Laser welding of 6061-T6 crack sensitive aluminium based alloy with pulse shaping

Pulsed laser material processing requires the optimization of many parameters depending on the thermo-physical properties of the material, the environment, laser and its process parameters, i.e. peak power, pulse duration, pulse energy, pulse repetition rate, power density and temporal pulse shaping. Pulse shaping works by combining the normal single welding sector (main sector) with sectors of lower peak power. This will slowly reduce the laser energy going into weld nugget, allowing slow cooling (Figure 1).

Figure 1: Example of pulse shape (Ramp down or cool down pulse)

Shaping the pulse can greatly affect the weld quality (i.e. cracking, porosity, etc.). The pulse shaping can also be optimized to improve the metallurgical properties and also has impact on the cosmetic appearance. Pulse shape is very beneficial, when welding:

• High carbon steels
• Crack sensitive alloys
• Dissimilar melting points materials
• Coated materials
• Painted materials
• Contaminated materials
• Powder metallurgy parts
• Highly reflectivity materials

Aluminium alloys

• Strain-hardened alloys: Most of these alloys can be laser welded autogenously. Sometimes filler metal is added during laser welding to improve strength and ductility.
• Heat-treatable alloys: These alloys are susceptible to hot cracking during welding, due both to their chemical composition and thermal strains induced in the metal during welding. To avoid hot cracking, a filler metal can be used to adjust the weld bead and careful joint and fixture design will help minimize thermal strains.

Table 1: Main aluminium based alloys

Table 2: Type of aluminium based alloys

Although most are considered weldable, some aluminium alloys are susceptible to weld metal or heat affected zone cracking, especially the case for both 2xxx and 6xxx series alloys. This cracking can be reduced or eliminated by the addition of the correct filler wire during welding, which reduces the freezing range of the weld metal and minimizes the tendency for solidification cracking.

6xxx series aluminium alloy

Aerospace packages (Figure 2) for microwave circuits, sensor mounts, or small-ordinance imitators are the most common examples of aluminium components that require laser welding. Aluminium alloy type 6061(AL-Mg-Si) is the material of choice because of economics, rigidity, and ease of machining. However, the material cannot be successfully laser welded to itself because the partially solidified melt zone cannot withstand the stress of shrinkage upon solidifying and cracks are formed (termed “SOLDIFIACTION CRACKING” or “HOT CRACKING”). The solution to this problem is to improve the ductility of the weld metal by using aluminum with high silicon-content such as alloy 4047 (Al 12% Si). This alloy is very ductile and difficult to machine into small complex shapes. Therefore, 6061 is usually employed as the package component with intricate features and 4047 is used as a simple lid that is relatively thin (typically less than 1mm). A 4047 ribbon can be inserted between 6061 components to produce excellent welds (Figure 3), but this requires a very labor-intensive step, unless round washers or other simple perform geometries can be employed.

Figure 2: Example of aluminium electronic package

Figure 3: Weld cross-section of 4047 to 6061, Weld penetration >1 mm

A technique, which is often advocated when joining 6061 aluminium alloy autogenously for electronic packages or any other applications by laser welding, involves pulse shaping.

At Prima Power Laserdyne, we have recently undertaken detailed laser welding work to develop pulse shapes to weld a range of crack sensitive aluminium alloys including 6xxx (Al-Mg-Si). Welding tests were carried out with 1 mm thick 6061-T6 aluminium alloy (Table 3 and Figure 4). Some of the results from the findings are highlighted in in the following sections.

Table 3: Chemical composition of 6061-T6 aluminium alloy (wt. %)

Figure 4: 6061-T6 base metal showing insoluble (Fe, Cr)3 SiAl12 and excess soluble Mg2Si particles (dark); Keller’s etchant

Generally, weld cracking in aluminum alloys can occur as a result of the aluminum relative high thermal expansion, large change in volume upon solidification, and wide solidification temperature range. The crack sensitive aluminium alloys (6xxx series) are known to be highly susceptible to weld cracks. This is also true with other conventional welding processes. The mechanism of crack occurrence in the laser welding is considered to be similar to that of the arc welding. However, severity of the weld metal cracking in the case of laser welding is much less than that in the case of arc welding due to the lower heat input with laser welding.

Figure 5 shows typical optical micrograph of the fusion boundary of 6061 aluminium alloy for the pulse shapes investigated. The most noticeable feature is the narrow heat-affected zone (HAZ) width, which was less than 2mm. This is related mainly to the low-heat input with pulsed laser welding that resulted in narrow weld zone and subsequently narrow HAZ.

Figure 5: Optical micrograph of fusion boundary of 6061 butt-welded joint; 5m/min

Optical microscopic examinations of the weld metal made with conventional pulse shape (Figure 6) exhibit severe solidification cracking. The micrograph revealed a cellular dendritic structure with an equiaxed zone formation along the centerline of the weld as shown in Figure 7a and Figure 7b for the 6061 alloy. Cracks were observed along the boundaries of the equiaxed zone formed along the weld centerline, believed to be solidification cracks. These cracks were observed only in weld metal while both HAZ and base material were free from cracking.

Figure 6: Conventional pulse shape from a QCW fiber laser

Figure 7a: boundary cracks in 6061-T6 aluminum alloy

Figure 7b: Optical micrograph of weld metal of 6061 aluminium joint made with conventional pulse shape

The welding tests carried out with pulse shaping (Figure 8) showed it was possible to eliminate solidification cracking in 6061-T6 aluminium alloy (Figure 9). Two sector pulse shape gave better control of the cooling rate during the welding, hence minimizing the tendency for solidification cracking. Further optimization of both laser parameters and welding speed is needed to reduce the top and bottom bead undercut respectively.

Figure 8: 2 sector ramp- down pulse shape

Figure 9: Optical micrographs of weld metal of 6061 aluminium joint made with 2 sector ramp- down pulse shape

The detailed study of laser welding with pulse shaping has shown that using temporal pulse shaping, it possible to laser weld 6061-T6 aluminum alloy without the need for filler material. It may be possible to use a similar type of pulse shape, i.e. either by decreasing or increasing the ramp-down gradient of the laser pulse power after the main welding pulse sector, to weld 2xxx (Al-Cu) aluminium based alloys, which are also prone to cracking and require filler material to eliminate cracking.

The future work on laser welding with pulse shaping will include:

• Devolve pulse shapes to weld
  • High carbon steel
  • Dissimilar materials
• Develop pulse shapes to improve the weld quality / mechanical properties
  • Reduce/eliminate porosity
  • Weld geometry