Laser welding has been widely used in the automotive, aerospace, electronic and heavy manufacturing industries to join a variety of materials. In the automotive industry, high-power lasers are used to weld many components such as transmissions, mufflers, catalytic converters, exhaust systems, and tailor-welded blanks.
However, a number of defects, such as porosity, surface holes, irregular beads, undercuts, humping, and solidification cracking, are often found in laser welds. Industrial laser users are always looking for economical methods to improve weld quality and relax the strict fit up requirement for work pieces.
Laser surface treatment techniques, including laser surface melting (LSM) and laser surface alloying (LSA), have been the subject of considerable interest as a means of enhancing the corrosion performance of aluminum and its alloys. Microstructural modification together with the incorporation of non-equilibrium concentrations of alloying elements resulting from relatively rapid rates of cooling compared with conventional surface treatment techniques provide the basis for property enhancement. Hereby the microstructural evolution in a range of laser surface treated aluminum alloys including LSM of Al-Cu, Al-Si, Al-Zn, Al-Fe and Al-transitional element alloy systems; LSA of Al-Ni, Al-Cr and Al-Mo is considered. It is shown that surface alloys with unique microstructural and compositional characteristics have been produced by these techniques and that in many cases promising improvements in hardness and critical pitting potential compared with conventional alloys.
Laser surface melting (LSM) with a CO2 laser can produce a rapidly solidified surface layer on metals and alloys since the action of the laser is limited to melting to a depth of a few hundred microns and the remainder of the material acts as an effective heat sink; the surface material resolidifies under a relatively high rate of cooling. This can modify both the microstructure and the distribution of the alloying elements in the melted layer and hence affect surface-related properties such as corrosion and wear resistance. This has particular potential in the treatment of aluminum alloys, since the added alloying elements which are essential for the enhancement of strength compared with unalloyed aluminum invariably lower the resistance of the metal to corrosion.
Aluminum-Copper Alloys (2xxx Series)
These alloys require solution heat treatment to obtain optimum properties; in the solution heat-treated condition, mechanical properties are similar to, and sometimes exceed, those of low-carbon steel. In some instances, precipitation heat treatment (aging) is employed to further increase mechanical properties. This treatment increases yield strength, with attendant loss in elongation; its effect on tensile strength is not as great.
The alloys in the 2xxx series do not have as good corrosion resistance as most other aluminum alloys, and under certain conditions they may be subject to intergranular corrosion. Alloys in the 2xxx series are good when some strength at moderate temperatures is desired. These alloys have limited weldability, but some alloys in this series have superior machinability.
Research work has been carried out on pure Al and Al-Cu binary alloys to characterize the modification of the surface microstructure resulting from LSM. Resolidification of the melted layer has been shown to be epitaxial commencing with a very thin layer of planar front growth. For alloys containing less than 5 wt. % Cu, the growth after the planar front region has been reported as dendritic with the dendrites being α-Al and with either CuAl2 precipitates or CuAl2-Al eutectic as the interdendritic phase.
In addition to the above binary alloys, laser surface melting has also been carried out on 2024 Al alloy (Al-4.4wt.%Cu-1.5wt.%Mg-0.6wt.%Mn). Kim and Weinmann have studied the formation of porosity during solidification after melting with a microsecond Nd:Glass laser. More recently, however, Milewski et al. used a Nd:YAG laser for LSM and the resulting microstructures were similar to those in the work reported by Munitz. Noordhuis and De Hosson studied the nucleation of precipitates in this alloy following LSM with a 1.3 kW CO2 laser. Other work concerning Al-Cu alloys has involved eutectic, hypoeutectic and hypereutectic compositions.
Aluminum-Silicon Alloys (4xxx Series)
The major alloying element in 4xxx series alloys is silicon, which can be added in sufficient quantities (up to 12%) to cause substantial lowering of the melting range. For this reason, aluminum-silicon alloys are used in welding wire and as brazing alloys for joining aluminum, where a lower melting range than that of the base metal is required. The alloys containing appreciable amounts of silicon become dark gray to charcoal when anodic oxide finishes are applied and hence are in demand for architectural applications.
Hegge and De Hosson used a 1.5 kW CO2 laser to melt the surface of Al-Si alloys containing 4, 7, 12 or 20 wt. % Si. At scan velocities of 1-250 mm/s, the 4 and 7 wt. % Si alloys showed microstructures which were cellular at the bottom of the track which, on progression towards the free surface, became dendritic and finally cellular dendritic. There was an interdendritic eutectic phase of Si plates in Al.
