Seiji Katayama

Solidification microstructure of laser welded stainless steels

The solidification microstructures of pulsed Nd:YAG laser or CW CO2 laser welds were studied in commercial SUS 310S and 304 (corresponding to AISI Types 310S and 304) austenitic stainless steels and in other several austenitic and duplex stainless steels. In the pulsed laser welding of Type 310S, neither grain growth nor recrystallization grain-refining was observed in the heat-affected zones (HAZ), and planar and cellular growth occurred epitaxially in weld metals from adjacent, incompletely-melted grains in the HAZ. According to the data relating a primary dendrite arm spacing to the cooling rate in Type 310S weld metals, the average local cooling rates within the single laser shot welds were extrapolated to be so rapid as to range from 5×104 to 5×106°C/s depending mainly on the pulse energy. In the case of Type 304, the pulsed laser weld metals were almost fully austenitic in contrast to GTA or CO2 laser weld metals containing about 5% residual delta(δ)-ferrite. It was significantly confirmed that almost fully austenitic structure of Type 304 could be produced with extremely high cooling rates obtained by CW CO2 laser welding process with low heat inputs at high traverse speeds. From the microstructural observation of several materials, it was further revealed that the solidification microstructures of pulsed laser welds were not consistent with the prediction from the Schaeffler diagram. In other words, the steels containing the calculated contents of less than 8% δ-ferrite or more than 30% δ-ferrite exhibited fully austenitic or fully ferritic microstructure, so that the compositional region indicating a duplex microstructure was very narrow in the case of pulsed laser weld metals. The disappearance of ferrite in the laser-welded microstructure was interpreted in terms of the change in the solidification process due to extremely high cooling rates accompanied with large undercooling. In addition, the change from duplex to fully ferritic structure was attributed to the suppression of the solid-state, ferrite-to-austenite transformation.The solidification microstructures of pulsed Nd:YAG laser or CW CO2 laser welds were studied in commercial SUS 310S and 304 (corresponding to AISI Types 310S and 304) austenitic stainless steels and in other several austenitic and duplex stainless steels. In the pulsed laser welding of Type 310S, neither grain growth nor recrystallization grain-refining was observed in the heat-affected zones (HAZ), and planar and cellular growth occurred epitaxially in weld metals from adjacent, incompletely-melted grains in the HAZ. According to the data relating a primary dendrite arm spacing to the cooling rate in Type 310S weld metals, the average local cooling rates within the single laser shot welds were extrapolated to be so rapid as to range from 5×104 to 5×106°C/s depending mainly on the pulse energy. In the case of Type 304, the pulsed laser weld metals were almost fully austenitic in contrast to GTA or CO2 laser weld metals containing about 5% residual delta(δ)-ferrite. It was significantly confirmed that almo...

Surface hardening of titanium by laser nitriding

Nitriding and related hardening of titanium(Ti) and its alloys were investigated as anew method of laser surface treatment by using a pulsed Nd:YAG laser of 1.06 μm wavelength with 3.6 ms pulse width. When a laser beam was irradiated on Ti plate in nitrogen(N) atmosphere, it was found that a flat and smooth surface covered with TiN nitride layer could be produced by selecting a good combination of laser irradiation conditions such as beam energy, power density and the distance from the lens focal point to the material surface. Compared with Ti base metal having a Vickers hardness(Hv) of about 200, the hardness of the laser-nitriding surface could exceed about 700 Hv at a single pulse shot and increase up to 1700 Hv at overlap of twenty shots, and besides the fusion zone could be hardened up to about 400 to 700 Hv. These hardening mechanisms were interpreted in terms of the formation of TiN nitride layer on the surface and the enrichment of nitrogen in the fusion zone. Nitriding and hardening phenomena were observed as well in the case of titanium alloys such as Ti-6Al-4V and Ti-6Al-6V-2Sn. Moreover, it was confirmed that nitriding and hardening of Ti and its alloys were feasible by the use of a CW CO2 laser.Nitriding and related hardening of titanium(Ti) and its alloys were investigated as anew method of laser surface treatment by using a pulsed Nd:YAG laser of 1.06 μm wavelength with 3.6 ms pulse width. When a laser beam was irradiated on Ti plate in nitrogen(N) atmosphere, it was found that a flat and smooth surface covered with TiN nitride layer could be produced by selecting a good combination of laser irradiation conditions such as beam energy, power density and the distance from the lens focal point to the material surface. Compared with Ti base metal having a Vickers hardness(Hv) of about 200, the hardness of the laser-nitriding surface could exceed about 700 Hv at a single pulse shot and increase up to 1700 Hv at overlap of twenty shots, and besides the fusion zone could be hardened up to about 400 to 700 Hv. These hardening mechanisms were interpreted in terms of the formation of TiN nitride layer on the surface and the enrichment of nitrogen in the fusion zone. Nitriding and hardening phenomena wer...

