5.5 ± 0.9-μm-thick pure Ni coating layer on the steel with a defect-free interface. Figure 1B shows a SEM micrograph of the cross section of the nickel-coated steel. The white layer on top of the steel is the Ni coating layer. The coating was of uniform thickness with a void-free interface. Energy-dispersive X-ray spectrometer (EDS) analysis of the electrodeposited layer on the steel showed a pure Ni coating layer. After the electroplating process, the prebent steel sheet was clamped against the magnesium sheet to make a singleflare bevel lap joint as shown in Fig. 2A. The filler metal was cut into pieces and preset on the workpiece at the weld interface with some flux before heating and brazing by the laser beam. An integrated Panasonic 6-axis robot and Nuvonyx diode laser system with a maximum power of 4.0 kW and a 0.5- × 12-mm rectangular laser beam intensity profile at the focal point was used for laser brazing. This energy distribution is more suitable for brazing processes compared with the nonuniform Gaussian-distributed circular beams generated by CO2 and Nd:YAG lasers (Ref. 17). The beam was focused on top of the filler metal. In order to limit oxidation, helium shielding gas was provided in front of the molten pool with a flow rate of 30 L/min from a 6-mm-diameter soft copper feeding tube. Laser brazing was performed using a range of laser powers, travel speeds, and beam offset positions. After laser brazing, transverse cross sections of the brazed specimens were cut and mounted in epoxy resin. The samples were then mechanically polished using 300, 600, 800, 1000, and 1200 grades of SiC grinding papers followed by polishing using a 1-μm diamond suspension. The polished specimens were etched to reveal the microstructure of the braze metal and AZ31B base material. The etchant was comprised of 20 mL acetic acid, 3 g picric acid, 50 mL ethanol, and 20 mL water (Ref. 18). Macro- and microstructures of the etched joints were examined using an optical metallographic microscope. The microstructure and composition of different zones of the joint cross section were determined using a JEOL JSM-6460 scanning electron microscope (SEM) and EDS. Phase characterization of the phases formed in the steel-fusion zone interface and on the fracture surfaces was carried out using X-ray diffraction (XRD) phase analysis in a Rigaku AFC-8 diffractometer with Cu target, 50 kV acceleration voltage, Fig. 3 — A laser-brazed Ni electroplated steel/AZ31B joint made using 8 mm/s travel speed and 2.2-kW laser beam power: A — Top bead; B — transverse section of the joint. and 40 mA current. A transmission electron microscope (TEM) foil of the steel-fusion zone interfacial region was prepared using a focused ion beam (FIB) and in-situ lift out technique. After attaching the TEM foil to a copper grid, final thinning was performed on the sample at an acceleration voltage of 30 kV, followed by 10 kV, and 1 kV for the final polishing step to get a 100-nm-thick TEM sample. The TEM studies were performed with a JEOL 2010F TEM WELDING JOURNAL 3-s WELDING RESEARCH Fig. 2 — A — Schematic of the laser brazing system used for joining AZ31 Mg and Ni electro-plated steel sheets in the single-flare bevel lap joint configuration showing the position of two thermocouples used for temperature measurements; B — schematic of the 5-mm-wide tensile shear test specimen. Table 3 — Composition of Ni Electroplating Solution and Electroplating Parameters Plating Solution Composition (g/L) Electrodeposition Parameters NiSO4•6H2O 263 Cathode current density 45–120 mA/cm2 Na2SO4 215 Time 5–20 min H3BO3 31 pH 3 Temperature 25°C Anode Graphite (8 cm2) Cathode Carbon Steel (6 cm2) A A B B
Welding Journal | January 2013
To see the actual publication please follow the link above