Akira Matsunawa

The role of recoil pressure in energy balance during laser materials processing

A theoretical analysis of the energy balance in the laser - metal interaction zone is carried out. The heat transfer due to the recoil-pressure-induced melt flow is taken into consideration. It is shown that, for the absorbed laser intensities typical in welding and cutting, the recoil pressure induces high-velocity melt-flow ejection from the interaction zone. This melt flow carries away from the interaction zone a significant portion of the absorbed laser intensity (about 70 - 90% at low laser intensities); thus, convection-related terms can be ignored neither in calculations of the energy balance in the interaction zone nor in calculations of the thermal field in the vicinity of the weld pool or cutting front.

Dynamics of keyhole and molten pool in laser welding

In laser and electron-beam welding, a deep cavity called a keyhole or beam hole is formed in the weld pool due to the intense recoil pressure of evaporation. The formation of the keyhole leads to a deep penetration weld with a high aspect ratio and this is the most advantageous feature of welding by high-energy-density beams. However, a hole drilled in a liquid is primarily unstable by its nature and the instability of the keyhole also causes the formation of porosity or cavities in the weld metal. In particular, the porosity formation is one of the serious problems in 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 article will describe the real-time observation of keyhole and plume behaviors in the pulsed and continuous-wave laser welding by high-speed optical and x-ray transmission methods, the cavity formation process and its suppression measures.

Laser ablation at solid–liquid interfaces: An approach from optical emission spectra

The emission spectra from the solid–liquid interface irradiated by a pulsed laser were studied. The solid target used in this study was graphite and boron nitride, and the liquid in which the target was immersed was water, benzene, n-hexane, and carbon tetrachloride. The results showed strong continuous spectrum immediately after a pulse shot, whereas after ≈100 ns later from the irradiation it was greatly reduced, and instead, the emission from small molecules dominated the spectra. The line spectra of small molecules observed in the later time range indicate the chemical reaction between the ablated species and the species originated from the liquid molecules. The intensity of the continuous spectrum was very prominent compared to what has been observed for solid–gas interfaces. This is due to rapid electron ion recombination or bremsstrahlung due to highly confined interface plasma.

High-speed simultaneous observation of plasma and keyhole behavior during high power CO2 laser welding: Effect of shielding gas on porosity formation

Laser welding can produce a deeply penetrated bead at high speed. However, in high power cw CO2 laser welding, the characteristic porosity is easily formed in the weld metal, but its formation mechanism has not been well understood. Therefore, the authors have conducted systematic studies of the elucidation of porosity formation mechanism and the development of preventive remedies. They have revealed that many bubbles are formed, mainly from the bottom tip of the keyhole by intense evaporation of the metal. It has also been revealed that the keyhole fluctuates frequently and changes its size and shape, corresponding to the intermittent bubble formation. The majority of bubbles are trapped at the solidifying front in the rear part of the molten pool. However, there are few reports that deal with the simultaneous observation of keyhole and plasma dynamic behavior as well as the formation of bubbles and porosity. In this study, therefore, the interrelationship between keyhole and plasma behavior was examined...

Unbounded keyhole collapse and bubble formation during pulsed laser interaction with liquid zinc

Suppression of pore defects in keyhole laser spot welding demands for a theoretical description of the fundamental process. Investigating the unbounded keyhole collapse in liquid Zn instead of a solid provided a simplified situation offering several advantages. Improved high speed x-ray transmission imaging due to an enlarged keyhole in the absence of violent melt motion was enabled, which also facilitated the development of a semi-analytical mathematical model. Good correspondence between the experimentally and theoretically obtained transient keyhole and bubble shape permitted physical analysis by the model. Characteristic timescales were identified for post-vaporization, vapour relaxation, cooling, collapse, bubble contraction, oscillations and buoyancy. Recondensation due to rapid cooling turns out to be responsible for shielding gas flow into the keyhole, finally maintaining a spherical bubble. Creation of a convergent keyhole is a possibility to avoid bubbles.

Effect of vacuum on penetration and defects in laser welding

The effect of vacuum on weld penetration and porosity formation was investigated in high-power cw CO2 and YAG laser welding. It was consequently confirmed in welding with both lasers that the penetration was slightly deeper in aluminum alloys and austenitic stainless steel with a decrease in the ambient pressure. It was also revealed that no porosity was present in the materials welded at lower pressures. The reason for no porosity formation in vacuum was examined by observing keyhole behavior, bubble and porosity formation situation, and liquid flow in the molten pool during high-power YAG laser welding under various conditions through the microfocused x-ray real-time observation system. It was confirmed in the coaxial Ar or He shielding gas that a lot of bubbles were generated near the bottom part of the molten pool from the tip of a fluctuated keyhole and resulted in large pores. On the other hand, under the vacuum conditions, no bubbles were formed in the melt pool from the keyhole, although the middl...