Formation mechanism of porosity in high power YAG laser welding

With the objectives of clarifying the formation mechanism of porosity and producing a sound weld bead, welding conditions of porosity formation were investigated in A5083 alloy and Type 304 steel welded with a high power YAG laser, and the behavior of a keyhole, bubbles and porosity as well as liquid flows were observed during laser welding through X-ray transmission imaging system using markers. It was confirmed that a lot of bubbles and pores were formed in 3.5 kW YAG laser weld beads produced in Ar, He and N2 gases except Type 304 in N2 gas. Porosity was reduced at high welding speed in Type 304 steel even in He and Ar gases. A lot of bubbles were formed by the evaporation of metals from the bottom tip of the keyhole and flowed upwards in front of the solid-liquid interface. Some bubbles disappeared out of the molten surface especially in A5083 alloy welded at low welding speed, but the majority of bubbles were trapped at the solidifying front of the weld beads in most cases. The shielding gas was also included in the porosity. This mechanism is similar to that in high power CO2 laser welding. Fast liquid flows occurred circularly from the bottom keyhole to the rear upper part of the molten pool, from the rear to the front near the pool surface, and from the top to the bottom behind the keyhole in weld molten pools of both A5083 alloy and Type 304 steel in He, Ar or N2 shielding gas. Slightly different flows were noticed in the molten pool of Type 304 steel between YAG and CO2 lasers.With the objectives of clarifying the formation mechanism of porosity and producing a sound weld bead, welding conditions of porosity formation were investigated in A5083 alloy and Type 304 steel welded with a high power YAG laser, and the behavior of a keyhole, bubbles and porosity as well as liquid flows were observed during laser welding through X-ray transmission imaging system using markers. It was confirmed that a lot of bubbles and pores were formed in 3.5 kW YAG laser weld beads produced in Ar, He and N2 gases except Type 304 in N2 gas. Porosity was reduced at high welding speed in Type 304 steel even in He and Ar gases. A lot of bubbles were formed by the evaporation of metals from the bottom tip of the keyhole and flowed upwards in front of the solid-liquid interface. Some bubbles disappeared out of the molten surface especially in A5083 alloy welded at low welding speed, but the majority of bubbles were trapped at the solidifying front of the weld beads in most cases. The shielding gas was also...

Pulse shape optimization for defect prevention in pulsed laser welding of stainless steels

A variety of stainless steels were exposed to pulsed Nd:YAG laser with the rectangular shape power to first investigate practical problems related to pulsed laser welding. Porosities, cracks and undercuts were found as weld defects. A large porosity was formed in a key-hole mode of deep penetration weld metal of any stainless steel. Solidification cracks were present in Type 310S with above 0.017%P and undercuts were formed in Type 303 with about 0.3%S. The conditions for the formation of porosity were determined in further detail in Type 316. With the objectives of obtaining a fundamental knowledge of formation and prevention of weld defects, the fusion and solidification behavior of a molten puddle was observed during laser spot welding of Type 310S through high speed video photographing technique and the behavior was estimated by using a heat-conduction and solidification theory model. It was deduced that cellular dendrite tips grew rapidly from the bottom to the surface, and consequently residual liquid remained at the grain boundaries in wide regions and enhanced the solidification cracking susceptibility. Several laser pulse shapes were investigated and optimum pulse shapes were proposed for the reduction and prevention of porosity and solidification cracking.A variety of stainless steels were exposed to pulsed Nd:YAG laser with the rectangular shape power to first investigate practical problems related to pulsed laser welding. Porosities, cracks and undercuts were found as weld defects. A large porosity was formed in a key-hole mode of deep penetration weld metal of any stainless steel. Solidification cracks were present in Type 310S with above 0.017%P and undercuts were formed in Type 303 with about 0.3%S. The conditions for the formation of porosity were determined in further detail in Type 316. With the objectives of obtaining a fundamental knowledge of formation and prevention of weld defects, the fusion and solidification behavior of a molten puddle was observed during laser spot welding of Type 310S through high speed video photographing technique and the behavior was estimated by using a heat-conduction and solidification theory model. It was deduced that cellular dendrite tips grew rapidly from the bottom to the surface, and consequently residual liqu...