At scan velocities of less than 5 mm/s the structure in the 12 wt. % Si alloys was totally eutectic, but between 5 and 50 mm/s the structure changed from eutectic to dendritic. A cellular dendritic microstructure was observed just below the surface which increased in depth with increasing scan rate up to 200 mm/s.
At scan velocities higher than 200 mm/s the microstructure of the entire melt depth was cellular dendritic. The resolidified structure of the fourth alloy, 20 wt. % Si, investigated by Hegge and De Hosson started dendritic, turned eutectic and finished as cellular for the whole range of scan velocities. They also reported that large areas of pure Si were found at the bottom of the melt pool in this alloy.
Aluminum-zinc alloys (7xxx Series)
Zinc, in amounts of 1 to 8% is the major alloying element in 7xxx series alloys, and when coupled with a smaller percentage of magnesium results in heat-treatable alloys of moderate to high strength. Usually other elements, such as copper and chromium, are also added in small quantities. Some 7xxx series alloys have been used in airframe structures, and other highly stressed parts. Higher strength 7xxx alloys exhibit reduced resistance to stress corrosion cracking and are often utilized in an overaged temper to provide better combinations of strength, corrosion resistance, and fracture toughness.
Lasek et al. and Synecek et al. have used CO2 lasers operating at 200-1400 W to melt the surface of Al-Zn alloys. In the heat affected zone (HAZ) of laser melted Al-30wt.% Zn, small spherical GP zones were seen to form in the partially melted grains. Epitaxial planar front solidification was seen to occur at the maximum melt depth, and the transition from planar front solidification to dendritic growth was preceded by a region of planar front growth that also contained solute enriched droplets that solidified and precipitated β-Zn spheroids on final cooling.
The presence of high residual stresses concentrated in the HAZ and at grain boundaries was also inferred by Lasek et al. because of the existence of shear bands in grains within the HAZ and also because of cracking along the grain boundaries into the melted region. As a result of grain boundary cracking, a high density of lattice defects near to grain boundaries was observed and this enabled the heterogeneous nucleation of equilibrium β-Zn particles during room temperature ageing.
Similar overall microstructures have been observed in Al-Fe alloys, containing less than 8 wt.% Fe to those reported previously for Al-Cu, Al-Si and Al-Zn alloys. In the investigation of intercellular precipitation, Carrard et al. found that the precipitates were quasi-crystalline in nature and also showed that they formed without any apparent orientation relationship with the α-Al cells. The size and morphology of the precipitates was found to be dependent on concentration and growth rate and they appeared as globules in some cases.
Other Surface Melted Aluminum Alloys: 3xxx Series and 5xxx Series
These alloys generally are non-heat treatable but have about 20% more strength than 1xxx series alloys. Because only a limited percentage of manganese (up to about 1.5%) can be effectively added to aluminum, manganese is used as a major element in only a few alloys. The major alloying element is magnesium and when it is used as a major alloying element or with manganese, the result is a moderate-to-high-strength work-hardenable alloy.
Magnesium is considerably more effective than manganese as a hardener, about 0.8% Mg being equal to 1.25% Mn, and it can be added in considerably higher quantities. Alloys in this series possess relatively good welding characteristics and relatively good resistance to corrosion in marine atmospheres. However, limitations should be placed on the amount of cold work and the operating temperatures permissible for the higher-magnesium alloys to avoid susceptibility to stress-corrosion cracking.
Mc Cafferty et al. have studied the effect of laser surface melting of Al 3003 (Al-1.2wt. % Mn) alloy. The melt depth was only about 10 μm and the microstructure was found to consist entirely of a banded structure alternating between pure Al and the MnAl6 phase. Juarez-Islas also studied laser surface melting of Al-Mn alloys, comparing the resulting microstructure with that from unidirectional solidification (UDS), and tungsten inert gas (TIG) weld traversing techniques. It was only possible to form extended α-Al solid solution without any microsegregation by laser surface melting techniques. The cell spacing of the α-Al dendrites was smaller in the LSM alloys than in the TIG welded and UDS samples, demonstrating that much higher growth rates can be achieved by LSM techniques.
Corrosion Properties of Laser Surface Treated Aluminum Alloys
Moore et al. studied the effect of surface modification by LSM using a CO2 laser on the pitting behavior of 2024 Al alloy but found no improvement in the pitting resistance because of cracks and pores that were present in the surface. Bonora et al. observed the effect of LSM on the corrosion behavior of pure Al using a Q-switched nanosecond pulsed ruby laser with energy density at the surface between 1 and 5J/cm2 giving an estimated cooling rate of the order of 1011 K/s. The pitting potential was unchanged after LSM but the current density was lower on anodic polarization under both potentiostatic and potentiodynamic control.