I've seen several other posts on this forum addressing aluminum porosity issues and I was looking to get some more information on what other possible causes there are. We weld aluminum piping - roughly 0.160" 2219 using the GTAW process. We are using DC current and 100% helium shielding gas. The material is abrasively cleaned and then chemically etched within 2" of the center of the weld joint. I think if we switch to VP that will give us some additional help because we will be getting a better cleaning action. However, I want to try to control all the other variables in the process to prevent porosity, such as material cleanliness, filler wire storage, equipment cleanliness, etc. Here are some of my questions..
1. Once aluminum is machined and ready to be sent to the welder for joint prep, how long can it sit before it starts absorbing contamination?
2. If the aluminum part cannot be welded within the time frame from question 1, how should it be stored? Should the part be in a low temperature oven?
3. Our current machine setup has a spool of aluminum filler wire that is somewhat exposed to airborne dust and contamination. However it does have the option to be purged with gas. This machine is not in an isolated area, and is close to an area where some grinding operations area. So, it's susceptible to contamination. Will purging this wire spool help push out some of this dust and contamination prior to being fed into the liner?
4. What is the proper way to prep the faying surface of the joint? After the edge is machined, we clean it and chemically etch it, but that's all we do. Is there a need to use a carbide or vixen file to dress the edge even more?
Thanks
We store some of our critical aluminum parts in argon boxes. They are basically a shelf closet with glass doors, rubber seals, and positive argon pressure on the inside.
I can't give a helpfull answer to #1 however the aluminum oxide starts to re-form immediatly after You prep it. I have read that by 24 hours it has formed an appreciable thickness and from that point the oxide coating thickness increases more slowly.
1 & 2. Like Dave said, the oxide layer begins to reform immediately in air after cleaning. An inert gas enclosure is one way of dealing with this, but absolutely do not put the material into an oven, especially if it's heat-treatable alloy.
3. Cover the spool. If the grinding operation is anything other than aluminum, isolate the entire area. Do not cross-contaminate tools. Take some time every shift to clean everything up.
4. Chemical etch followed by water rinse and scrubbing with a clean stainless wire brush (not rotary) should do fine.
I caution against using abrasives, especially those made from aluminum oxide. Particles of the abrasive grit can become embedded in the base metal, becoming impurities. I take it you are purging the root with helium as well, right?
I also am baffled by "VP"....what is it?
Good luck!
Thanks for the info.
3. The majority of all the grinding operations done in the facility are aluminum. However, we do have a tooling area in the facility that does a lot of structural steel work. So, it's possibly that the airborne metallic particles can make their way to our machine.
In regards to the abrasive cleaning, they use a scotch brite type abrasive wheel to break down the initial part of the oxide layer. Then they proceed with chemical etching and DI water rinse.
At this time we are not purging the root of the weld, however I believe we should be.
Currently we are just using a straight DC current, which is good for penetration but it's not getting a very good cleaning action. The machine we have has the ability to use the "variable polarity" mode. This is similar to a square wave power source, but it allows you to individually adjust your primary and reverse current and the amount of on/off time for each. If we used this we'd get the good cleaning action of a typical AC aluminum weld, but we'd be able to control the heat a lot better.
For sure I think a switch to AC will help. All of our aluminum welding here is done with Miller VP power settings.
Yeah, switching to the variable polarity will hopefully help us. Unfortunately, for now we are locked into using a procedure for straight DC current. Theoretically we should be able to run this procedure without forming porosity if all the cleanliness variables are controlled. So far we've controlled the cleanliness of our filler material and the edge prep of the joint. However, these sections of aluminum piping did sit outside for a period of a week after they were machined - when it was raining. They were brought in afterwards and sat inside for about a week. The welder noticed that he had to chemically etch it twice in order to get a good appearance at the joint. I'm concerned that when this material sat outside, it most likely sucked in moisture and formed a hydrated aluminum oxide layer. Even though we chemically etch it, is it likely that moisture or other contamination could've been sucked further into the aluminum material - due to the expansion/contraction stresses of the rain and temperature differences?
Thanks.
Hello Zeek, also, depending upon the types of air pollution around you, you could have picked up additional items to fight, ie: acid rain, etc. maybe an issue, maybe not, I believe you already have a pretty good understanding of the pitfalls that can result from lack of a stringent control of the materials. DC straight welding procedures on aluminum would call for an even higher degree of control on storage and cleaning of the materials than those associated with similar AC procedures. Best of luck and regards, Allan
I don't think " moisture or other contamination could've been sucked further into the aluminum material " but Your people did notice a well developed oxide layer from the parts stored longer. I think the chemical etch and stainless wire brushing immediatly before welding is probably about all You can do, short of the AC or VP as Your machine calls it.
The stainless brushes work great when new & sharp, replace them when they loose effectiveness. Use the dull ones for general work.
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