Phenomena of welding with high-power fiber laser

This study was performed with the objective of obtaining a fundamental knowledge of welding phenomena and mechanisms in producing deep and narrow weld beads in Type 304 stainless steel with a high-power fiber laser beam. Welding penetrations and defects were classified in weld beads made with the laser of about 115 and 360 Pm in beam diameter at the power of 6 kW. A focused high-power fiber laser beam could produce deeply penetrated weld beads with narrow widths, and the weld penetrations and bead widths were extremely deeper and narrower, respectively, at higher laser power density with the smaller beam diameter. Molten pool and keyhole behavior, bubble/porosity generation and melt flows, resulting in characteristic weld beads and defects, were observed during welding through high-speed video camera and the X-ray transmission real-time imaging apparatus. Consequently the formation situations and mechanisms of porosity, sound weld beads and underfilled weld beads were interpreted in terms of bubbles formation from a keyhole tip at low welding speeds, the formation of a long molten pool at moderate speeds and the production of spattering and a narrow, short molten pool at high welding speeds, respectively.This study was performed with the objective of obtaining a fundamental knowledge of welding phenomena and mechanisms in producing deep and narrow weld beads in Type 304 stainless steel with a high-power fiber laser beam. Welding penetrations and defects were classified in weld beads made with the laser of about 115 and 360 Pm in beam diameter at the power of 6 kW. A focused high-power fiber laser beam could produce deeply penetrated weld beads with narrow widths, and the weld penetrations and bead widths were extremely deeper and narrower, respectively, at higher laser power density with the smaller beam diameter. Molten pool and keyhole behavior, bubble/porosity generation and melt flows, resulting in characteristic weld beads and defects, were observed during welding through high-speed video camera and the X-ray transmission real-time imaging apparatus. Consequently the formation situations and mechanisms of porosity, sound weld beads and underfilled weld beads were interpreted in terms of bubbles forma...

Beam-plume interaction in pulsed YAG laser processing

Experimental studies on beam-plume interaction was conducted when a pulsed Nd:YAG laser was irradiated on a Titanium target in the pressure range of 10 to 3×105 Pa. In this paper are described the energy dissipation processes of incident beam, constituent and fluidmechanical structure of plume, absorption and scattering of incident radiation in plume, and their effects on energy transfer of laser beam to target material.Experimental studies on beam-plume interaction was conducted when a pulsed Nd:YAG laser was irradiated on a Titanium target in the pressure range of 10 to 3×105 Pa. In this paper are described the energy dissipation processes of incident beam, constituent and fluidmechanical structure of plume, absorption and scattering of incident radiation in plume, and their effects on energy transfer of laser beam to target material.

High power laser cutting of CFRP, and laser direct joining of CFRP to metal

CFRP possesses an extremely high specific strength, and thus the development of cutting and joining technology of CFRP is expected for the increase in applications in various industrial fields. To obtain fine cut quality in CFRP, high-quality disk lasers are applied to various types and kinds of CFRP. It was understood that fine cut quality could be produced in some CFRP with a small focused laser beam at an extremely fast speed. Joints between Type 304 plate (of 3u2005mm thickness and 30u2005mm width) and CFRP sheet (of 2u2005mm thickness and 20u2005mm width) were produced with a disc laser. The joint possessed a high shear tensile load of about 4800 N, and the fracture occurred in the CFRP near the lap joint.CFRP possesses an extremely high specific strength, and thus the development of cutting and joining technology of CFRP is expected for the increase in applications in various industrial fields. To obtain fine cut quality in CFRP, high-quality disk lasers are applied to various types and kinds of CFRP. It was understood that fine cut quality could be produced in some CFRP with a small focused laser beam at an extremely fast speed. Joints between Type 304 plate (of 3u2005mm thickness and 30u2005mm width) and CFRP sheet (of 2u2005mm thickness and 20u2005mm width) were produced with a disc laser. The joint possessed a high shear tensile load of about 4800 N, and the fracture occurred in the CFRP near the lap joint.

