The Hot Spot
Addressing challenges of resistance welding on aluminum
The use of aluminum has been rapidly growing and has made its way into a myriad of metalworking processes. It’s been applied in aerospace, which requires complex assemblies and ultra-tight tolerances. And it’s become key in the automotive industries' ongoing quest to lightweight vehicles for improved fuel efficiencies. In fact, the average automobile rolling off production lines today requires about 6,000 spot welds according to Tom Snow, chairman of the board for Tennessee-based T.J. Snow, a supplier of resistance welding machinery, supplies and service.
“It’s a very important process to the automotive industry. And now there’s a move toward making hoods and other larger components from aluminum in order to reduce more weight,” says Snow. “Traditionally aluminum is hard to resistance weld, but it’s not impossible. For manufacturers that are used to welding mild steel or are transitioning to welding aluminum, it becomes challenging because the plastic range of the material is much narrower than the plastic range of a comparable thickness of mild steel. So it requires a more controllable process.”
Resistance spot welding may seem like a simple process—it’s basically applying a current through two pieces of metal to fuse them together. But depending on the material type and thickness, there are very precise parameters and conditions that must be upheld to ensure repeatability and consistency. To dial in a quality spot weld, it’s all about the pressure applied between the upper and lower electrodes, the quality of the current used, the time it’s applied and the shape of the electrode used.
“Aluminum is problematic for a number of reasons. It takes a great deal of current, far more than steel. And the alloys of aluminum vary so much and it has a lower resistivity,” says Tim Brady, Amada’s product manager of combination punch and laser machines, and former product manager of Amada’s fabrication equipment division, which includes resistance welding technology. “The surface condition is also very important. Sometimes when fabricating parts, the surface could have an oxide layer or be dirty. It’s extremely critical that the aluminum surface be clean and smooth.”
As fabrication moves more towards using exotic materials and complex resistance welding processes, welding suppliers are introducing more advanced controls and providing inverter-style mid-frequency direct current welding equipment.
“Benefits of mid-frequency direct current (MFDC) controllers include that the electrical energy is delivered to the welding gun more efficiently, producing less magnetic losses and electrical energy is drawn from all three phases, unlike the older AC (alternating current) equipment,” says Ted Jorin, Lincoln Electric’s segment specialist.
When welding with alternating current (AC), during each 60 hz. cycle the weld current passes through zero creating a cooling effect that is not beneficial to producing a spot weld in aluminum. “MFDC controllers produce electrical current that goes up to a programmed peak level for a programmed duration and you avoid the cooling effect,” explains Jorin.
Brady agrees. “If you’re talking about aluminum, it almost exclusively requires the inverter style power supply. It’s just a superior way to get the current to the material. You have to be able to control the amount of current, the time that you have the current going through the material and the pressure that you’re squeezing the material together with. Those are the three main things that need to be controlled carefully to get a consistent spot weld.”
Aluminum is a good electrical conductor so it causes challenges with resistance welding.
“Resistance welding is based on the resistance to current flow,” says Jorin. “By going with a mid-frequency with DC, you can deliver hotter energy more consistently than passing through zero. Secondly, with steel you can use multiple pulses and cycle multiple times. You can start building a weld nugget in steel, stop for a cycle and then hit it again to grow that nugget and make it larger. With aluminum, once you establish an initial nugget and get a decent current path, it’s very hard to grow it any bigger because the material now conducts even better.”
Snow points out that steel has a wide plastic range, and several combinations of the three main machine settings —pressure, current and time—can usually make an acceptable weld.
“But when you’re spot welding aluminum you really need to zero in on the optimum settings to do the job. A lot of applications require a weld to not only be strong, but also look good, so aluminum has more stringent requirements.”
The melting temperature of steel versus aluminum is also very different. “Aluminum has a shallow plastic zone compared to steel. It will go to liquid much faster after it reaches plastic and can have a tendency to squeeze out of the joint and spurt out in the form of spatter, which is called expulsion,” says Jorin. “It’s more difficult to weld. The range of parameters is tighter with aluminum because you have to get it to that stage where it starts to melt, but you don’t want it melting so much that it expulses from your weld joint.”
Another consideration is with the electrode shape, of which there are many possibilities, and the condition of the tip. Brady says there are customers that have to dress their electrodes as often as every ten spots to get consistent welds. “Types of electrodes vary and as you weld it will get residue on it that will start to affect the weld,” he says. “Aluminum does have a problem with electrodes sticking to the material. That’s why cleaning is so important. When welding, some of the aluminum can start to collect on the electrode and if you’re not set up right, that part can actually get stuck on the tip and you’ll have to break it off the electrode. It’s a common struggle. But you need to dress the electrode properly. And some use a conductive tape or material that will allow the metals to bond together.”
Recent advancements in controls and monitoring have allowed users to set and save parameters that greatly help with maintaining consistency.
“One of the features of our new control for our ID40IV welder is that it actually monitors the current and voltage, which is how you would derive the resistivity of the weld. You can do a test weld and once you have the conditions the way you want it, the controls will monitor each spot weld. If the conditions or resistivity vary outside of a certain threshold, the system will alert the operator that the electrode is getting dirty or there’s a problem with the material,” says Brady.
The ID40IV also features a camera that will document setups to show what electrodes were used and how they were set on the machine. It saves this data for future reference, and operators can change or add welds using the touch screen control. “It will highlight the spots for the operator so they know exactly where they need to go for each spot and they can easily cycle through a part,” says Brady.
Lincoln Electric, which designs and builds custom systems for resistance welding, also offers advanced controls that monitor how much energy is being delivered and can dynamically adjust themselves to hold a particular level that is set. There are also built-in sensors to alert the operator if the tip needs to be cleaned.
“The controllers will monitor the amount of voltage being delivered to the spot weld gun and look at the current flow that voltage is creating. Many will also have a force transducer to document how much pressure is being used,” says Jorin. “If you monitor all of those and that remains consistent during the weld, you’ve delivered the appropriate amount of energy. If the voltage and current are off, it will alert the operator that the weld did not achieve the appropriate parameters and it needs to be checked.”
Snow says that if customers aren’t getting the results they want with resistance aluminum welding, they often revert to other processes such as using rivets.
“There’s no better, quicker, cheaper or stronger way to blend materials than resistance welding, but it has to be done properly. One of the challenges is being able to look at a resistance weld, ensure that it’s been made properly and the weld strength is there.”
Today’s advancements in controls have clearly made the process easier for resistance welders. But as Snow says, if you’re not a skilled welder, you won’t be able to set the machines up properly.
Knowledge and skill still reign supreme for this widely used welding process, especially if you’re dialing in on alloys such as aluminum. SMT