October 26, 2011
Your customer wants two pieces of metal welded together. Why should you bother with a welding procedure specification? Because conformity leads to quality and enhances safety.
The challenge for any welding operation is to make the weld the same each time. No doubt you can weld it once, but can you describe all the critical steps in enough detail for another welder to follow your procedures? The intent of any welding procedure qualification record (PQR) is to identify those variables that control the quality of the weld and may have an effect on the metal surrounding the weld. From these records, you can develop a welding procedure specification (WPS).
The list of variables goes on and on, but here are some important details it includes:
The type of metal you are welding likely determines the type of power you will use. It also affects your choices of welding method. The metal’s thickness range determines the best electrode diameter.
Current should be set based on the electrode size, workpiece thickness, type of metal transfer from the electrode, and the electrode feed speed. At lower currents metal is transferred by short circuiting. As current increases, the transfer changes to globular transfer, and then can become a spray of metal droplets.
For thin titanium, imagine you are welding with a GMAW process and then transfer that setup to GTAW. The challenge is to be consistent in feeding the filler next to the tungsten electrode. The rate should be as consistent as possible. Filler metal can cool the weld pool if it is fed too quickly. As the electrode or filler metal wire size decreases, the variation in feed rate becomes less important. This make sense; if you are adding less metal per minute to the weld pool, there should be less temperature change within the pool.
Bead size is related to the size of the electrode. Smaller beads need smaller wires.
Higher current means higher speeds, just as long as you can keep pace in a manual process. Automated processes can use that as an advantage, feeding more wire more quickly than a manual process.
Time is money, so if the machine can feed faster than you can, it may make sense to consider automating the welding process. If you are overheating the part and it begins to warp—stop. Skip welding may be a way to reduce the amount of distortion the weld creates in your part.
Voltage varies as the arc length increases, but there are other effects. When voltage increases, penetration decreases, and the bead width increases. The type of metal transfer may also change from short circuiting to globular to spray transfer. Shielding gas may affect the voltage and change the transfer as well.
In manual welding, keeping the electrode extension distance consistent from welder to welder and day to day may be one of the more challenging tasks in a production shop. Working in a comfortable, supportable, ergonomic work position goes a long way toward achieving consistent-quality welds.
Welding technique, forehand or backhand, can affect the weld bead width. When the electrode is angled in the direction of welding, that is forehand. When the electrode or gun is opposite to the direction of welding, the position is backhand. Usually the angle is between 10 and 20 degrees of vertical. Backhand welding produces a narrower bead and a stable arc with a convex bead and deeper penetration. Forehand welding produces a wider bead with less penetration. A vertical welding position is used most often with automated welding. GTAW of titanium with a filler wire can be done with the filler in the forehand position and the torch in the backhand.
Titanium needs pure argon as a cover.
Shielding gas prevents oxygen, nitrogen, and hydrogen from reaching the weld pool. Oxides, nitrides, and carbides are combinations of metal and gases. These compounds are ceramic in character—hard and brittle with very low fracture resistance when compared to the metals that make up these compounds. Titanium can pick up the carbon from a shielding gas containing carbon dioxide. Pure titanium needs pure argon as a cover.
Hydrogen acts differently. It dissolves at high temperatures easily. When the metal cools and the hydrogen can no longer remain dissolved in the metal, it can form bubbles in the same way carbonated beverages respond when you shake the can and open the container quickly. The difference between your favorite beverage and the metal is that the bubbles can move in the liquid and not in the metal. Extreme pressure is created within these minute bubbles, causing microscopic cracking to relieve the pressure. Hydrogen relief consists of keeping the metal at a high enough temperature to allow the hydrogen to diffuse out of the metal. How hot and how long the metal needs to cook depends on the metal and the amount of hydrogen that has been absorbed. One thing is consistent - the longer the delay between the absorption and the relief cycle, the more likely the cracking will be sufficient to affect the fatigue strength.
Short circuiting or dip transfer is fine for thin sheets. Globular or spray transfer can work for automated welds, although short-circuit transfer is used as well.
Electrode extension is part of the process, too, and when the electrode is too close, the shielding gas can become restricted.
An extension that is too long creates a different set of problems. Arc instability, shallow penetration, and a poorly formed weld bead may be signs the electrode is extended too far.
Spatter can build up on the nozzle, creating a swirling gas flow and mixing the gas with the air around the nozzle. Porosity usually is a sign of this type of gas contamination. Some specifications require cleaning/inspecting the nozzle on a recurring schedule.
Some parts need cooling to prevent the final assembly from being warped. Water cooling is used in the extreme, but sometimes an air cool or fan is specified. Some procedures will specify the length of bead that can be allowed (skip welding) and using the part as its own heat sink can be effective in reducing part distortion.
Welding near a braze joint may require special weld preparation. It is important to remove all of the braze before beginning the weld. Those characteristics that make a metal a good filler for brazing also make them a bad filler for welding. Weld-brazing or braze-welding is not the same as welding over a braze, which is never a good idea.
Since the days of Vasily Petrov, who discovered in 1802 that welding could be performed with an electric arc, welders have needed to control and duplicate the welding process. ASME, AWS, and other countries’ welding societies developed procedures to capture and record all of the pertinent information. Quality control tests such as tensile testing, impact testing, metallographic analysis, nondestructive penetrant testing, and guided bend testing might all be a part of qualifying a welding process.
A PQR should be a part of any new welding procedure in your shop. If a problem subsequently occurs with the welds, these records provide a baseline for how the welding procedure was initially developed. Specifications are a part of almost every manufacturing process. An effective WPS can be developed only from the information gathered during the qualification procedure.