Understanding some of the metalurgical considerations associated with various metals aids in the laser welding process
Laser beam welding is a fusion joining process that uses the energy from a laser beam to melt and subsequently crystallize a metal, resulting in a bond between parts. Laser beam welding can be successfully used to join many metals to themselves as well as to dissimilar metals. Main applications are related to welding steels, titanium, and nickel alloys.
Laser welding of steels
Low-carbon steels are readily laser weldable provided that sulfur and phosphorus levels are kept below 0.04%. A higher content can promote solidification cracking. In low-carbon steel the welding zone is martensitic and exhibits increased hardness, the level of which depends upon carbon and alloying elements contents. The structure of the heat-affected zone depends upon initial material state. In the case of annealed (normalized) condition there is fine martensite in the place of former pearlite grains and conserved ferrite colonies, appearing as if austenite transformation occurs separately in these two locations. This is due to the typically short thermal cycle of laser welding and lack of time for carbon diffusion to occur throughout the whole material. However, time is enough for carbon redistribution to proceed in pearlite that results in martensite formation with eutectoid carbon content upon cooling.
The heat-affected zone in steel after standard heat treatment (quenching and tempering) consists of two areas: a re-quenching zone and a tempering zone. In the first area tempered martensite undergoes reverse martensite transformation when heating up and martensite transformation upon cooling that results in increased hardness. In the tempering zone, temperature is not high enough for austenization, and additional tempering proceeds.
Knowing these metallurgical features
is essential for choosing correct welding parameters and materials. At Alabama
Figure 1. Laser welded lever shaft assembly composed of two steels.
As carbon content in steel increases, martensite created in the weld zone, as well as in heat-affected zone, becomes more brittle. Reduced toughness combined with a high level of residual stress may lead to cracking in the weld or heat-affected zone, and also have an adverse effect on weld embrittlement and other alloying elements.
When the carbon equivalent is less than 0.4, steel is readily weldable, but when it exceeds this value, special precautions should be taken, for example, pre-heating and/or use of filler material to change the metallurgy of the weld. Often pre-heating is a preferred method, because it affects not only the weld zone but also the heat-affected zone. Pre-heating can be performed with the use of an induction heater, hot air blast, or auxiliary laser beam. Pre-heating reduces the cooling rate, resulting in a formation of different phases other than martensite (bainite, pearlite) that eliminates cracking. There are no ready-to-use recipes on pre-heating temperature and dwell time. They are dependent on the material to be welded, part geometry, and mass. Proper choice of pre-heating and welding regimes requires experimentation.
Austenitic stainless steels are good candidates for laser welding if their sulfur and phosphor contents are kept low. Specific laser beam welding features, such as high welding speed, small heat-affected zone, and low material exposure at elevated temperatures are beneficial for welding this type of steel, because prolonged periods at high temperatures may lead to chromium precipitation on grain boundaries and reduction of corrosion resistance. Alabama Laser has a great deal of experience in laser welding different grades of austenitic stainless steel: 304, 316L, 309, which are used for corrosion probes.
Martensitic stainless steels produce relatively brittle welds due to their high carbon content. To avoid cracking, the same measures used for high carbon steels may be performed; pre-heating and use of filler material (in particular, austenitic wire or powder).
Alabama Laser has designed a precision micro-wire feeder that assures full control over the wire feeding process. When utilizing wire feeding, the wire diameter should be smaller than the size of the weld pool (1 mm) and directed precisely into the weld. The correct wire feed rate is determined by the gap, wire cross section, and required welding speed.
Normal practice is to deliver the wire at a 45° angle, intersecting the surface slightly ahead of the laser beam thus preventing excessive spatter.
Powder feeding has an advantage because more alloys are available than with wire. However, obtaining a constant mass flow rate of powder over time is a problem. Selection of the method of material delivery to the weld joint depends upon the specific task to be performed and available resources.
Nickel and its alloys are widely used in industry due to their corrosion, heat, and creep resistance. There are two major types of nickel alloys, solid solution and precipitation hardenable.
The solid solution type is readily weldable with conventional methods as well as with laser welding. Alabama Laser has welded a number of corrosion probes made of solid-solution-type alloys (Monel 400, Inconel 600, 825) as well as heat exchanger fin assemblies of Monel 400.
The precipitation-hardenable type is generally difficult to weld due to cracking. Strain-age cracking can occur under some combinations of temperature and stresses in the heat-affected zone during or after welding. Alloys of the Nb-Al-Ti system are generally less susceptible to this type of cracking.
Some tips on laser welding of precipitation hardenable alloys include proper fit-up and attention to start and closeout of the weld. If post-welding heat treatment is necessary, weldments should be annealed first. Also helpful are a good inert gas shield and a convex bead shape.
Titanium alloys are widely used in welded structures due to their high specific strength and corrosion resistance. The main difficulties in welding titanium alloys are high reactivity with gases at elevated temperatures, especially in liquid state, that produce weld embrittlement, and rapid grain growth at elevated temperatures. The second drawback is easily overcome with laser beam welding, because high welding speed and temperature gradients lead to a short material exposure to elevated temperatures in a narrow heat-affected zone and suppressed grain growth.
Special precautions should be taken to protect the titanium weld area from the ambient atmosphere, including the use of welding grade argon or argon-helium mixture as a shield gas and the use of trailing and back-up shields. The function of trailing and back-up shield coverage is to shroud the weld and adjacent hot material with inert gas until the surface temperature cools to 300°-400°C. Dendrite structure of the weld zone in a cross section is a good indication of improper shielding.
The weld structure of titanium alloys consists generally of a’ martensite, which differs from martensite in steels by its higher toughness and lower susceptibility to cracking. Short thermal cycles in laser welding accounts for smaller a’ martensite needles as compared to arc welding and, consequently, enhanced mechanical properties. A distinctive feature of laser beam welding of titanium alloys is a very small heat-affected zone. This is due to a low diffusion rate of a’ stabilizing elements as compared to a carbon diffusion rate in steels.
Another factor, which has a major impact on mechanical properties of the weld, is its chemical composition. A number of publications indicate the superiority of laser welding over the electron beam and arc welding with regard to fatigue strength. Laser welding is considered superior because of its typical short welding cycle, which causes little or no evaporation of the alloying elements.
When welding dissimilar metals certain considerations with regard to their properties must be taken into account; difference in melting point, heat conductivity, reflectivity, possible formation of brittle phases, and wettability. To achieve better welding performance the laser beam can be shifted towards the material with higher melting point, heat conductivity, and reflectivity.
A number of projects related to laser welding of dissimilar metal have been accomplished at Alabama Laser. The most significant is the laser welding of wear-resistant nose tips to steel guide bars of chain saw blades for Sandvik Windsor Corporation (see Figure 2). Previously an oxyacetylene process was used to deposit a layer of Stellite 6 on the surface of the steel. This process was labor intensive, hazardous, led to excessive waste of expensive Stellite 6, and required extensive post-welding machining operations. Laser welding of Stellite 6 tips to steel bars produced sound welds. Laser welding has many benefits; the manufacturing environment is safer because of significant reduction of subsequent grinding, productivity is increased by eliminating the need to pre-heat and post-weld cool, the fusion bonding achieved with laser welding between the Stellite and the steel bar has excellent resistance to mechanical and thermal shock.
Figure 2. Wear-resistant alloy welded to chain saw blade.
Laser beam welding opens up many opportunities for designing and economically joining machine parts. Various branches of industry, which use laser beam welding include electronics, automotive, and food processing.
Don Johnson, Wayne Penn, Sergey Bushik