Observation of keyhole behavior and melt flows during laser-arc hybrid welding

Hybrid welding was carried out on Type 304 stainless steel plate under various conditions using a YAG laser combined with a TIG arc, and the effects of various welding conditions on the penetration and porosity formation were investigated. In most cases the hybrid weld beads were deeper and wider in comparison with the YAG laser ones; however, this fact did not always take place. It was consequently found that a laser beam should be shot inside the molten pool made readily with a TIG arc alone to produce deeper weld beads effectively. It was also confirmed that the hybrid weld beads had a smaller amount of porosity than the YAG laser ones. In order to understand these reasons, keyhole behavior and melt flows were observed through the X-ray real-time transmission apparatus. It was consequently revealed that the keyhole was enlarged near the top surface but was still small in diameter in the middle and bottom parts. Nevertheless, keyhole behavior was rather stable inside the molten pool and only a few bubbles were generated. There was a strong melt flow toward the rear part of the molten pool from the keyhole tip. Moreover, the melt flow was steady and fast backwards along the bottom fusion boundary of the molten pool. It is therefore considered that the backward melt flows conveyed the heat to the rear molten pool, and the melt pool was elongated, resulting in the formation of a wider weld bead.Hybrid welding was carried out on Type 304 stainless steel plate under various conditions using a YAG laser combined with a TIG arc, and the effects of various welding conditions on the penetration and porosity formation were investigated. In most cases the hybrid weld beads were deeper and wider in comparison with the YAG laser ones; however, this fact did not always take place. It was consequently found that a laser beam should be shot inside the molten pool made readily with a TIG arc alone to produce deeper weld beads effectively. It was also confirmed that the hybrid weld beads had a smaller amount of porosity than the YAG laser ones. In order to understand these reasons, keyhole behavior and melt flows were observed through the X-ray real-time transmission apparatus. It was consequently revealed that the keyhole was enlarged near the top surface but was still small in diameter in the middle and bottom parts. Nevertheless, keyhole behavior was rather stable inside the molten pool and only a few bubbl...

Characteristics of LAMP joining structures for several materials

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 (Laser-Assisted Metal and Plastic joining method). Recently it has been reported that the LAMP joining between several metals and engineering plastics could produce sufficiently strong joints. In this research, several LAMP joints was observed or analyzed with scanning electron microscope (SEM), transmission electron microscope (TEM), in order to obtain fundamental knowledge of LAMP joining structure. It was considered that not only the anchor (mechanical bonding) effect but also chemical bonding were considered as bonding mechanisms of LAMP joining.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 (Laser-Assisted Metal and Plastic joining method). Recently it has been reported that the LAMP joining between several metals and engineering plastics could produce sufficiently strong joints. In this research, several LAMP joints was observed or analyzed with scanning electron microscope (SEM), transmission electron microscope (TEM), in order to obtain fundamental knowledge of LAMP joining structure. It was considered that not only the anchor (mechanical bonding) effect but also chemical bonding were considered as bonding mechanisms of LAMP joining.

Collaboration of physical and metallurgical viewpoints for understanding and process development of laser welding

In Matsunawa laboratory at JWRI of Osaka University, laser welding phenomena were interpreted and laser weldability was evaluated from collaboration of welding physical and metallurgical viewpoints. It was observed by TEM that ultrafine particles were formed during laser welding, and then Prof. Matsunawa particularly suggested that Inverse Bremstrahlung and Rayleigh Scattering should exert an important role in the interaction between laser beam and laser-induced plasma or plume during CO2 laser welding and YAG laser welding, respectively. He was also expecting the effect of pulse-shaping on YAG laser spot weldability, which was confirmed by experiment. A microfocused X-ray tranmission real-time imaging system was newly constructed, by which keyhole behavior, melt flows, and bubbles and pores formation mechanisms were elucidated, and some remedies for prevension of porosity were proposed. Thereafter, in succeeded Katayama laboratory, the welding phenomena were more clearly observed by improved high speed video camera and illumination system, and the plumes induced with high power YAG or fiber laser were mainly in the vapor or weakly ionized state depending upon the power, and their interaction to a fiber laser beam was better understood by visualization of probe laser beam behavior. The importance of melt flows was also demonstrated and confirmed to understand YAG laser-TIG hybrid welding mechanism, which Prof. Matsunawa would like to know. The fact is that these research results were obtained not by one professor but by collaboration of many students and researchers.In Matsunawa laboratory at JWRI of Osaka University, laser welding phenomena were interpreted and laser weldability was evaluated from collaboration of welding physical and metallurgical viewpoints. It was observed by TEM that ultrafine particles were formed during laser welding, and then Prof. Matsunawa particularly suggested that Inverse Bremstrahlung and Rayleigh Scattering should exert an important role in the interaction between laser beam and laser-induced plasma or plume during CO2 laser welding and YAG laser welding, respectively. He was also expecting the effect of pulse-shaping on YAG laser spot weldability, which was confirmed by experiment. A microfocused X-ray tranmission real-time imaging system was newly constructed, by which keyhole behavior, melt flows, and bubbles and pores formation mechanisms were elucidated, and some remedies for prevension of porosity were proposed. Thereafter, in succeeded Katayama laboratory, the welding phenomena were more clearly observed by improved high speed v...