The rapid pace of technological advancement for industrial robots, welding equipment and feature-rich software has revolutionized the welding industry, meeting challenges head on for a variety of manufacturing applications. From hollow-arm robots and robust positioner options to dynamic wire feed systems and more, these assets add the flexibility needed to handle unique workpieces as well as the ability to accommodate certain items, such as dissimilar metals.
Not surprisingly, these technology upgrades have prompted the perfection or creation of welding methods to successfully execute a range of applications for manufacturers across the globe. Proven to work, here are the most popular welding processes to consider for robotic systems.
Widely used on a large variety of materials, arc welding brings high quality and consistency to the components produced. This method yields extremely strong, continuous fusion of the weld joint, and the consumables used can be adjusted to the material type. There are also many power source process programs that match AC or DC voltage and current settings to alter the arc transfer with an ideal electrical waveform for the material type and thickness being welded. Overall, the high-quality equipment needed is relatively cost effective and easy to service.
Gas tungsten arc welding (GTAW) uses a non-consumable tungsten electrode to conduct current while the filler metal is introduced into the arc to the material being welded. It is usually associated with being a “precision” process.
When used, filler metal is fed into the puddle externally, while manual operators skillfully balance the melting of base material with the addition of filler metal that results in defined solidification lines. This forms a “ripple” in the bead, and is often referred to as the “layered penny” or “stacked dime” look. This form of welding can also be used to form autogenous welds where the base metal of the part is sufficient to create the weld joint, and no additional filler metal is introduced.
Gas metal arc welding (GMAW) uses a wire electrode that also acts as the filler metal to form the weld deposit, allowing for fast travel speed and providing a process tolerance for robotics that can be maintained at plus or minus half the wire diameter. Capable of welding materials that are less than 1 mm thick, weld cosmetics for GMAW are generally accepted as a smooth, uniform bead shape with solidification lines that are not sharply defined. This process cannot be used for autogenous welds as the weld relies on the filler metal as the acting electrode.
In recent years, the capability to combine the consistency and quality of the GMAW production process with the cosmetics of the GTAW weld appearance has benefitted aluminum welding. Widely popular with automobile manufacturers for lightweighting, the use of aluminum alloys reduces vehicle component weight to better improve fuel economy.
Another popular robotic welding method frequently found in the automotive industry is resistance (spot) welding. This method is accomplished by passing an electrical current between layers of metal while applying pressure with no filler metal being added. This creates a strong metal bond called a “nugget” at the point of execution. While a simple procedure in nature, it can be more advantageous than arc welding for certain applications, providing attractive benefits when it comes to:
- Cost effectiveness – the lack of filler metal or shielding gas can rapidly reduce the cost per weld and weight.
- Safety – minimal sparks and spatter as well as no arc flash to reduce help to simplify safeguarding, saving on costs.
- Cleanliness – spot welded parts are dual-layered, essentially hiding the weld within the body of the part, while providing consistent and clean aesthetics.
- Consistency – lower heat input lends itself to less part deformation, and parts have very high repeatability. There is also no filler metal metallurgy to consider, making the process relatively similar for dissimilar parts.
- Fabrication time – not using filler metal and lower heat input, among other variables, can facilitate fast torch travel speeds and cycle times with the potential to reduce fabrication time nearly in half for certain parts. A typical spot weld squeeze only takes a brief partial second, allowing a robot to move from spot to spot very quickly.
Spot welding is well-suited for thin, stamped steel or aluminum parts in high volumes. The process can easily accommodate a variety of gun sizes and styles for different parts, and the use of servo-motor guns provides consistent force and welds. For many manufacturers, it is an ideal automation tool to keep in the proverbial tool bag for increasing competitive edge.
For rotary spot welding, single-spot welding can be modified to implement a variation that uses wheel-like electrodes to run along a continuous line for a longer seam-type weld. This is ideal for parts that need to be liquid-tight, such as radiators or steel drums.
Projection welding is another popular type of resistance welding. Physical projections from the part help control the flow of the arc, and multiple projections can be processed in a single cycle. Often used for spot welding nuts or studs onto sheets for adding a threaded anchor point, projection welding is ideal for stamped parts.
