New developments in advanced welding

New areas of research in laser welding

6.3.1 Laser welding of steels, Zn-coated steels and

stainless steels

Low carbon steel sheets of approximately 3 mm or less in thickness are subjected to tailored blank welding with CW lasers. In order to achieve high formability and stamping ability of a steel weld, or in the use of low-carbon sheets and wires, a slight increase in heat input by lower-speed laser welding or hybrid welding is adopted. A beam shaping, scanning or spinning, and/or high speed laser welding could be used. These methods can suppress or narrow a hard weld as well as minimizing or eliminating an underfilled level of the weld bead surface.40

In the welding of steels with a high content of carbon, attention is paid to the occurrence of solidification cracking, and sometimes, cold (hydrogen embrittlement) cracking and the formation of hard, brittle, fragile martensite and/or cementite. It is noted in Japan that even mild steels with extremely fine grains (of about 1 mm in diameter) are hard and strong due to the microstructure-hardening effect, according to the Hall-Petch equation (s = so + k/d1/2; where s = yield stress; d = grain diameter; and so and к = constants). They are equivalent to the properties of high tensile strength (HT) steels.41,42 However, the problem is that the heat-affected zone (HAZ) of weld beads becomes soft due to the coarsening of grains and disappearance of strain hardening. Consequently, rapid cooling during laser welding will maintain the mechanical properties of the HAZ by the suppression of grain coarsening and the formation of hard martensite.41,42 Laser welding of HT steels is also under investigation for such items as cars and pipes worldwide.43,44 In HT steels, the low temperature toughness of weld joints can be maintained
by decreasing the hardnesses of weld beads and the HAZ. It should be noted that it is rather difficult to evaluate the toughness of a welded joint with an extremely narrow hardening zone.

Zn-coated steels are used in industry because of low prices and higher corrosion resistance. Sound laser weld beads with good surface appearance are easily produced in lap welding with a correct gap (about 0.1 mm depending upon the Zn layer thickness45) as well as in butt-joint welding.46 In the case of a lap joint with a wide gap (for example, 0.5 mm for 1 mm thick sheets47), weld beads are formed separately in the upper and lower sheets. Thus the control of a gap or its absence are desirable for the production of a sound weld. It is, however, known that evaporated Zn causes spatters or porosity easily in laser lap welding of steel sheets with a rather thick Zn-coated layer without a gap.45 The formation and characteristics of weld beads were investigated using ultra-high speed video and microfocused X-ray transmission imaging system and monitoring signals, as shown in Fig. 6.15.46,47 In welding of lapped joints without a gap between sheets at low speeds, some bubbles of Zn vapors come into the molten pools from the peripheral lapped part of the HAZ, resulting in large pores or wormholes. On the other hand, at high welding speeds in sheets without a gap, spattering occurs easily, resulting in heavily underfilled weld beads. It has been reported that sound weld beads can be produced in laser lap welding without a gap under the following conditions: with an elongated beam,43,48 by properly tilted beam irradiation at a high power,49 under the irradiation conditions of optimum pulse width and repetition,46 by using Cu insert sheet50 equivalent to the use of Cu-Si wire in laser blazing,11,12 or by the selection of the hybrid welding with a laser arc.51,52 It is important to reduce the harmful effect of Zn vapor affecting the melt in the molten pool.

Steels are generally welded in an inert gas shielding. It has been shown that in the laser welding of normal carbon and HT steels with high-affinity alloying elements with oxygen CO2 gas shielding is more effective in reducing porosity than is Ar or He gas shielding.53 Also, austenitic stainless steels can be welded where the shielding gases are Ar, He, N2, or a mixture of these.19,20,54 Pores are easily formed in a deeply penetrated weld bead made with a CO2 or YAG laser with Ar or He shielding but are almost eliminated in N2.20,54 In - situ observation of X-ray transmission reveals that the generation of bubbles can be suppressed and some bubbles can shrink and disappear in the molten pool in N2 shielding gas.20 The reason for reduced porosity is attributed to the higher solubility of N in the molten pool and the easier combination of N with Cr vapor during welding. The N content in the weld fusion zone is higher with a CO2 laser than with a YAG laser due to the formation of N plasma.54 Solidification cracks are absent in YAG laser welds but are present along grain boundaries of the austenite phase under some conditions with a CO2 laser in N2 gas. Attention should be paid to the use of N2 gas in CO2

6.15 Schematic presentation of characteristic laser weld bead formation and welding phenomena of Zn-coated steel sheets.

laser welding of austenitic stainless steels. This is because the absorbed N is such a strong austenitizing element that the solidification process varies from the primary solidification of the ferrite phase to that of austenite phase; consequently, microsegregation of P and S along grain boundaries increases to lower the solidification temperature and to widen the solidification brittleness temperature range (BTR) leading to enhanced cracking sensitivity.54

