K Nishimoto

Elucidation of melt flows and spatter formation mechanisms during high power laser welding of pure titanium

This study was undertaken to clarify the relationship among the laser-induced plume ejected from the keyhole, the melt flows in the molten pool, and the formation mechanisms of spatters ejected from the molten pool during 10 kW laser welding of a pure titanium plate. High-speed video camera observation results showed that laser-induced plumes occurred at intervals of about 0.5 ms, and that the maximum plume ejection velocity reached 250 m/s. Three-dimensional X-ray transmission in-situ observation of the weld molten pool with tungsten carbide tracers revealed that the melt flowed mainly along the bottom of the molten pool from the keyhole tip to the rear part and then from the rear to the front near the surface of the molten pool at high speeds, while the melt in front of a keyhole flowed upward along the keyhole wall at a velocity of less than 0.6 m/s and then was accelerated to 2.1 m/s at the height of about 2 mm above the keyhole inlet. One-way upward melt flows were continuously piled up at the tip of the elongated melt, resulting in spattering as droplets from the molten pool due to the strong ejection of laser-induced plumes. Spatters were formed from part of a molten metal elongated around the keyhole inlet, and approximately 20 ms was required to form spatters. About 80% of spatters were generated from melts of the front or sides of the keyhole at the speeds of less than 50 mm/s. When the welding speed increased from 50–100 mm/s to 300 mm/s, the ratio and the size of spatters occurring from the rear part of a keyhole increased from 20% to 80% and became smaller than 1 mm.

Deterioration of weldability of long-term aged HP heat-resistant cast steel containing Nb, Mo, and W

Summary The purpose of this study is to clarify the mechanism of weld cracking in the HAZ of long-term aged HP heat-resistant cast steel containing Nb, Mo, and W during repair. The results obtained may be summarised as follows: The results obtained in butt welding cracking tests using as-cast specimens and aged specimens (aged for 247 Msec) suggest that ductility-dip cracking occurs in the HAZ of the aged specimens and that this cracking mainly propagates through the microconstituents precipitated at the dendrite grain boundaries. The results obtained in the elevated temperature high-speed tensile tests of aged specimens and as-cast specimens in the temperature range 673–1473 K suggest that the aged specimens sustain an especially heavy loss of ductility in the temperature range below 773 K. The results obtained in the elevated temperature high-speed tensile tests and small-sized U-groove restraint cracking tests at 773 K using aged specimens and specimens aged at 1323 K suggest that the hot ductility of ...

Effect of grain size on heat affected zone cracking susceptibility.Study of weldability of Inconel 718 cast alloy (2nd Report)

Introduction A previous report has shown that Inconel 718 cast alloy faces the problem of liquation cracking in the heat affected zone (HAZ) and clarifies that HAZ cracking is mainly due to grain boundary liquation associated with constitutional liquation of the Laves cluster. These results suggest that, to improve the HAZ cracking susceptibility, it is basically necessary to find a way of reducing the grain boundary liquation caused by constitutional liquation of the Laves cluster. For this purpose, it is effective to reduce the amount of Laves cluster present at the grain boundaries in the base metal. The extent to which the Laves cluster is formed at the grain boundaries varies depending on the alloy chemistry, granular structure, heat treatment, etc. To improve HAZ cracking in Inconel 718 cast alloy, it is necessary to clarify the effects of these factors on the HAZ cracking susceptibility. Previous research intended to establish the relationship between the HAZ cracking susceptibility and grain size among these factors has been mainly focused on the forged alloy. That is to say, Thompson et al, Lucas et al and Robinson et al have shown that the HAZ cracking susceptibility of Inconel 718 forged alloy can be improved by grain refinement. However, relatively few quantitative research efforts have so far been mounted to determine the relationship between the HAZ cracking susceptibility and grain size of Inconel 718 cast alloy, and many obscure aspects concerning the mechanism of HAZ cracking susceptibility improvement remain to be clarified. To obtain fundamental data relating to improvement of HAZ cracking, Inconel 718 cast alloy specimens with different grain sizes are subjected to the spot varestraint test and the isothermal liquation test, and the effect of the grain size on the HAZ cracking susceptibility is investigated. Based on these results, the mechanism of HAZ

Effects of laser focusing properties on weldability in high-power fiber laser welding of thick high-strength steel plate

The effect of various high-power laser-welding parameters on obtaining deep penetration welds without weld defects has been investigated. However, there are no studies on the effect of laser focusing. In this study, high-power fiber laser welding of a 12-mm-thick high-strength steel plate was performed by using two optics systems with different power density distributions and focus depths (2 or 4 mm) to investigate the effects of laser focusing properties on weldability. Full penetration welds without weld defects were obtained with the 4 mm focus depth optics system at low welding speeds of 25–50 mm/s. High-speed video and X-ray transmission images showed that the behavior of the molten pool on the top surface during laser welding was more stable for the 4 mm system than for the 2 mm system. The keyhole was stable with no large fluctuations, and no bubbles were formed in the keyhole. This result was attributed to keeping the power density within 50–120 kW/mm2 to maintain a stable keyhole shape during the...