Capable of creating strong, repeatable weld seams at a relatively high speed with remarkable precision, robotic laser welding is implemented for greater productivity on shop floors throughout the Americas, giving manufacturers the ability to weld materials that were once seen as non-weldable. Ideal for medium- to large-volume production runs, this process uses a focused laser beam to provide precision heat input and to target weld seams. A good choice for a wide selection of metals with varying thickness, robotic laser welding can be performed in one of several ways.
Heat conduction welding: Used with modulated or pulsed lasers (less than 500 W), heat conduction welding entails heating metal above its melting point while avoiding vaporization. Simply put, the laser beam moves after the weld puddle is created. Faster than traditional arc welding processes, this method is ideal for producing smooth, aesthetic welds that require minimum weld strength.
The most common form of heat conduction welding is remote laser welding (RLW), which fuses two thin sheets of metal via the use of a contact-free laser welding technique. The laser head utilizes a longer standoff of 100 mm to 150 mm for the beam to move from the head to the point of focus on the part. Lap welds are usually used as little to no gap is the target and a light-tight enclosure is required.
Deep penetration welding (keyhole welding): Used with high-power lasers (greater than 105 W per mm2), this process entails melting and heating metal until it vaporizes, leaving a deep and narrow “keyhole.” As the laser beam travels along the weld path, molten metal flows around the keyhole-like opening and solidifies in the seam.
The use of a laser seam stepper is ideal for large parts requiring extended reach. This highly precise method uses a laser inside a “picker” or C-gun, where the laser head comes down to the part before the C-gun applies pressure, creating vertical fusion between two pieces of metal. This high-speed process does not require a light-tight enclosure and efficiently reduces cycle time. Tight fitting and reliable lap welds are used.
Most scanner-based RLW applications and fixed-position RLW tasks use keyhole-type welds, as well.
Occasionally, keyhole welding is combined with an arc welding process like GMAW, GTAW or submerged arc welding (SAW). In doing so, keyhole welding creates very deep and narrow joints that have a higher depth-to-width ratio than standard arc welds.
The process serves to improve weld tolerances as well as porosity and cracking issues. Fillet and butt welds are typically used.
Joining dissimilar metals
Aside from laser welding, there are several other popular options for the robotic welding of mixed metals that manufacturers may find more beneficial, especially when it comes to cost savings and cycle time.
Flow drilling is best for joining dissimilar sheet metals or extruded parts. This very clean, single-sided process utilizes a spinning screw to generate heat and friction during the drilling process. This melts the base metals and uses the screw as filler metal, resulting in high shear and pullout strength.
Friction stir welding (FSW) is an innovative solution for joining alloys or dissimilar metals from 0.5 mm to 65 mm thick. FSW is a solid-state joining process (where the metal is not melted) that entails a spinning router applying pressure and friction to fuse metals. Proven effective on electric vehicle and aerospace components that necessitate superior weld strength, this process excels at continuous weld seams, and it requires minimal consumables and no filler metal.
Utilizing a traditional GMAW- or GTAW-like process, adhesives or brazing typically employs silicone bronze filler metal to braze materials, creating a metallurgical bond between two dissimilar metals, including dissimilar steels or steel with aluminum. A low melting point filler metal is flowed onto the base part or joint without distortion. Often used to fill voids or to bond or strengthen parts, adhesive or brazing is ideal for parts with deep gaps.
Friction element joining is a newer technique. This method enables the joining of lightweight materials, like aluminum, with ultra-high-strength steel. Ideal for demanding joining processes found with automotive structural parts, this method uses a steel friction element to penetrate the upper layer (e.g., aluminum), then welds onto the base plate (e.g., steel).
The friction, which is generated by appropriate process control and the application of mechanical forces, acts directly on the friction element. The produced frictional heat acts on the element and the base plate without reaching the melting temperature. An adhesive bond is only created between the friction element and the base plate.
Self-pierce riveting is a cost-effective method for joining low-carbon steels, brass, aluminum and stainless steels. This process is excellent for metal-to-metal fastening applications. Self-piercing rivets punch their own holes through harder materials and are clinched in one operation, thus eliminating the cost of pre-drilling or pre-punching holes.
From new material requests to challenging design specifications, this arsenal of robotic welding processes gives manufacturers the ability to accommodate evolving customer demands. While the optimal solution for a given task depends on compatibility, cost and cycle time, the effective use of any of these processes has the potential to increase throughput and product quality.