A pure copper layer was formed on the stainless-steel substrate with low-dilution layer by a Wire-based Laser Metal Deposition method (W-LMD) with a blue diode laser. Pure copper is an important material for realization of a carbon-neutral society because of excellent properties such as electrical and thermal conductivity. As the demand of copper increases, a wide variety of processing methods are required with high yields. Thus, the LMD method, which is one of the Additive Manufacturing (AM) technologies, is focus on this study. The W-LMD is the process in which a metal wire is fed to the laser irradiation point and melted and solidified to form a layer. Conventional W-LMD with near-infrared fiber laser has formed a layer of Fe-based alloy, Ti-based alloy, Ni-based alloy, and so on. However, it was difficult to form a pure copper layer since light absorption rate of copper in this region was less than 10%. Therefore, we focused on blue light at the wavelength 450 nm. It is because light absorption rate of copper in blue region is 60%. Thus, the high-power blue diode laser has been developed and installed the W-LMD system. In this study, the pure copper layer was tried to form on the SS304 substrate with our W-LMD system. As the results, copper layer was formed with a layer thickness of 388 μm and a dilution thickness of 22 μm at the output power of 500 W, stage sweep speed of 10 mm/s, and wire feeding rate of 10 mm/s.
Laser welding in vacuum with a high-power laser is performed for a stainless steel plate which forms a keyhole inside the molten pool, resulting in a deep penetration without spatters. The spatter is one of the defect factors for laser welding, such as thinning of weld bead, and pore generation. In order to elucidate the mechanism of spatter free process, in situ observations were conducted to investigate dynamics of molten pool and keyhole in vacuum laser welding with a highspeed video camera and X ray transmission system. A stainless steel type 304 (SS304) is widely used in several industries such as automotive industry, chemical plants, petrochemical industry, etc. due to its excellent properties such as high corrosion resistance, hardness, low-temperature toughness, and thermal stability. The stainless steel type 304 (SS304) was set in a vacuum chamber and then a disk laser with an output power of 6 kW was irradiated and scanned on it to form a weld bead under aimed pressure. At the same time, the behavior of molten pool and keyhole were captured with real time observation system and an X ray transmission system, respectively. From the results, it was found that this fluctuation of molten pool affected the generation of spatter.
The 1.5kW fiber coupled blue diode laser with a fiber core diameter of 400μm was newly developed and pure copper plates were performed bead on plate welding tests. Laser welding has been usually employed near infrared ray [IR] laser with approximately 1000nm wavelength, such as a fiber laser, a disk laser, and a diode laser. However, it is difficult to weld pure copper with the IR laser due to the low light absorption rate of copper. In the blue region, on the other hand, copper has the high light absorption rate and the fluctuation of this rate with rising temperature is small. Thus, a blue diode laser is suitable for welding copper. Pure copper plates are used in this study. The high speed and high quality copper welding is required for several industries since copper is essential for various products such as fuel cells, automotive motors, and busbars for realization of a carbon-neutral society. Although blue diode lasers have been created worldwide, the keyhole welding was difficult because of the insufficiency of laser power density. Therefore, we developed the 1.5kW high-power blue diode laser and irradiated 10mmw×30mml×2mmt pure copper plates with sweep speed of 25mm/s and spot diameter of 300μm. The melting and solidification dynamics were observed with high-speed video camera and spectrometer for elucidation of welding mechanism with the blue diode laser. As the results, one of fluctuation factors was found to be presence of neutral Cu and CuO in the laser plume, which may cause instabilities due to interference them with the laser.
Laser welding, which has many advantages such as high-speed process, deep penetration, narrow weld width, remote control etc., has been industrial applied for several industries. The formation of molten pool, and the mechanism of spatters generation have not been sufficiently investigated. In this study, we performed bead-on-plate welding tests on SS304 plates using disk laser by changing atmosphere pressure. The laser welding was observed in real time using the glass transmission method, and the shape of the molten pool was observed. As a result, it was found that the length of the molten pool was a factor for spatter suppression.
Theoretical and experimental studies have been carried out in order to improve the understanding of the mechanism of pore formation in keyhole laser spot welding in a qualitative and also quantitative manner. A semi-analytical mathematical model of the keyhole collapse illustrates the different characteristic time scales of the contributing physical processes: post-vaporization (order of magnitude: 100 ns typically), excess keyhole vapor relaxation flow 10 μs), inertia driven collapse (100 μs), followed by bubble contraction, re-condensation and rising (10 ms), and re-solidification (10 ms). The conditions of the keyhole just before switching off the laser beam, observed by X-ray imaging, are essential for the subsequent collapse mechanism. In case of a bottleneck-shaped keyhole, which can easily form due to the paradox of vapor flow inversion, bubble formation is likely to occur due to necking. When the thermally contracting bubble is trapped by the re-solidification front, a pore is formed. The model is complementary to high speed X-ray observations of the keyhole shape, particularly in liquid Zn that enables investigation of keyhole and bubble formation not constrained by surrounding solid.