Laser pressure welding of aluminium and galvannealed steel

Dissimilar metal joints of galvannealed steel and commercially available pure aluminium (A1050) sheets were produced by changing the laser power and the roller pressure by the laser pressure welding method. In this method, the YAG laser beam was irradiated into a flare groove made by these dissimilar metal sheets. In addition, the laser beam was scanned at various frequencies and patterns through the fθ lens using two-dimensional scanning mirrors. Then the sheets were pressed by the pressure rolls to be joined. The compound layers in the weld interface were observed by optical microscope, and the layer thicknesses were measured. The thicknesses were in the range of 7–20 μm. The mechanical properties of welded joints were evaluated by the tensile shear test and the peel test. In the tensile shear test, the strengths of the joints produced under the most welding conditions were so high that the fracture occurred through the base aluminium sheet. In the peel test of the specimens subjected to the laser beam of 1200–1400 W power under the roller pressure of 2.94 kN, the specimen fracture took place in the base aluminium sheet. Even if the compound layer was thick, high joint strength was obtained. In order to know the reason for such high strength of joints with thick compound layers and the joining mechanism, the compound layer was observed by the HR-TEM. The TEM observation results revealed that the main phase in the compound layer was the solid solution of Al + Zn. Moreover, the intermetallic compound was identified as FeAl, Fe2Al5, Fe4Al13, and Fe2Al5Zn0.4 phase by electron diffraction. The Fe3Zn10 (Γ phase) of Fe–Zn intermetallic compound was confirmed on a Fe base material. It is assumed that the joining areas were heated in a range of 782°C more than 665°C, a melting point of Al, by laser irradiation because the δlk phase aspect was not confirmed. Because the surfaces of A1050 and Zn plated layer were melted thinly, the layer was over 10 μm thicker. The reason for the production of high strength joints with the relatively thick intermetallic compound layer was attributed to the formation of (Al + Zn) phase with finely dispersed intermetallic compounds.

Laser Direct Joining Between Stainless Steel and Amorphous Polyamide Plastic

Joining of the dissimilar materials is necessary and important from a manufacturing viewpoint. Therefore, the authors have developed a new laser direct joining method between metals and plastics. In this research, such joining was applied to join Si3 N4 ceramic and PET engineering plastic, although the metal was replaced by the ceramic. The shear strength of the joints was 3100 N, which was strong enough to elongate PET plates of 2 mm thickness and 30 mm width. It was confirmed that this laser joining process was effective to directly produce a strong dissimilar material joint of a ceramic and an engineering plastic.Copyright

Mechanical properties of laser-pressure-welded joint between dissimilar galvannealed steel and pure aluminium

Dissimilar metal joints of Zn-coated Galvannealed steel (GA steel) and commercially available pure aluminium (A1050) sheets were produced by changing the laser power and the roller pressure by the laser pressure welding method. By this method, the YAG laser beam was irradiated into a flare groove made by these dissimilar metal sheets. In addition, the laser beam was scanned at various frequencies and patterns through the fθ lens using two-dimensional scanning mirrors. Then the sheets were pressed by the pressure rolls to be joined. The compound layers in the weld interface were observed by an optical microscope and the layer thicknesses were measured. The thicknesses ranged from 7 to 20 μm. The mechanical properties of the welded joints were evaluated by the tensile-shear test and peel test. In the tensile-shear test, the strengths of the joints produced under the most welding conditions were so high that the fracture occurred through the base aluminium sheet. In the peel test of the specimens subjected to a laser beam of 1200–1400 W power under roller pressure of 2.94 kN, the specimen fracture took place in the base aluminium sheet. Even if the compound layer was thick, high joint strength was obtained. On the other hand, the specimen fractured in the weld interface at a laser power of 1500 W. The results of X-ray diffraction on the peel test specimen surface identified that the intermetallic compound on the GA steel side was Fe2Al5Zn0.4. Moreover, the aluminium parts adhering to the GA steel side were confirmed. These results suggest that the fracture in the peel test occurred between the compound layer and A1050 and partly in the base aluminium. A micro-Vickers hardness test was performed to examine the hardness distribution in the compound layer. The hardness values near A1050 and GA steel were about 100 and 470 Hv, respectively, which suggests that the compound layer should not necessarily consist of brittle intermetallic compounds. It is therefore concluded that laser pressure welding could produce high strength joints of GA steel and A1050 dissimilar materials.

