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Despite the growing interest in using powder metallurgy shape memory alloys, there is limited research on their joining and welding aiming to expand their applications. In this research, we utilized a high-power, high-brightness disk laser welding process to join spark-plasma-sintered Ti-51 at.% Ni shape memory alloy and studied the primary factors that affect the strength and functionality of the created sound welds such as intermetallics and martensite (B19′) phase formation. The introduction of high-power laser could impede the formation of B19′ and undesirable intermetallics such as TiNi3 and Ti2Ni; however, the desired Ti3Ni4 would also be suppressed and post-weld heat treatment was necessary to induce them again. The microstructure of fusion zones was significantly altered, producing coarse columnar grains with centerline and equiaxed weld centers. Additionally, high-power laser welding of powder metallurgy Ti-Ni alloy triggered amorphous phase formation in the welds due to rapid cooling. It was found that a laser power of 3 kW and welding speed of 6 m/min are optimal welding parameters, since the produced weld solidified along favorable [001] direction of strain recovery in Ti-Ni shape memory alloys.

The objective of this research is to reveal relationship between melt flows and spatter reduction by angle of incidence and defocusing distance in partial penetration welding of an SUS304 stainless...

In laser cutting, industry practitioners have long demanded higher cutting speeds and better quality cuts. There is thus much interest in the elucidation of related processing phenomena. Cutting quality is known to be affected by ejection behavior of generated melt, which itself is thought to be influenced by the profile of the laser cutting front. It would thus be desirable to directly observe the cutting front in real time, but this is not easily done in practice, mainly because laser cutting speeds are relatively high and visible light generally cannot pass through workpieces. In this study, we used an X-ray transmission system to perform fluoroscopic observation of cutting fronts in real time during actual processing. Further, we discuss the effect of the cutting parameters (cutting speed, etc.) on cutting front profile and melt ejection. We find that both the profile of the cutting front and the ejected melt behavior were stable when a sound cut was produced. We directly confirmed that the cutting front profile changed along with cutting speed.

With the objective of reducing the operating costs, argon-rich shielding gas was replaced by 100% CO2 gas in hybrid laser-arc welding of shipbuilding steel. The welding parameters were optimized to obtain buried-arc transfer in order to mitigate spatter formation. Sound butt joints could be successfully produced for plates of 14 and 17 mm thickness in one welding pass. Subsequently, the welded joints were subjected to different tests to evaluate the influence of CO2 shielding gas on the mechanical properties of the welded joints. All tensile-tested specimens failed in the base material, indicating the higher strength of the welded joints. The impact toughness of the welded joints, measured at −20 °C, reached approximately 76% of that of the base material, which was well above the limit set by the relevant standard. The microstructure of the fusion zone consisted of grain boundary ferrite and acicular ferrite uniformly over the plate thickness except for the joint root where the microstructure was chiefly ferrite with an aligned second phase. This resulted in higher hardness in the root region compared with the top and middle parts of the fusion zone.

An innovative hybrid laser-arc welding technique has been developed for one-pass welding of 25 mm thick steel plates. The parts to be welded were assembled into a butt joint configuration with a square groove and 2.5 mm air gap. A backing method was applied using ceramic strip or submerged arc welding flux. Cut-wire particles were inserted into the joint gap to protect the backing material from laser radiation and in the same time help to fill the gap. Laser power density was optimized to obtain full penetration joints without damaging the backing material. Defect-free joints with adequate fusion zone profiles and good mechanical properties were successfully produced. The technique was then applied to weld 50 mm thick plates in two welding passes. Using the same joint design, one welding pass was overlaid from one side of the joint and a second pass was deposited from the other side. In this case, a backing plate of any material could be used since the laser radiation does not reach the bottom side of the joint. Keywords: Hybrid laser-arc welding, Thick plate, Cut-wire, Ceramic backing, Flux backing

A series of trials to fusion weld commercially pure titanium (CPTi) to stainless steel 304 (SS304) have been conducted using laser beam welding (LBW) and gas tungsten arc welding (GTAW). Neither technique produced adequate weld joints with LBW showing a more promising result, while GTAW yielded separation of the workpieces immediately after welding. Cracking and fracturing took place mainly on the SS304 side, which was explained by the differences in the materials&rsquo, thermal properties. Various intermetallic phases formed during welding that were identified using energy dispersive X-ray spectroscopy (EDS) and electron backscattered diffraction (EBSD) technique and were compared with an isothermal ternary phase diagram of Fe-Cr-Ti. Their corresponding hardness values are reported and correlated with alloy compositions.

