Kevin Pagano, segment engineering manager at Miller Electric and Deryck Hart, director of automotive NA, Hobart Brothers Company, discuss welding equipment advancements
In automotive applications, galvanised steel has become an increasingly viable option for manufacturers aiming to build lighter-weight vehicles capable of offering greater gas mileage without sacrificing quality or safety. Due to the protective layer of zinc oxide on its surface, the material offers excellent corrosion resistance as well as high strength, even at thinner gauges.
Many automotive manufacturers use galvanised steel in the 1.6-4mm range for components such as frames, engine cradles and suspension links, and use even thinner measurements for other areas of the vehicle. In fact, it is not uncommon to find components like body skins as thin as 0.7mm.
However, the most positive attributes of galvanised steel are also the very things which make the material so challenging to weld. Its thinness increases the possibility of burn-through, while the zinc oxide coating contributes to defects when welded incorrectly. Should these issues arise, manufacturers risk sacrificing quality and productivity, which can adversely affect their competitive edge and their bottom line.
Recent advancements in filler metal technology as well as welding equipment are now providing automotive manufacturers with new ways to address these challenges and to improve their overall galvanised steel welding operations.
Most automotive manufacturers use one of three types of galvanised steel: hot-dipped, galvannealed, or electrogalvanised. The hot-dipped galvanised steels tend to be the most challenging to weld but are also the most prevalent in the industry due to their lower cost. As a result of the specific galvanising process, this material tends to have an uneven surface thickness, which makes the completion of consistent welds particularly challenging.
To obtain its zinc oxide finish, the material passes through molten zinc at high temperatures (over 427°C or 800°F), after which the zinc undergoes a series of chemical reactions, for example with oxygen and carbon dioxide. The result is the tough zinc oxide surface which provides a protective barrier and acts as a sacrificial anode for corrosion resistance.
Most automotive manufacturers use the gas metal arc welding (GMAW or MIG/MAG) process, either pulsed or constant voltage (CV), with a solid wire for welding hotdipped galvanised steels. However, both processes have proven difficult when it comes to gaining consistency in weld quality at similar travel speeds as those used to weld mild steel.
Spatter is one troublesome issue that arises and is typically the result of the shorter arc lengths associated with CV modes of welding. Yet porosity is by far a greater problem and the travel speeds used during the welding process directly impact this weld defect.
The faster the travel speed on hot-dipped galvanised steel, the faster the weld pool tends to freeze. This is especially problematic because zinc vaporises at a much lower temperature than steel. The temperature differentiation can lead to gas pockets becoming trapped because the weld solidifies before the zinc gas can escape. In this situation, the welds may have a good outward appearance but significant subsurface porosity.
In some cases, the porosity appears as small pockets, but it can often span the entire length of a weld joint in the form of a defect referred to as ‘piping’ or ‘worm tracking’. In other cases, there is the potential for porosity to create a linear path which can lead to an ‘unzippering’ effect during cyclic loading.
At certain levels, porosity or worm tracking is acceptable. The range varies according to each manufacturer, but is typically based on American Welding Society (AWS) specifications. As a rule, individual instances of porosity need to be separated by at least their own diameter and the total length of porosity (sum of diameters) should not exceed 6.4mm in any 25mm of weld. The maximum diameter of any instance of porosity should also not exceed 1.6mm. Internal porosity is generally limited to less than 25% of the area being inspected. If a defect detected in the subsurface of the weld exceeds the allowable limits for a given component, it leads to rework or scrapping of those parts.
To compensate for these issues, automotive manufacturers often lower travel speeds. While this solution may address quality in a positive way, it can also affect productivity negatively. Slower travel speeds mean fewer welds and lower net throughput. It can also affect capital outlay and real estate requirements, as well as shift production bottlenecks.
One of the ways in which automotive manufacturers can more readily address the challenges of welding hot-dipped galvanised steel is to pair their current pulsed MIG welding process with metal-cored wires designed specifically for this material. While this combination offers marked improvements over the speed and performance of solid wire, it is also slower than pairing metal-cored wires with a power source offering an optimised pulsed MIG/MAG process designed for welding galvanised steel.
Metal-cored wires are a type of tubular wire consisting of a metal sheath filled with metallic powders, alloys and arc stabilisers. As opposed to solid wire, metal-cored wires carry higher current densities (at equivalent amperage settings), making it possible to put more weld metal in a joint in less time during the welding process. Due to the fast travel speeds these wires offer, they are often chosen for robotic welding applications like those found in automotive manufacturing.
Recent advancements in metal-cored wires, specifically those carrying the AWS classification E70C-GS, provide significant advantages when welding hot-dipped galvanised steel. These wires feature formulations which allow them to weld with a direct current electrode negative (DCEN) or straight polarity.
Operating in straight polarity offers two distinct advantages when welding this material with metal-cored wire. These are:
- A softer arc penetration that prevents burn-through on thinner gauged material and creates an improved penetration profile
- Sufficient arc energy to vaporise the galvanised zinc coating. This action allows enough time for zinc vapours to outgas from the weld pool to drastically minimise porosity in the subsurface of the weld and on its surface.
Like other metal-cored wires, these also feature arc stabilisers which help to improve metal transfer from the wire to the weld, effectively reducing spatter and necessitating little to no post-weld cleaning which could slow down the overall throughput. The manner in which the wires also mitigate zinc vaporisation further prevents arc instability that could lead to poor weld quality, rework or rejected parts.
The pulsed MIG waveform process optimises these results by aptly controlling the deposition of the wire across the arc at controlled intervals in the welding cycle. The power source switches welding output rapidly between a high peak current and a low background current to create the pulsing effect. The peak current then pinches off spray-transfer droplets and propels them toward the work piece. The combination of this action with the formulation of specific E70C-GS metal-cored wires for galvanised steels helps to control the pace at which the weld pool freezes, allowing the zinc vapours to escape more readily. In addition, the pulsed MIG process generates lower heat input than the standard CV process used to weld with solid wire, making it less likely to burn through the thinner material.
Other benefits of combining metal-cored wires for galvanised steels with the pulsed MIG process include:
- Improved T-joint and downhill welding
- A fine ball transfer that creates a broad arc pattern and wide weld bead with good gap-bridging abilities
- The ability to weld a range of material thicknesses (1.2-4.0mm)
- The ability to weld in multiple positions.
The combination of the two technologies has also proven beneficial in addressing the variable surface thickness of hotdipped galvanised steels, adjusting for that unevenness and still providing quality welds that are strong yet ductile. These additional features can benefit the automotive industry as manufacturers continue to seek a means to improve their welding operations and gain a competitive edge.