Cavitation erosion mechanism of NiTi coatings made by laser plasma hybrid spraying

Abstract NiTi intermetallic compound is known to be a high cavitation erosion-resistant material. The erosion behaviors of NiTi intermetallic compound coatings made by laser plasma hybrid spraying (LPHS) were studied. The erosion resistance of the NiTi coatings depended strongly on the chemical composition of the coatings and the laser irradiation condition, because the NiTi phase and the microstructure of the coatings were varied by those conditions. The best erosion-resistant coatings were obtained with the conditions of nickel composition richer than that of equiatomic NiTi and irradiated laser power density greater than 6×10 8 W/m 2 . The cavitation erosion resistance of the coating was about 380-fold that of a titanium alloy substrate. High erosion resistance of these coatings was obtained due to a combination of two factors, namely, super elasticity due to austenite-phase NiTi and high work hardenability due to excess soluble Ni.

Direct observation of keyhole behaviour during pulse modulated high-power Nd:YAG laser irradiation

When a high-power laser beam is irradiated on the surface of material, it is well known that a cavity, called a keyhole, is generated in the molten material. To examine the effect of laser beam pulse shape on the keyhole behaviour, we observed it directly with an in situ microfocused x-ray transmission imaging system. As a result, it was found that the keyhole began to be generated after about 0.6-0.7 ms and became deepest after about 1.5 ms from the laser irradiation initiation and collapsed about 1 ms after laser power declination when a pulse modulated laser beam of 1.1 ms rise time and 4.6 kW peak power was used. It was also understood that the generation and the collapse of the keyhole was repeated, synchronizing with laser beam irradiation of 100 Hz. The transition of the keyhole depth and the cross sectional area had good agreement with the transition of the irradiated beam shape. Moreover, it was observed that the porosity was frequently generated at the beam shape in which the laser power was decreased rapidly from a high peak power.

Problems and solutions in deep penetration laser welding

Abstract Keyhole dynamics in the high power continuous wave laser welding of aluminium alloys, in particular the role of keyhole instability in porosity formation, is discussed on the basis of high speed X-radiography observations. Motion in the weld pool was observed by introducing fine tungsten particles into the weld pool. A depression which moved periodically up and down the back wall of the keyhole gives rise to porosity, through bubbles formed at the bottom of the keyhole. The depression appears to be related to non-uniform evaporation on the front wall of the keyhole; both the keyhole and the weld pool are strongly disturbed by the dynamic pressure of the metallic vapour jet. The characteristic spherical and elongated pores were found to contain predominantly metal vapour and entrained shielding gas. Keyhole fluctuation in continuous wave laser welding can be suppressed by controlled pulse modulation, provided a suitable pulse frequency and duty cycle are selected.

Porosity formation mechanism and its prevention in laser welding

During laser welding intense evaporation develops from the molten pool surface due to the high power density at the focusing point and the molten pool surface is deeply indented by the reaction force; the welding process proceeds with a deep hole, commonly called a keyhole. The formation of this type of keyhole makes feasible deep penetration welding with a narrow bead width and these are the special features of high energy density power source welding. The penetrated hole on the liquid surface is inherently unstable and its instability increases with increasing ratio of the hole diameter to the depth, in other words, the aspect ratio. In keyhole laser welding, the penetration depth increases with increasing laser output when the welding speed is constant; simultaneously, it is known that, during this process, there is a frequent occurrence of porosity (also called as cavity gas pocket) which is a characteristic of laser welding. However, the initiation mechanism of this has not been identified. Conventionally, the keyhole formed during CW laser welding is considered to be relatively stable and all the mathematical models concerning the keyhole welding phenomena have been constructed with the assumption that the evaporative reaction force, hydrostatic pressure and surface tension pressure (Laplace pressure) are in equilibrium under the quasi-steady state. However, it is extremely difficult to explain porosity formation using such an assumption. From the results of research concerning the occurrence of porosity in laser welding and its suppression, the authors have come to believe that the distinctive porosity formation during welding with pulsed and continuous lasers is caused by the unstable phenomena of the keyhole. Consequently, an attempt was made to observe directly unstable phenomena of keyhole. At present, there are numerous examples of investigation regarding the plasma behaviour and its characteristics during laser welding; however, there is no paper describing a comprehensive investigation into the relationship between the plasma behaviour and the keyhole dynamics from the viewpoint of solving the mechanism for the occurrence of defects and the correlation between the related emission spectrum and the acoustic spectrum. The following describes the results of observation of keyhole dynamics using equipment with a high time/ place resolving power and porosity formation mechanisms.