In filler metal qualification tests where argon/oxygen shielding gas mixtures are used, why and how do you leave some wiggle room in contracts to make adjustments as the situation warrants, instead of stipulating how welding will take place with a particular blend?
Argon-oxygen shielding gas mixtures are frequently used during fabrication of low-alloy, carbon steels. These blends of shielding gases also are used when conducting conformance tests for low-alloy, ER90S-D2 solid wire MIG electrodes to the requirements of AWS A5.28, Specification for Low-Alloy Steel Electrodes and Rods for Gas Shielded Arc Welding. Specifically, this specification calls for an argon shielding gas that is comprised of one percent to five percent oxygen.
The goal of this column is not to promote the merits of this type of shielding gas or debate over whether or not they should be selected over other blends such as argon/CO2. Rather, I want to focus on situations that could arise during filler metal qualification tests when argon/oxygen shielding gas mixtures are used and implementation of solutions that will aide in meeting requirements.
But, first, I would be remiss if I did not provide a little background on why anyone would want to use argon/oxygen shielding gas. One advantage of this type of shielding gas is that the addition of oxygen to argon shielding gas lowers the spray transition current in the gas metal arc welding process, which allows spray arc transfer at lower average output currents than argon/CO2 mixtures.
The reason for this reduction in transition current that has been given in some texts is that oxygen decreases the surface tension of the molten metal on the end of the solid wire. This would then make the molten droplets from the electrode detach more easily, resulting in a finer droplet spray and a more stable arc. Furthermore, oxygen reduces the surface tension of the weld pool and allows it to flow more easily to areas of higher surface tension. (Mother Nature likes to reduce surface tension whenever it can.)
In the AWS A5.28 specification, test conditions for solid electrodes to be used during filler metal qualification testing are provided. These test conditions are to be implemented, for example, when a product lot of a solid MIG wire, such as SuperArc® LA-90, is being tested for conformance. In the table showing the test conditions, the welding parameter ranges to be followed for both .045 in and 1/16 in electrodes are outlined. For solid wires with diameters other than .045 in and 1/16 in, the test conditions are to be agreed upon between the purchaser and supplier of the welding filler material.
Let’s now discuss a situation where a particular testing lab has written a welding procedure specification (WPS) for a .045 in ER90S-D2 electrode around the test conditions provided in AWS A5.28. Although ranges have been given in AWS A5.28 for all of the welding parameters, this particular testing lab has decided to fix the wire feed speed in the WPS at a point value of 450 ipm in order to take the variability out of the process.
In conjunction with this wire feed speed, the WPS specifies a contact-tip-to-work distance range of 7/8 in to 1 in. What this essentially means is that a welder will naturally tend towards using a 7/8 in CTWD in order to increase the output current and maximize puddle control while welding in the flat position.
A third constraint is that the travel speed range is permitted to be 11 ipm to 15 ipm. So with a deposition rate of over 12 lb per hour (determined mathematically at a wire feed speed of 450 ipm using a .045 in diameter solid wire), the weld bead size approaches the equivalent of a 5/16 in fillet. As I’ve detailed in the past, a considerable amount of operator skill is required to deposit a weld nugget of this size.
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