This article focuses on the many aspects of gas selection in laser welding techniques, including laser beam interaction, protection efficiency, bead performance, and equipment used to deliver standard gas mixtures and flows.
There is an urgent need for speed in the highly competitive automotive market. From the consumer side, they are more concerned about horsepower. However, in manufacturing, speed is the total of production and productivity. American automakers are gradually losing market share for a number of reasons, including body design, cognitive quality, and cost of ownership.
Although the discussion of body design is not considered in this paper, strategies to improve quality and productivity are the focus of discussion. Both can be achieved by a hybrid processing technique that combines laser welding with conventional metal gas arc welding (GMAW) for welding.
Laser parameters such as wavelength, beam quality, spot size, power density, depth of focus, and beam positioning are critical to successful soldering. Other parameters include routine replenishment and pulse transfer of the GMAW energy source, positioning of the GMAW wire, angle of contact, and chemical properties of the wire. In addition, the surface condition of the base material oxide, the design of the joint, the width of the weld, and the type and flow of shielding gas also have an impact on the quality and performance of the hybrid welding process.
The following is a detailed description of the effects of gas selection on many aspects, including laser beam interaction, protection efficiency, bead performance, and equipment used to deliver standard gas mixtures and flows.
Hybrid laser processing combines a secondary energy source into the weld pool area. Hybrid processing technology has made the advantages of laser welding specific. These advantages include improved welding speed, limited heat-affected areas, narrow welded joints and a superior weld bead shape. As a secondary energy source, GMAW improves the overall processing energy efficiency, reduces the cost of equipment and improves the ability to weld gaps. In addition, it reduces the cooling rate and improves the energy coupling efficiency of aluminum.
Secondly, although the equipment is more complicated, by reducing the size of the cavity required for welding, the energy cost of the GMAW is reduced, thereby reducing the cost of the entire machine. Depending on the desired result, the GMAW wire feed position can be determined before or after the laser beam. A higher welding speed can be achieved by means of a trailing GMAW wire feed. The GMAW wire is fed into the molten pool produced by the laser so that the secondary energy required to melt the wire is reduced.
In addition, when the filler wire reaches the tail, the arc of the GMAW generates a plasma that evaporates the substrate material, thereby creating a depression at the front edge of the weld pool. This depression in the molten weld pool reduces the total depth that the laser beam must penetrate, thereby improving penetration performance.
It has been well documented that vapor particles ejected from keyholes or weld areas cause attenuation (scattering and absorption) of the laser beam, thereby reducing the beam energy coupled to the substrate material. 1 The scattering and absorption of the laser beam reduces the speed and depth of the weld. 2 The glue layer determines that the larger the particle, the more severe the attenuation effect.
The æ°¦ shielding gas brings the smallest average vapor particle size. This shows that pure tantalum is the best choice for controlling particle size for CO2 or YAG laser welding. We must admit that helium does have a higher ionization rate and a lower plasma formation voltage than argon, but its molecular weight is small. Therefore, the helium shielding gas requires a large flow rate to ensure efficient discharge of metal vapor from the laser beam path. Since the unit cost of helium is higher than argon, this increases the average cost per foot during the welding process.
In order to optimize the shielding gas to suppress the plasma, to vent the vapor particles and to reduce the unit cost, we consider using up to 40-50% of the argon mixed gas. The higher the specific gravity, the smaller the flow rate required for the mixed gas to discharge the vapor particles. The mixed gas also provides a longer inert atmosphere during the curing of the weld pool, resulting in a higher welding speed. It also reduces the amount of trapped gas, thereby reducing the scrap rate due to porosity.
Secondly, the reduction in the curing rate promotes the growth of crystal grains and the reduction of internal stress, which increases the fatigue strength. Since the aspect ratio (weld depth/width) is high and the weld cracks generated by the subsequent stress are almost eliminated, the addition of the GMAW filler metal results in an increase in the width of the weld face.
Appropriate addition of a small amount of carbon dioxide and/or oxygen in the mixed gas or as a secondary protective gas for the GMAW process can further improve the performance of the bead. The helium-argon mixed gas tends to produce a higher arc voltage, and the resulting bead has a wider profile and higher arc stability.
Therefore, 3-10% of carbon dioxide can be added to stabilize the transfer and contraction of the arc. In some cases, 1-5% oxygen can be added to achieve superior arc stability while achieving better bonding (wetting) at the weld edge. Compared with carbon dioxide mixed gas, oxygen has a higher ionization rate and higher thermal conductivity performance, and it is easy to provide a wide and shallow penetration distribution.
After the finalization of the mixed gases for the required quality and productivity standards, it is also necessary to consider how to economically transport them to the place of use. Users can use a low-cost liquid argon supply by mixing these shielding gases at the production site. Why not pay for argon, carbon dioxide or oxygen without paying the price of a premixed high pressure helium cylinder?
Argon can be economically transported through a liquid argon bottle to meet monthly consumption of up to 35,000 cubic feet, which is equivalent to 87,500 cubic feet of mixed gas per month. If the monthly consumption of argon is greater, batch supply can be used to optimize the cost level. The analysis also needs to take into account the factors such as fill loss, monthly equipment costs, contractual restrictions on bulk supply, and shipping costs.
On the other hand, helium is usually supplied through a high pressure Tube Trailer or a cylinder group. On-site mixing requires a mixing system that accurately regulates tiny components from 0-100%. The total mass system can be monitored by placing an analyzer at the outlet of the mixer and alerting once the mixing ratio is out of tolerance. Existing software and alarm systems can transfer this information to a desktop computer or send it to a farther place by fax or email.
A properly designed hybrid laser gas delivery system allows the user to achieve higher welding speeds and correspondingly higher productivity. Focusing on the parameters of the shielding gas, such as type, flow, and impact angle, will improve the quality of the weld and reduce the beam absorption and scattering effects.
Continuous development of auxiliary welding technology, combining methods such as GMAW and laser technology, enables users to take advantage of both technologies and benefit from them.
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