This article briefly summarises previous reports as well as some recent results on welding of various titanium alloys. Titanium is available in a wide range, hence, their properties and characteristics (including welding characteristics) may also varied significantly. The alloys studied represented three major areas; they are (1): unalloyed or commercially pure titanium (CP Ti), (2): near α and α+β alloys, and (3): β alloys. From our preliminary results, it can be reported that the structure of fusion zone (FZ) and heat affected zone (HAZ) of alloys from area (1) may change but their hardness remained, arguably, the same as the as-received material. Alloys from area (2) showed an increase in hardness in the FZ and HAZ areas due to the formation of α-prime or martensite, while alloys from area (3) experienced a reduction in hardness associated with dissolution of α phase leaving only retained β in the FZ. Further investigations are required to confirm these findings. Such information are very useful so that appropriate post welding heat treatment (PWHT) can be applied to improve their properties and performance.

This paper describes a variety of fundamental research results of laser welding which the authors have recently performed. The behavior and characteristics of a laser-induced plume were elucidated. Especially, in remote welding with a fiber laser, the effect of a tall plume leading to shallow weld was interpreted by considering the interaction of an incident laser beam against the zone of a low refractive index from the Mickelson interferometer results. The laser absorption in the plate was higher in the case of a smaller focused beam of fiber laser, lower welding speed and higher power, and the reason was interpreted by considering the size and location of a keyhole inlet and a beam spot. High power tandem laser beams could produce deep penetration, and laser welding in vacuum was developed for production of deeply penetrated welds. Laser direct joining was also developed for joining of metal to plastic or CFRP.

High-power and high-brightness lasers such as fiber laser or disk laser are promising as desirable heat sources for deeply-penetrated welds, owing to their superior power density. However, their tightly-focused beam spots have low gap tolerance in butt welding. The objectives of this research are to investigate penetration characteristics, to clarify welding phenomena and to develop high quality welding procedures in butt welding with several gaps of zero to 0.4 mm for 590 MPa-high-strength steel plates with a 10 kW fiber laser or a 16 kW disk laser. The butt welding was performed on 12 mm-thick plates under several welding conditions including using a cold wire or a hot wire. In the case of 0.4 mm gap, underfilling and burn-through welds were obtained by the conventional 10 kW fiber welding and with using a cold wire. On the other hand, welding defects were greatly reduced with a hot wire. Moreover, a 16 kW high power disk laser with a hot wire could produce sound fully-penetrated welds to suppress not only the welding defects but also oxygen content in the weld fusion zone. The weld penetrations were stable in spite of several gaps from zero to 0.4 mm, owing to removal of excessive melt depending on the gap size from bottom keyhole inlet in the molten pool by a strong laser-induced plume.Consequently, it was revealed that laser power sufficient to produce a full-penetrated keyhole using a hot wire was effective to the improvement of gap tolerance and suppression of oxygen content in the weld fusion zone in butt welding of thick high-strength steel plates with a high-power and high-brightness laser.

High power fiber lasers of high beam quality have been developed, and remote welding with a high-power fiber laser has been expected to be one of new high-speed welding procedures in automobile industries. In particular a long-focal-distance focusing optics is beneficial to the welding of a wide area. However, features and effects of a laser-induced plume in such welding process are not fully understood. Therefore, the study was undertaken to investigate weld penetration characteristics with or without removal of a laser-induced plume, to clarify welding phenomena and to develop a stable welding procedure in 4 kW high-power fiber laser welding of a 1.5 mm-thick zinc-coated steel sheet using the focusing optics with a lens of 1,250 mm in focal length. The welds obtained at the focal position without removal of the plume were so unstable that the full penetrations were changed to partial penetrations during laser welding. To fundamentally understand the remote welding phenomena, refractive-index distribution above the laser-irradiated area was visualized by the Michelson interference method. The observation results of fringe patters through a high-speed video camera demonstrated that the incident laser beam was defocused and refracted by a wide formation range and complicated behavior of small refractive-index values derived from the high-temperature metal vapor of the plume, leading to the formation of an unstable penetration weld. It was also confirmed that the removal of the laser-induced plume and the corresponding widespread field of low refractive-index values by using a fan was essential to the stable production of fully-penetrated welds in the high-power fiber laser remote welding with the long-focal-distance focusing optics.