High power CO2, YAG and LD-pumped solid-state lasers have been developed to produce a deep penetration type of high-quality, high-performance and high-speed weld joints. However, porosity is easily formed in such deep keyhole-type weld beads. The authors have developed microfocused X-ray transmission imaging system, and revealed keyhole behavior and porosity formation mechanism in high power laser welding. This paper will describe a summary of porosity formation mechanism and prevention procedures during cw laser welding of aluminum alloys. Especially, many bubbles were formed by the evaporation of the metals from the bottom tip of the keyhole and flowed upwards according to the liquid flow near the solid-liquid interface inside the molten pool. The majority of them were trapped and captured at the solidifying front of the weld beads, leading to the formation of porosity. Moreover, it was revealed that the shielding gas was chiefly included in the porosity. Main melt flows were observed as a function of welding speed. As the speed was increased, vapor plume was ejected from the keyhole inlet more and more normal to the plate surface, and consequently induced the upward flow of the keyhole-surrounding liquid. On the basis of the above knowledge, full penetration welding, properly pulse-modulated laser welding, vacuum or low pressure welding, welding using the tornado nozzle, very low or high speed welding, and so on were investigated, and it was consequently confirmed that these procedures were beneficial to the reduction in porosity.
In order to understand laser keyhole welding phenomena, pulsed or continuous wave laser was irradiated on a molten metal of Sn or Zn, and spattering and keyhole evolution were observed by high-seed video camera and X-ray transmission method, respectively. It was confirmed from the observation of the surface that a keyhole was initiated to form by far earlier in the molten metal than in the solid metal. According to the X-ray transmission real-time observation result in Zn liquid metal, bubbles were predominantly generated from the tip of a keyhole, which is the same formation mechanism as we revealed in general laser welding. Furthermore, simplified numerical calculation demonstrated that surface tension should affect the formation of such bubbles.
A deep cavity called keyhole is formed in the laser weld pool due to the intense recoil pressure of evaporation. The formation of keyhole leads to a deep penetration weld with high aspect ratio. However, a hole drilled in a liquid pool is primarily unstable by its nature and the instability of keyhole also causes the formation of porosity in the weld metal. The porosity formation is one of the serious problems in the very high power laser welding, but its mechanism has not been well understood. The authors have conducted systematic studies on observation of keyhole as well as weld pool dynamics and their related phenomena to reveal the mechanism of porosity formation and its suppression methods. The paper describes the real time observation of keyhole and laser plasma/plume behaviors in the high power CW CO2 laser welding by the high speed optical and X-ray transmission methods, cavity formation process and its suppression measures.
With the objectives of obtaining a fundamental knowledge of laser welding technology inside and outside the spacecraft in space, pulsed YAG laser spot welding was performed on the metal plates in Ar gas atmosphere or a vacuum in the falling microgravity apparatus equipped with the fiber-delivered laser focusing optics. The influence of gravity or microgravity on penetration and welding defect formation was further clarified by comparing the welds made in the normal flat and overhead positions. Almost all results of weld penetration and defect formation under microgravity were similar to those under normal gravity except the welding result of aluminum alloy A5083 subjected to the high power density laser, and were between normal gravity and overhead position welding results. Welding in a vacuum was characterized by the formation of a narrower and cone-shaped bottom in any alloy weld. Porosity was easily formed in any deeply penetrated weld metal under high power density welding with a rectangular pulse-shaped laser, and could be reduced by utilizing pulse-controlled laser even under microgravity.
It is well known that porosity is easily formed in high power laser welding, which is quite a serious problem to be solved. At present, there are few reports studying interrelationship between keyhole and plasma behavior with the objective of understanding the effect of shielding gas on porosity formation. In this study, therefore, the relationship between keyhole and plasma behavior was observed directly by using two synchronized ultra high-speed cameras and X-ray transmission observation system. In the case of He gas, metallic plasma was continuously formed, and the keyhole was always open. It was observed that many large bubbles, which were formed from the tip of a keyhole, were trapped at the solidifying front in the rear part of the molten pool, and lead to the porosity formation. On the other hand, in the case of N2 gas, big nitrogen plasma was formed above the weld bead periodically, and its absorption of laser caused the disappearance of metallic plasma and keyhole. This periodical interval and duty were different among materials used and affected the bubble and porosity suppression beneficially.
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