New developments in advanced welding
Nd:YAG laser welding tips: process development
5.6.1 Check solid solubility of the major constituents
This is not a final factor in determining feasibility but a quick check is worthwhile. Almost all materials are alloys, i. e. combinations of many metals,
so looking at each constituent is not practical or worthwhile. If certain metals are required, try them at the prototype level.
5.6.2 Are these alloys being welded by another process?
If the metals to be laser welded cannot be welded by processes such as tungsten inert gas (TIG), they might not be good candidates for laser welding. Check with a welding engineer or someone with a considerable degree of experience with the alloys in question.
5.6.3 Check any platings and coatings
The platings and coatings include phosphorus in electrodeless nickel plating, galvanising, lead and tin in solder coatings, oxide coatings, carburised surfaces, and paint or anodised coatings. Any coating can cause a problem - phosphorus, lead and tin lead to the formation of brittle intermetallics, oxide coatings cause weak welds and porosity, the high carbon content of carburised surfaces results in brittle steels and organic contaminants from paint cause porosity and other problems. Zinc with its low boiling point can cause metal loss and porosity in galvanised steels. If unsure of the effect of platings and coatings, weld the samples without the coatings to determine their effects. Be careful of variations in coatings and platings from batch to batch.
5.6.4 Is the fit-up acceptable?
Laser welding is usually performed without any filler metals so gaps must be filled with metal from the adjacent area and bridging a gap requires extra laser energy. Generally, a gap no more than 10% of the thickness of the thinnest component is allowed. This can be relaxed for thicker materials greater than 1 or 2 mm but might need to be reduced to 5 % or lower for materials less than 0.2 mm. Gap problems show themselves as very concave weld beads or failure to bridge the gap between parts. It is also much more difficult to start a laser weld in a large gap area in comparison with seam welds through a short section of high gap area where bridging can sometimes be maintained.
5.6.5 What is the throughput requirement?
High throughput welding jobs usually require multiple laser sources and/or the very high welding speeds of CW lasers. Fast spot welding is best done with pulsed lasers. Optimise the laser welding process to meet all requirements of strength, distortion, heat input, etc. After the weld process has been developed throughput decisions can be made, such as the fastest laser source, automation and special beam delivery that will not compromise weld and part quality.
5.6.6 Is there a total heat input requirement?
Only pulsed lasers can produce welds using extremely low average power no matter what the penetration. Are there glass to metal seals, plastic, or electronic components with a maximum temperature rating? Lasers have much lower heat input than most other welding technologies so supermodulated CW sources might have sufficiently low heat input.
5.6.7 Is there a beam delivery constraint?
Nd:YAG lasers, both CW and pulsed, can be delivered with fibres or conventional mirror delivery. Long focal length lens requirements can favour mirror-based beam delivery but usually a design can be produced for either beam delivery. Talk with a laser applications engineer if the standard beam delivery systems will not work.
5.6.8 What type of weld joint is required?
Lasers can produce butt, lap, and fillet joints. Know the benefits and drawbacks of each and try to design the weld joint to make the best use of the technologies.
Look for intermetallic forming constituents from platings or free-machining alloys. If there are dissimilar materials, weld each material separately as a bead-on-plate test. The material that cracks in this test is usually the crack contributor. If both materials weld without any problems, look for solid solubility problems or high stress in the weld zone such as that found with fillet welds or poor joint fit-up. Try to have material certificates for each metal in use, each time it is used, especially in the prototype stages; the prototype builder may use incorrect alloys.
Look for high carbon constituents that produce brittle phases at the heat affected zone (HAZ) in iron alloys. Steels such as 1035-1070 or stainless steels such as 430-440 have this problem. Try using a CW laser for high carbon content steels or a pre-heat to reduce post weld cooling. Some super alloys also exhibit this type of cracking due to precipitation hardening in the HAZ. Hastelloy 713C and waspalloy exhibit this type of cracking.
5.6.11 Extensive cracking throughout the weld?
Some alloy combinations will show cracking throughout the weld, both centreline and transverse cracking, as well as cracks that move into the parent metal. This is very common for high-strength steels and for some aluminium alloys. A pre-heat is required for the high-strength steels to eliminate the problem. For aluminium alloys the addition of another aluminium alloy at the weld joint such as 4047 or a 5000 series alloy is required to overcome the cracking. Some 300 series stainless steels generally considered very easy to weld will show this problem and it is due to the very high cooling rate of pulsed laser welding and a slight variation to the nominal alloy combination or chromium and nickel and their ‘equivalents’. Moving to a CW laser or pulse shaping will help with this but good material control is the best method.
5.6.12 Pinholes and/or random cracking?
Look for very low melting-point contaminants such as lead, zinc, tin, or other organics such as solvents, plastics, oils and fibres. Some plating contains organic brighteners that contribute the volatile constituents. Re-welding over the affected area can reseal over the holes since many of the volatile components are boiled out of the melt puddle on the first pass and the second pass will seal the unit. Initial part cleanliness and good housekeeping are the best solutions here.
Check with a welding engineer or other welding expert. Try other processes to determine if they also show the same effect to try to determine potential causes. Talk with material suppliers concerning best practices with their alloys and any issues with storage, heat treatment, or condition of the materials you have.