The behavior and effect of a plasma plume on the weld penetration are greatly different between CO2 laser welding and YAG, disk or fiber laser welding. The effects of the power and the power density on the weld penetration are elucidated. Spattering leading to the formation of underfilled weld beads is controlled by inclining the laser beam. Porosity is formed from bubbles generated from the tip of the keyhole at low welding speed or from the middle part of the keyhole at high laser power density. Cracking easily occurs in pulsed spot welding of aluminum alloys.

Fiber laser has been received extensive attention in welding of thick section materials in terms of high power intensity and high beam quality. This paper investigated the influence of fiber laser welding conditions, including welding speed, beam spot size and defocused distance, on the weld bead geometry in welding of thick section stainless steels. The result showed that the beam spot size had significant influence on the penetration depth, and the penetration depth at 0.1 mm beam spot size was shallower than that at 0.2 mm beam spot size. The maximal weld penetration at 10 kW laser power and 0.3 m/min could be obtained at the defocused distance of about -7 mm. Wine-cup weld bead geometry with excessive wide bead surface was formed in range of low welding speeds (lower than 1 m/min). As a result, the decrease in welding speed induces considerable increase in bead width, but only slight increase in penetration depth. In order to obtain deeper penetration at low welding speed range, it is important to enlarge and deepen the keyhole opening and suppress the plume/plasma. Thus an ultra-deep penetration laser welding method, which is characterized by using a gas jet nozzle to modify the keyhole and melt flow behavior, was developed. With two high-speed CMOS cameras and X-ray transmission apparatus, the behaviors of the keyhole, the melt flow and the plume in gas jet assisted laser welding were observed. The effects of the gas jet parameters and welding conditions on the weld bead geometry were investigated. By adoption of the gas jet, the penetration depth was increased from 18.2 mm to 24.5mm in the conditions of the laser power of 10 kW and the welding speed of 0.3 m/min. Moreover, the instability of penetration depth induced by thermal lens effect was also discussed.

Recently 10 kW fiber lasers with high beam quality have been developed to achieve the power density of about 1 MW/mm2. A plume induced during welding with such an ultra-high power density laser was in a weakly ionized state. The objectives of this research are to obtain a fundamental knowledge of interaction between a laser beam and an induced plume and to evaluate the effect of this ultra-high power density laser on the weld penetration. The observation result of the plume induced during bead-on-plate welding of a 20 mm thick Type 304 plate with 10 kW laser showed the growth of about 12 mm height at the repetition cycle of around 0.5 ms. The interaction was investigated by the power meter measurement or the high-speed behavior observation of a fiber laser probe beam (wavelength : 1.09 μm), which passed vertically through the plume. The attenuation of the probe laser beam was measured to be about 4%, which was not mainly caused by Inverse Bremsstrahlung but by Rayleigh scattering due to ultrafine particles and partly by absorption and reflection due to spatters. The probe beam observed was refracted at 0.6 mrad angle in average, which was much lower than the 90 mrad divergence of the focused fiber laser beam. Moreover, a stable laser welding process could be produced at such ultra-high power density that 12 mm deep penetration was obtained even if the laser peak power was decreased 1 ms periodically from 10 kW to 8.5 kW. It was consequently considered that the interaction between a focused 10 kW fiber laser beam and a weakly ionized plume was too small to exert the reduction in weld penetration.

This study was undertaken to obtain a fundamental knowledge of the generation behavior and internal state of a plume induced during bead-on-plate welding of a 20 mm thick Type 304 stainless steel with a 10 kW fiber laser beam of a 130 μm spot diameter, on the basis of 40,000 f/s high-speed video observation and spectroscopic analysis. This 0.9 MW/mm2 ultra-high power density laser produced a 12 mm deep partial-penetration weld with 1.3 mm narrow bead width at 3 m/min welding speed. According to the high-speed observation pictures, laser-induced plumes were repeatedly generated from a keyhole at the interval of about 0.5 ms period to reach 12 mm height at the maximum. The plume was also characterized by bright blue emission. The spectroscopy indicated that the plume was emitted from the line spectra of neutral atoms such as iron (Fe), chromium (Cr) and manganese (Mn), which were alloying elements of Type 304. However, ionized spectra of alloying elements and line spectra of argon (Ar) neutral atom were not apparently detected under these welding conditions. Furthermore, the temperature and ionization degree of the laser-induced plumes were calculated to be 6,000 K and 0.02, respectively, by the Bolzman plots and Saha's equation. Consequently, the plume induced with a 10 kW fiber laser of extremely high power density was judged to be in a state of weakly ionized plasma.

