Fiber lasers have been receiving considerable attention because of their advantages of high power, high beam quality and
high efficiency, and are expected as one of the desirable heat sources for high-speed and deep-penetration welding. In
our researches, therefore, the effects of laser powers and their densities on the weld penetration and the formation of
sound welds were investigated in welding of Type 304 austenitic stainless steel, A5052 aluminum alloy or high strength
steel plates with four laser beams of about 0.12 to 1 mm in focused spot diameter, and their welding phenomena were
observed with high-speed video cameras and X-ray transmission real-time imaging system. It was found that the laser
power density exerted a remarkable effect on the increase in weld penetration at higher welding speeds, but on the other
hand at low welding speeds deeper-penetration welds could be produced at higher power. Laser-induced plume behavior
and its effect on weld penetration, and the mechanisms of spattering, underfilling, porosity and humping were elucidated,
sound welds without welding defects could be produced under the improved welding conditions. In addition, importance
of the development of focusing optics and the removal of a plume during remote welding will be emphasized in terms of
the stable production of constant deep-penetration welds and the reduction in welding defects in high power laser
welding.
A face welding technology of a titanium sheet on a steel plate for adding corrosion resistance was investigated. In the welding tests, 1800 W YAG laser beam was irradiated at different locations near a touching line of both plates through f:θ lens and was scanned within 13 mm width along the line at the frequency of 10 Hz, 20 Hz or 40 Hz. Twin rolls were set to load forces on both plates subjected to welding and simultaneously for pulling them out. The tensile shear test was carried out on each piece. Consequently, the highest strength was obtained at the frequency of 40 Hz. SEM observation and EDX analyses revealed that the formation of intermetallic compounds was smallest in this weld.
Hybrid welding was carried out on Type 304 stainless steel plate under various conditions using YAG laser combined with TIG arc. During arc and laser-arc hybrid welding, arc voltage variation was measured, and arc plasma, laser-induced plume and evaporation spots as well as keyhole behavior and liquid flow in the molten pool were observed through CCD camera and X-ray real-time transmission apparatus. It was consequently found that hybrid welding possessed many features in comparison with YAG laser welding. The deepest weld bead could be produced when the YAG laser beam of high power density was shot on the molten pool made beforehand stably with TIG arc. A keyhole was long and narrow, and its behavior was rather stable inside the molten pool. It was also confirmed that porosity was reduced by the suppression of bubble formation in hybrid welding utilizing a laser of a moderate power density.
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.
In high power YAG laser welding of steels, a rectangularly modulated beam with high peak power is usually used to get deep penetration. On the other hand, many spatters and solidification cracks are generated when some aluminum alloys are welded with a rectangularly modulated beam because of its high heat conductivity, high reflectivity, low surface tension, large contraction, wide solidification temperature range, etc. Therefore, a properly modulated beam or a continuous beam is usually used in aluminum alloy welding, although the penetration depth is shallow. In this research, sinusoidal wave or rectangularly modulated wave of YAG laser combined with TIG arc was tried to improve the weldability of A6061 aluminum alloy. As a result, when TIG arc was superimposed behind the YAG laser beam, deeply penetrated weld beads with good surface appearances were produced without spatter losses and cracks.
CO2 laser welding of thin aluminum sheets was performed at welding speeds of up to 20 m/min to investigate the weldability, weld pool dynamics and mechanical properties of the weld bead of aluminum alloys. High-speed camera observation of weld areas showed that the thickness of the keyhole-front-face decreased to 100 μm under high-speed welding conditions and the weld pool became unstable. The focal length was optimized to increase the spot power density and thereby easily melt the aluminum sheets. Using a 76-mm focal length lens, which corresponds to 11 MW/cm2 power density, we obtained a keyhole mode weld bead with a depth of 1.3 mm at 20 m/min welding speed at 2 kW laser power. It was also possible to reduce the heat affected zone (HAZ) width to only 1.6 mm when the welding speed was 20 m/min. The HAZ width decreased as welding speed was increased. The tensile strength test of A6N01 weld beads showed that the fracture strength increased as the welding speed was increased up to 16 m/min, probably because the soft region of weld specimens was decreased. On the other hand, solidification cracks formed in the weld bead center at higher speeds, resulting in decreased strength.
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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