overlapping, the ferritetoaustenite transformation can be limited only to the grain boundaries. Keywords duplex stainless steel, microstructure, pulsed laser welding, solidi?cation Introduction Duplex stainless steels (DSS) are widely used in petro chemical and chemical processings because of the bination of corrosion resistance and advantageous mechanical proper ties. The wrought alloys microstructure at room temperature is posed of austenite and ferrite phases (Ref 1, 2). However, the microstructure resulting from a fusion welding process can be signi?cantly different because of the cooling rates involved (Ref 35). Figure 1 depicts a typical DSS alloy that would solidify pletely into ferrite and then, while cooling through solid state transformation, it partially transforms into austenite (Ref 1, 2). Considering the paratively higher cooling rates involved in welding processes, the weld metal and the HAZ microstructure could contain higher amounts of ferrite phase than the base metal. This also can affect the mechanical and corrosion resistance properties of DSS welds (Ref 27). Welding DSS alloys with continuous power laser has been the subject of previous research studies (Ref 810). It is shown 2 that the low heat input and consequently high cooling rates can lead to the formation of higher a/c ratio. On the other hand, pulsed laser can provide further controls on power and heat input. However, there can be questions on how the microstruc ture of a DSS alloy is affected by the rapid pulsating nature of the heat source, since consecutive melting and solidi?cation of weld spots would occur (Ref 1113). In the present study, the focus is on the evaluation of the microstructure in different regions in the weld metal of a DSS and also analyzing the effect of variation in weld travel speed and pulse frequency. Experimental Procedure Beadonplate laser welding was applied on 2mmthick mercial SAF 2205 DSS plate. The base metal chemical position is given in Table 1. Laser welding machine was IQL10, with a pulsed Nd:YAG laser connected to a puter controlled working table and with a maximum mean laser power of 400 W. The available range for the laser parameters were 11000 Hz for pulse frequency, 040 J for pulse energy, and ms for pulse duration. During laser welding, argon shielding gas with a coaxial nozzle was used to protect the heated surface from oxidation. Work pieces were polished and cleaned with acetone to be prepared for welding. The welded samples were observed in cross sections from three different perpendicular directions (top, transverse, and longitudinal). The etchant was Beraha ( K2S2O5 20 mL HCl in 100 mL solution). The wrought base metal consisted of 55% ferrite and 45% austenite as measured by image analysis, with an average hardness of 280 HV as measured by a 500 g load. After establishing the range of parameters to achieve an acceptable weld appearance, the experiments were carried out with varying travel speeds and pulse frequencies, as shown in Table 2. Overlap factor ? was calculated by the Eq 1 (Ref 12, 13). 100)1( ???? vTD fvOf (Eq 1) where T is the pulse duration, v is the welding speed, f is the laser frequency, and D refers to the laser spot size on the work piece measured as 177。 mm. and Discussion 3 Figure 2 shows the top view of welds at a low and high overlapping. As observed from the ?gure, the weld spots are clearly distinguishable from each other specially at lower overlapping. When the time (or distance) between two pulses increases, high cooling rates can cause the earlier spots to solidify pletely before coincident of the next pulse (Ref 11). On the other hand, when the time (or distance) between two pulses decreases, the former spot temperature can still be high enough to the extent that semisolid condition is dominant and the next pulse can raise the temperature to a degree which can almost disappear the fusion line. The solidi?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.%