This paper deals with the behavior of a keyhole and a laser-induced plasma observed during laser welding of several metals with a single YAG laser or superimposed two beams through the ultra high-speed video camera, with the objective of understanding enhanced beam absorption due to two different wavelength laser beams. Moreover, reflected YAG laser beams were measured by photodiodes, and the relationship between the keyhole behavior and the reflected-laser photodiode signal was investigated to clarify rapid welding phenomena. It was observed that the behavior of a keyhole and a laser-induced plasma during laser welding was considerably different between a single beam and combined two different beams. And the reflected light intensity during welding with the superimposed two different laser beams lowered in the earlier stage than that with a single pulsed YAG laser beam.

It is important to understand the liquid surface configuration, meniscus configuration in brazing and soldering process. In this study, meniscus configuration formed on circular rod base metal during brazing and soldering has been investigated using computer simulation. Meniscus configuration is determined by combining the Laplace equation and hydrostatic pressure. The meniscus configuration on the plane surface could be determined by simple equation, however, on the curved surface it could be obtained only by the numerical analysis. In the present study the meniscus configuration on circular rod was simulated by Runge-Kutta method. The effects of radius of rod on meniscus rise height and its configuration were investigated. It was found that the meniscus height became small with decreasing the radius of rod. Meniscus configuration of various brazing metals having different surface tension and density were also simulated. Within the present estimation, aluminum brazing filler metal offered the largest meniscus rise height, whereas, Sn-Pb solder gave the smallest one. Similarly, the meniscus configuration depending on the contact angle and the volume of liquid metal was also derived by the present method, therefore, this method is effective to obtain the accurate meniscus configuration of brazing filler metals and to prevent brazing defects such as overor in-sufficient brazing.

To obtain fundamental knowledge of pulsed laser welding phenomena, especially evaporation mechanism of different aluminum alloys, the authors investigated the structure and composition of evaporated particles of Al-Mg alloys in air and in the Ar atmosphere during pulsed laser welding. It was found that ultra-fine particles of 5 to 100 nm diameter in a globular or irregular shape were formed in laser-induced plume and the main structure was MgAl2O4. The composition of particles was different depending on the power density of a laser beam ; namely, under the low power density irradiation conditions, magnesium was predominant in the particles, while aluminum content increased with an increase in the power density. These results were attributed to evaporation phenomena of metals with different boiling points and latent heats of vaporization. On the other hand, under the assumption that the mixed gas of different atoms and the ionized species was an ideal one, the number density of laser-induced plasma species was obtained by Saha's equation. It was confirmed that the number density depends upon the plasma temperature and total pressures.

In the field of visualization and CG (Computer Graphics), it is necessary to propose new methods to present objects precisely and to display images in real time. However, these methods need a great deal of computing time and memory. Recently, many methods, using a massive parallel computer, have been reported to satisfy these conditions. The purpose of our research is to develop practical visualization algorithms for massive parallel computers. In this paper, a parallel ray tracing algorithm is proposed, because the ray tracing algorithm has a great deal of parallelism and is able to take advantage of the high performance of massive parallel computers. The ray tracing algorithm has some parallel approaches, i.e. pixels parallel, objects parallel, rays parallel and so on. We propose a hybrid parallel ray tracing algorithm to include these parallel approaches and allowing experiments to compare processing time between each parallel approach. In conclusion, we point out problems with hybrid algorithms on massive parallel computers.

Vacuum brazability of aluminum was investigated using various Al-Si-Mg powder brazing filler metals made by atomization. The effects of atomizing atmosphere and powder size on brazability were studied. After brazing a part of used filler metals gave no contribution for the fillet formation and gathered on the base metal, showing non-wetting among filler metal powders. Brazability was evaluated by the percentage of the filler metal remaining as fillet by measuring the weight change before and after brazing. After brazing of the powder filler metal, voids were found in the formed fillet. Although the completely sound fillet was not obtained, the fine powders and air atomized powders gave better brazability than coarse powders and those atomized in argon gas atmosphere. The difference in vaporization characteristics of magnesium in the filler metals seemed to be responsible for the void formation and poor brazability of the powder filler metals. Mixing of the filler metals with the Al-Mg powder was found to be effective for the improvement of the fillet formation, however, completely sound fillet was not obtained. It is believed that both the control of magnesium vaporization rate and the improvement of quality of vacuum are important to obtain the sound fillet using powder filler metals.

