Feb_March_AMP_Digital

A D V A N C E D M A T E R I A L S & P R O C E S S E S | F E B R U A R Y / M A R C H 2 0 2 0 2 1 (b) (a) welding layer had a preheating effect on the next layer, which caused the HAZ to become wider. The base-metal microstructure (Fig. 3a) consists mainly of a large amount of polygonal ferrite grains (white) and a small amount of uniform- ly distributed pearlite (black). The weld zone microstructure (Fig. 3b) consists of coarse columnar crystals distributed in a direction parallel with the thermal diffusion direction and perpendicular to the fusion line due to heterogeneous nucleation. Columnar crystals grew from the base metal grains to the inside molten pool and are consistent with the orientation of maximum temperature gradient owing to the minimal require- ment for nucleation in that direction. The crystallization mode is termed epi- taxial solidification. Lath ferrite is dis- tributed along the grain boundaries, acicular ferrite and side lath-plate fer- rite grew intragranularly, and the grains welding wire was used as filler met- al. Dimensions of the groove root were 3 mm high and 1.5 mm wide with a 4-degree single angle of the groove. An IPG YLS-6000 laser machine (IPG Photonics Corp.) was used with a 6-kW maximum output power and the laser head fixed on an ABB 6-axis link- age manipulator robot. The wire feeder was a Fronius MIG welding machine and argon shielding gas was used during the welding process. A 4-mm OD and 1.25-mm ID wire-feed pipe was special- ly made to ensure that the welding wire could be successfully delivered to the narrow-gap groove. The angles of the wire-feeding tube and shield-gas tube were set at 45° and 60°, respectively, with respect to the horizontal line. Ad- ditionally, because the wire exiting from the welding machine was curved, a wire-feeding module with a three-roller wire-straightening device was applied to improve welding stability, as shown in Fig. 1. Table 1 lists the main parame- ters for laser welding with filler wire. RESULTS OF METALLO- GRAPHIC EXAMINATION Specimens of the weld joint were polished and etched using a 5% HNO 3 + C 2 H 5 OH solution for examination by optical microscope. Mechanical testing (tensile and impact) was conducted in accordance with European standards BS EN 895, “Destructive tests on welds in metallic materials. Transverse tensile test” and BS EN 875, “Destructive tests on welds in metallic materials. Impact tests. Test specimen location, notch ori- entation and examination.” Tensile tests were conducted on two specimens in a universal test machine. Charpy V-notch Impact tests with the notch located at the weld centerline were conducted at −20 ° C and −40 ° C. Figure 2 shows a macrograph of the cross section of the laser weld- ed joint. Three different zones are ob- served including the weld zone (WZ), heat-affected zone (HAZ), and base metal (BM). The joint has a narrow HAZ (1 to 2 mm wide), which means that la- ser welding with filler wire had low heat input during welding, while at the over- lap zone of weld bead, the previous contain a small amount of pearlite (dark spots). In the crystallization process, lath ferrite precipitated along original austenite grain boundary, which is also called primary ferrite. However, side lath-plate ferrite grew from the side of austenite grain to the inside of the crys- tal. Acicular ferrite and pearlite were obtained during subsequent cooling. The temperature at the center of the weld is the highest and becomes lower the farther away from the weld during welding. Due to experiencing different thermal cycles, the HAZ can be divided into three different zones: over- heated zone (OHZ), recrystallization zone (RCZ), and partial recrystallization zone (PRCZ), as shown in Fig. 4(a-d). The temperature of the recrystal- lization zone usually ranged from A 3 to 1150 ° C. Figure 4(c) reveals that grains in the recrystallization zone are very fine with a microstructure consisting of a TABLE 1 — PARAMETERS FOR LASER WELDING WITH FILLER WIRE Welding pass Laser power, kW Welding speed, mm/min Defocusing length, mm Wire feed rate, m/ min Gas flow, L/ min 1 4.3 0.36 10 2.0 10 2 4.8 0.30 15 4.0 3 4.8 0.30 15 4.0 4 4.8 0.30 15 4.0 5 5.5 0.30 30 5.0 Fig. 2 — Cross section macrostructure of SMA490BW plate ultranarrow-gap weld joined using laser welding with filler wire. Fig. 3 — Microstructure of (a) base metal and (b) weld zone.