双相不锈钢脉冲激光焊接焊缝金属显微组织的发展 毕业论文外文翻译.doc
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1、 英文原文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
2、 travel speeds and pulse frequencies. In terms of the solidication 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 deve
3、lop 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 involve
4、d 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, solidicationIntroductionDuplex stainless steels (DSS) are widely used in petro- chemical and chem
5、ical 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 signi
6、cantly 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 cooli
7、ng 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
8、the subject of previous research studies (Ref 8-10). It is shownthat the low heat input and consequently high cooling rates canlead 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 m
9、icrostruc- ture of a DSS alloy is affected by the rapid pulsating nature of the heat source, since consecutive melting and solidication 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
10、 also analyzing the effect of variation in weld travel speed and pulse frequency.Experimental ProcedureBead-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
11、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 noz
12、zle 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
13、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
14、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). (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 0.9
15、0.1 mm. 3.Results and DiscussionFigure 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 c
16、ause the earlier spots to solidify completely 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 rais
17、e the temperature to a degree which can almost disappear the fusion line. The solidication 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 patt
18、ern point of view, two zones can be identiedZone I is the part of the weld metal which is remelted by the next pulse before becoming 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
19、is not affected by the next pulse heat and is solidied mainly from the base metal. In this part, the grain boundaries were relatively ner and more jagged.Fig. 1 Pseudo binary section of Fe-Cr-Ni system at 70% ironTable 1 Chemical composition (in wt.%)ElementCSiMnPSCrNiMoFewt.%0.031.01.420.0230.00522
20、.315.483.34Bal.Between zones I and II, there exists a very narrow band of material which is affected by the heat of the next pulse welding, i.e., the HAZ of zone I in zone II. In Fig. 2(a), this region is marked as 3, and from a solidication pattern point of view, it is a part of zone II. The develo
21、pment of the observed solidica- 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 competing routes exist for the extraction of heat from
22、 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
23、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 solidication orientation. The preferential solidication orien- tation is also affected by the orientation of the grains on which the
24、 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 almos
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