It is known that oxides of copper, nickel, tin, etc. are reduced in hydrogen atmosphere at soldering temperature. In expectation of this hydrogen reduction effect, dull nickel electrodeposits were formed under excess current density to obtain nickel films containing hydrogen and their solderability was measured by meniscograph. The hydrogen content of the nickel electrodeposits, and their crystal structure, composition and morphology were analyzed and correlations between these parameters and solderability investigated.The results are summarized as follows.(1) Nickel electrodeposits formed under excess current density showed a marked improvement in solderability.(2) The crystal structure of the nickel electrodeposits was fcc and the preferential orientation changed with current density, but no correlation was found between solderability and orientation.(3) There was good correlation between the solderability of the nickel electrodeposits and their hydrogen content.

High power and high brightness disk lasers have been developed, and it is revealed that a deeper weld bead can be produced with an increase in the power density. It is furthermore expected that sound deeper penetration welds can be produced under vacuum, although those can hardly be made in a gas shielding at 1 atm. Thus, a vacuum chamber using a high beam quality laser was developed to produce sound ultra deep penetration welds. Therefore, this study was undertaken on Type 304 stainless steel and A5052 aluminum alloy under low vacuum produced by rotary pumps. The observation technique of welding phenomena was utilized to obtain a fundamental knowledge of laser weldability under low vacuum. As a result, the welds of both metals under low vacuum were over twice deeper than those under the atmospheric pressure. And it was found that the effect of ambient pressure on the penetration depth became significantly at low welding speed. Especially, over 0.6 m/min, the penetration depth with A5052 began to decrease below the pressure which produced the deepest penetration weld. In order to clarify welding phenomena under low vacuum, the laser-induced plume and the keyhole behavior in the molten pool were observed by a high-speed video camera. It was found that under low vacuum the plume was stable and the molten pool became narrower and steadier compared with those under atmospheric pressure. Especially, in A5052 the plume shifted back and forth, and the keyhole expanded and contracted backward to the laser beam spot.

Effect of curvature of convex and concave planes on the fillet size formed after brazing of aluminum was investigated in relation to the brazing method. The fillet size was measured by throat thickness and vertical fillet leg length on the inside (concave plane) and outside (convex plane) of an aluminum pipe braze specimen. Fillet size estimated by the Laplace's equation was larger on the inside than that on the outside. In vacuum brazing inside fillet was larger than outside one under limmited test conditions, while in dip brazing inside fillet was always larger than outside one, which agreed well with the prediction of the Laplace's equation. The discrepancy between the calculation and experimental results in vacuum brazing was attributed to the poor flowabilty of molten filler metal.

Vacuum brazability of aluminum hollow receptacles has been investigated in relations to the degree of close and magnesium content in brazing sheet claddings. The degree of close was varied by changing the hole diameter in brazing sheet attached to top and bottom side of a cylindrical aluminum pipe. Outside fillet formation was only dependent on the magnesium content of brazing sheet, however, fillet at inside was dependent on both degree of close and magnesium content. At relatively completely closed circumstances, relatively large fillets were obtained irrespective to magnesium content. At open circumstance or outside, large fillets were obtained except brazing sheet with 0.5%Mg. The lowest brazability was obtained at medium degree of close. Auger electron spectroscopy on brazing sheets and aluminum pipe after brazing revealed that the existence of the surface layer with much MgO and/or thick Al2O3 were responsible to the poor brazability in medium degree of close.

The authors have developed a new chamber for laser welding under the low vacuum conditions achieved by using rotary pumps. High-power disk laser bead-on-plate welding was performed on Type 304 stainless steel or A5052 aluminium alloy plate at the powers of 10, 16 and 26kW at various welding speeds under low vacuum. The sound welds of more than 50 and 70mm in penetration depth could be produced in Type 304 at the pressure of 0.1kPa, the speed of 0.3 m/min and the power of 16kW and 26kW, respectively. Similar penetration was achieved in A 5052 aluminum alloy. Welding phenomena under low vacuum were also understood by observing the behavior of a keyhole inlet, a molten pool, melt flows and a plume ejected from a keyhole through high speed video cameras. Low interaction between a laser beam and a plume under low vacuum was confirmed by using probe laser beam method.