Aluminum alloys are used in a wide range of industrial applications because of their low density and good structural properties. The aluminum alloys can be divided into two main groups: cast and wrought alloys and further sub-divided into alloy series, as shown in Table 1.
Table 1: Classification of aluminum alloys
|Cast aluminum alloys||Wrought aluminum alloys|
|4- digit alloy series||Main alloying elements||4- digit alloy series||Main alloying elements|
|1xxx||Pure (99%)||1xxx||Pure (99%)|
|3xxx||Silicon, Copper, Magnesium||3xxx||Manganese|
|7xxx||Zinc||6xxx||Magnesium & silicon|
|8xxx||Others||8xxx||Others e.g. Lithium|
Laser welding is a key joining technology because it can offer distinct advantages over conventional joining techniques, such as TIG, MIG, resistance spot welding, mechanical fastening, and adhesive bonding.
There are two laser welding mechanisms: (1) keyhole and (2) conduction. Keyhole welding (Figure 1) is widely used because it produces welds with high aspect ratios and narrow heat affected zones.
|Figure 1: Keyhole mode welding; 5xxx series aluminum alloy; 3 kW average power|
Conduction welds are usually produced using low-power focused laser beams in applications requiring relatively shallow penetration and low aspect ratio (Figure 2). For example, conduction welding is used to lap weld thin sheets for which the strength of the joint is proportional to the diameter of the weld at the interface of the two materials.
|Figure 2: Conduction mode welding; 5xxx series aluminum alloy; 400 W average power|
Challenges To Laser Welding Can Be Overcome
The main challenges associated with laser welding of aluminum alloys are high reflectivity of the material to the laser wavelength, high thermal conductivity, and the volatility of low boiling point constituents of a number of the alloys. These and other material related problems can lead to weld quality issues including weld and heat affected zone cracking; weld porosity; degradation in the mechanical properties; and inconsistent welding performance.
Detailed experimental studies performed at Prima Power Laserdyne have demonstrated capability for welding a range of aluminum alloys both with and without filler material. These studies have also involved welding of various dissimilar material combinations used in the aerospace and automotive industries.
Welding trials involved both continuous wave (CW) and Quasi Continuous Wave (QCW) fiber lasers. Laser and processing parameters involving a range of laser power, continuous wave and pulsed output, power density, welding speed, and others process variables were used. The resulting welds were evaluated using metallography and mechanical tests
The result was a range of laser and processing parameters that produce good quality welds for different aluminum alloys.
Welding Aluminum Alloys To Other Alloys
The challenge in joining aluminum with other metals and alloys is greater, though worth the effort given the number of opportunities for joining dissimilar metal components. In particular, welding dissimilar metals and alloys is challenging because of the differences is physical and chemical properties, such as the melting and boiling points, thermal conductivity, density and coefficient of expansion.
These differences often lead to the formation of brittle intermetallics phases, which are detrimental to the mechanical strength and ductility of the welded joints.
One of the dissimilar material combinations that was tested involved titanium and aluminum. The demand for dissimilar metal joints of titanium to aluminum alloy has arisen in industry, especially in the transportation vehicle industry. However, it is well known that fusion welding of titanium to aluminum alloy is very difficult in part because the crystalline structures of titanium and aluminum are different. There are also major differences in the physical properties important in welding (for example, melting point, thermal conductivity, and coefficient of thermal expansion).
Figure 3 shows a photomacrograph of the weld between a Ti alloy (T-6Al-4V) (top) and 3003 aluminum alloy (bottom) made using 1.7 kW CW fiber laser power. It is apparent from the weld profile that the weld is wide though penetration into the lower aluminum sheet is small.
|Figure 3: Photomacrograph of weld joint between Ti (top) and Al (bottom) alloys.||Figure 4: The root of the weld joint between Ti and Al alloys.|
Figure 4 shows a close-up view of the area of the weld in which the two materials are mixed. At the root of the weld, a zone approximately 150 μm wide where aluminum had melted but not mixed with the remainder of the weld pool is apparent. The interface between the mixed molten metal and the melted Al was ‘fluffy’ with a swirl patterns indicating where there was variable mixing of the melted sheets.
The Ti-Al binary equilibrium phase diagram (Figure 5) highlights that on the Al-rich side of the weld, there is virtually no solid solution of titanium and formation of various intermetallic compounds such as Ti3Al, TiAl, TiAl2 and TiAl3. These intermetallics compounds are very hard and brittle at room temperature and often can lead to very poor tensile strength of the welded joint.
Work is in progress to develop approaches to reducing the formation of intermetallic compounds at the interface. Look for an update in a future issue of the newsletter.
|Figure 5: Ti-Al binary equilibrium phase diagram|