Laser pressure welding of Zn-coated steel and pure aluminum

Dissimilar metals joints of Zn-coated steel sheets and pure aluminum were produced using the laser pressure welding method by changing the laser power and the roller pressure. In this method, dissimilar metals sheets were set between the twin rolls. These sheets were opened to make up the wedge-shaped-gap. A 2 kW YAG laser beam was irradiated into a wedge-shaped-gap by a f:θ lens and scanned at various frequencies and patterns using two dimensional scanning mirrors. Then the sheets were pressed by the pressure rolls to be joined.The laser pressure welding experiments conducted by changing the laser power and the roller pressure indicated that welding is possible under all conditions. The intermetallic compounds were observed by optical microscope, and the layer thicknesses were measured. When the laser power was 1300 W, the intermetallic compounds layer thickness was about 15 µm. Very small voids were observed in the welded interface at the laser power of 1500 W, and the intermetallic compounds layer thickness was about 7 µm. The tensile shear strength and the peel strength of welded joints were evaluated. In the tensile test, the strengths of the joints yielded in most welded conditions were so high that the fracture occurred in the aluminum parent metal. In the peel test, at the laser power of 1200-1400 W and the roller pressure of 2.94 kN, the specimen fracture occurred in the aluminum parent metal. On the other hand, the specimen fracture occurred in the welded interface when the laser power was 1500 W. In the TEM observation results, the Zn phase was formed in the aluminum parent metal. Moreover, the intermetallic compound was identified as Fe4Al13 phase by electron diffraction. The joint strength was high even if the intermetallic compound layer was thick. The reason for such high strength is attributed to an intermetallic compound dispersed finely in the (Al+Zn) phase.Dissimilar metals joints of Zn-coated steel sheets and pure aluminum were produced using the laser pressure welding method by changing the laser power and the roller pressure. In this method, dissimilar metals sheets were set between the twin rolls. These sheets were opened to make up the wedge-shaped-gap. A 2 kW YAG laser beam was irradiated into a wedge-shaped-gap by a f:θ lens and scanned at various frequencies and patterns using two dimensional scanning mirrors. Then the sheets were pressed by the pressure rolls to be joined.The laser pressure welding experiments conducted by changing the laser power and the roller pressure indicated that welding is possible under all conditions. The intermetallic compounds were observed by optical microscope, and the layer thicknesses were measured. When the laser power was 1300 W, the intermetallic compounds layer thickness was about 15 µm. Very small voids were observed in the welded interface at the laser power of 1500 W, and the intermetallic compounds layer thic...

Transient Liquid Insert Metal Diffusion Bonding of Nickel-Base Superalloys

Transient Liquid Insert Metal Diffusion Bonding (TLIM bonding) consists of three processes, viz., a dissolution process of base metal, an isothermal solidification process and a homogenizing process. These processes are considered theoretically based on results of current researches.

Full penetration welding of thick high tensile strength steel plate with high-power disk laser in low vacuum

ABSTRACT This study was undertaken in order to investigate the effect of reduced ambient pressure from an atmospheric pressure (101 kPa) to 0.1 kPa on one-pass full penetration welding of thick high-tensile strength steel plate of 23 mm thickness. A 16 kW disk laser of 1030 nm in wavelength was employed to weld HT980 grade plates at the speed of 5–25 mm/s. In partial penetration welding, it was revealed that humping phenomena occurred easily. Full penetration welding of the high-tensile strength steel plates could not be achieved at 101 kPa. On the other hand, full penetration welding was obtained at the welding speed of less than 20 mm/s at the pressure of less than 10 kPa. Especially, at 0.1 kPa, and 17 and 20 mm/s, sound weld joints without defects were obtained. According to the observation results of a keyhole inlet and a surface molten pool during welding with a high-speed video camera, the melt in front of a keyhole was smaller and the behaviour of a keyhole and a plume was much more stable at 0.1 kPa than at 101 kPa. Moreover, in the full penetration welding, spattering was suppressed under the proper conditions. Such phenomena became more stable in fast welding. It was revealed in laser welding of thick high-tensile strength steel plates that the formation of narrow I-shaped weld beads by achieving full-penetration welding in low vacuum was essential for the production of sound welds without defect.