In pulsed YAG laser spot welding of stainless steels, the microstructure at room temperature, i. e. the ferrite-to-austenite phase ratio of the weld metals, is quite different from that of normal TIG weld fusion zones, as shown in Schaeffler diagram in Fig. 6.16.55,56 For example, although TIG weld fusion zones show duplex microstructure with about 5 % and 30 % residual delta(S)- ferrite content, the weld metals produced with a pulsed YAG laser with several ms irradiation duration generally exhibit almost fully austenitic and ferritic microstructure, respectively. This is interpreted in terms of rapid solidification and the subsequent rapid cooling.55,56 In the case of austenitic stainless steels producing TIG weld metals in AF mode (the primary austenite and subsequent eutectic or peritectic ferrite solidification process), in pulsed or high-speed CW laser weld metals, a fully austenitic microstructure is formed. This is caused either by the primary solidification of the austenite phase without subsequent transformation,55 or by primary ferrite solidification with a reduced level of microsegregation due to the rapid solidification

effect and the complete solid-state transformation from the ferrite to the austenite during subsequent rapid cooling.56 There is a possibility of solidification cracking only in the case of the primary solidification of the austenite phase.

6.3.2 Laser welding of aluminum or magnesium alloys

Aluminum and magnesium alloys have received much attention due to their light weight, attractive surface appearance, and other suitable properties. They are widely used in many industries - such as electrical, electronics and transportation.

Laser welding of aluminum alloys is generally difficult because of high light reflectivity (low coupling efficiency), high thermal diffusivity (conductivity), easier formation of welding defects such as porosity in deeply penetrated welds and hot cracking in pulsed spot welds.21,22,24 Melting is enhanced by utilizing a high power density laser or by forming a AlN phase in N2 shielding gas during CO2 laser welding. 57,58 Moreover, deeper penetration can be obtained in those alloys with a larger content of volatile elements, such as Mg, Zn and Li, as shown in Fig. 6.17.59 However, porosity can easily occur in aluminum alloy weld metals with a higher content of magnesium. In the case of high power laser welding of wrought aluminum alloys, many bubbles are generated from the bottom tip of a keyhole, resulting in the

formation of porosity. In aluminum alloys more bubbles are generated from a keyhole tip and float upwards depending on the melt flows in the molten pool. Gas constituents inside the large pores in A5083 alloy welded with a CO2 and with a YAG laser under given conditions were analyzed with a drilling Q-mass system in a vacuum and the results are summarized in Table 6.2.60 The shielding gas is mainly present in the pores and both hydrogen gas and nitrogen are detected in the pore atmosphere. The hydrogen content is high in the CO2 laser welds and increases with time before analysis takes place; hydrogen must invade the pores by diffusion during and after welding. It is therefore concluded that bubbles leading to porosity are formed by intense evaporation at the keyhole front near the bottom of the molten pool and the shielding gas is entrained into the keyhole and bubbles result.

In an inert shielding gas with a small amount of H2, a great number of small pores are formed in aluminum alloy weld beads made with a CO2 laser. Therefore, the use of a pure inert gas and the polishing of the plate surface, where the oxygen and hydrogen content are normally high, can decrease small-sized porosity caused by hydrogen. Several procedures such as correct pulse modulation, using moderate power density, twin laser beams, hybrid welding process and full penetration welding can reduce porosity in weld fusion zones of wrought aluminum alloys.60 The formation mechanisms and porosity prevention are the same for magnesium wrought alloys. In some cases porosity occurs so easily that it is difficult to reduce or eliminate it. Such cases include the laser welding of casting, die-casting, thixomolding and working with powder metallurgy products of aluminum and magnesium alloys. These often show small-sized porosity, blowholes, hydrogen or oxygen gas-enriched areas and other such features.

The susceptibility to hot cracking such as solidification cracking in the weld metal and liquation cracking in the HAZ can be expressed as a function of alloy content, as indicated in Fig. 6.18.60 Laser welding processes have a marked effect on a tendency towards hot cracking. Solidification cracking occurs easily in pulsed YAG laser spot weld metals and in CW laser high­speed weld beads of aluminum and magnesium alloys, while weld beads

without cracks can be produced in welding of 2 or 3 mm thick sheets with a CW laser. These are interpreted through the formation of a wider solid- liquid mushy zone and the rapid loading rates of augmented tensile strains to grain boundaries.

The mechanical properties, such as tensile and fatigue strength, of aluminum alloy joints are chiefly affected and degraded by the size of large pores, cracks, and the degree of underfilling. Furthermore, softening of HAZ due to annealing and overaging phenomena during welding can also decrease the mechanical properties of work-hardening and age-hardening materials. 22,61,62