Development of Weld Metal Microstructures in Pulsed Laser Welding of Duplex Stainless Steel F. Mirakhorli, F. Malek Ghaini, and M.J. Torkamany (Submitted October 30, 2011) 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 solidification pattern, the weld microstructure is shown to be composed of two distinct zones. The presence of two competing 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 ferrite-to-austenite transformation can be limited only to the grain boundaries. Keywords duplex stainless steel, microstructure, pulsed laser welding, solidification 1. Introduction Duplex stainless steels (DSS) are widely used in petro- chemical and chemical processings because of the combination of corrosion resistance and advantageous mechanical proper- ties. The wrought alloys microstructure at room temperature is composed of austenite and ferrite phases (Ref 1, 2). However, the microstructure resulting from a fusion welding process can be significantly different because of the cooling rates involved (Ref 3-5). Figure 1 depicts a typical DSS alloy that would solidify completely into ferrite and then, while cooling through solid state transformation, it partially transforms into austenite (Ref 1, 2). Considering the comparatively 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 2-7). Welding DSS alloys with continuous power laser has been the subject of previous research studies (Ref 8-10). It is shown 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 solidification of weld spots would occur (Ref 11-13). 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. 2. Experimental Procedure Bead-on-plate laser welding was applied on 2-mm-thick commercial SAF 2205 DSS plate. The base metal chemical composition is given in Table 1. Laser welding machine was IQL-10, with a pulsed Nd:YAG laser connected to a computer controlled working table and with a maximum mean laser power of 400 W. The available range for the laser parameters were 1-1000 Hz for pulse frequency, 0-40 J for pulse energy, and 0.2-20 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 (0.7 K 2 S 2 O 5 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 ƒ was calculated by the Eq 1 (Ref 12, 13). O f ¼ 1  m=f D þ mT    100 ðEq 1Þ 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 0.9 ± 0.1 mm. F. Mirakhorli and F. Malek Ghaini, Department of Materials Engineering, Tarbiat Modares University, Tehran, Iran; and M.J. Torkamany , Iranian National Centre for Laser Science and Technology (INLC), PO Box 14665-576, Tehran, Iran. Contact e-mails: mirakhorli.f@gmail.com. farshidmalek@yahoo.com and mjtorkamany@ inlc.ir. JMEPEG (2012) 21:2173–2176 ÓASM International DOI: 10.1007/s11665-012-0141-3 1059-9495/$19.00 Journal of Materials Engineering and Performance Volume 21(10) October 2012—2173