Preheating of aluminum with respect to porosity only

Date 07-11-2015 15:37

Welding involves finding a balance between the good versus the bad. It is a never ending battle of good against evil. There is no such thing as a perfect weld. After all, the weld is a discontinuity.

We preheat carbon steels and high strength steels to reduce the cooling rate in hopes of mitigating the formation of undesirable microstructures. That undesirable microstructure is typically Martensite. Maintaining high interpass temperature allows much of the hydrogen to effuse from the weld before it has an opportunity to cause damage. Another means of reducing the potential to develop Martensite is to use steels with low carbon and low carbon equivalencies.

Aluminum does not form Martensitic microstructures, i.e., Body Centered Tetragonal crystalline structures, and we do not alloy aluminum with carbon. So, preheating is of no benefit in that regard.

Most aluminum alloys used for the construction of "parts" are either heat treatable or nonheat treatable. The heat treatable alloys are heated to a high temperature to allow the alloying elements to go into solution, the metal is quenched to prevent the alloys for coming out of solution, and then, the alloy is aged either naturally or artificially. The aging allows some of the alloying elements to come out of solution and "clump" together. The clumping strains the surrounding atomic lattice and thereby strengthens the alloy system. Subjecting the alloy system to elevated temperatures (as when it is welded) allows the alloying elements to precipitate to the grain boundaries where they weaken the alloy. This is called over aging. To mitigate over aging, the weld and surrounding base metal are often cooled between weld beads to prevent or to at least mitigate over aging.

Nonheat treatable aluminum alloys as strain hardened by rolling at room temperature or by physically stretching the base metal. the benefits of strain hardening can be and often are lost in the HAZ when the material is welded. The longer the material is held at high temperature, the wider the HAZ and the more damaging welding is.

One must ask two questions; what are the benefits derived from preheating? What is the damage that can be caused by preheating? Do the benefits of preheating out weigh the damage caused by preheating?

As for the hydrogen argument; the hydrogen comes from something. It doesn't come from air. The problem is to identify the source of hydrogen and then eliminate the source. Is the welder welding without gloves? Make the welder wear clean oil free gloves. Are air powered tools being used to prepare or clean the base metal? Where is the air exhausted once it goes through the air tool? Is the exhaust directed toward the base metal? Bad idea, compressed air comes from oil lubricated compressors, a source of hydrocarbon. Are the surface oxides and hydroxides completely removed from the weld zone and adjacent surfaces? Hydrogen can be liberated from the hydroxides when heated. Are the tools and fixtures oil free? Tools and fixtures are often contaminated from handling them without wearing gloves or from oily mists in the air if there are machine tools operating in the same work space. Did workers use bee's wax on grinding disks to keep them from loading when grinding aluminum? If so, wax is a hydrocarbon that will introduce into the weld if it is not completely removed. There are many sources of hydrogen and each must be discovered and eliminated.

Best regards - Al

By waqasmalik (**)

Date 07-11-2015 20:04 Edited 07-11-2015 22:21

Yes thank you.

Please find the attached file.

At last pages, the author shows the effect of welding variables on porosity developed because of hydrogen absorption into weld pool ( subsequently leaving the solidifying metal) from the arc having known value of hydrogen content. When describing the effect of increasing the current on rate of absorption the author writes that increasing the current ( Keeping all other parameters constant) brings the molten puddle in larger size, hence hydrogen in the arc has more ground to attack, this increased the porosity.

Also, the author shows the effect of increasing welding speed on the porosity, he writes that by increasing the welding speed ( Keeping all other parameters constant) porosity decreases because less time is available for hydrogen pick up, its not that the rate of absorption decreases by increasing welding speed.

I am quoting all this because many a times i have read in many of the posts at this and other forums that don't preheat aluminum, increase the current.instead. Then i read this paper which was showing that increasing the current, increases the porosity so question came into my mind that experts recommend the increase of current to weld aluminum but it makes the weld vulnerable to porosity as per this paper. So, i posted a question here.

After reading this paper and reading the first paragraph of your last comment " Balance between good and bad" i think i have understand the concept. We have to find optimum values of variables to qualify the welding procedures. Like increasing the current but also increasing the welding speed to catch benefits from both changes and achieve desired results.

Attachment: Researchpaper.jpg (338k)

By 803056

Date 07-11-2015 22:57

Very interesting information.

What publication does this come from? I am an advocate of increasing the welding current rather than applying preheat. Again, it is a balance between good and evil.

Al

By welderbrent (*****)

Date 07-11-2015 23:17

I have often used, and held the opinion that, some pre-weld application of oxy-fuel flame to an aluminum piece could be beneficial. I have learned alot about that from a positive and more so negative side here on this forum.

I don't necessarily think of it as pre-heat as cleaning. But, there are also borderlines where in order to get the weld puddle to be what is needed when using a small machine a little EXTRA heat when first starting the arc sure does help to keep from getting a lack of fusion for the first 1/2" or so (same goes when using those little 120v wire feed welders on material that is pushing the envelop for their upper limits on carbon steel). It is not that the part actually needs PRE-HEAT but the machine needs some help starting and/or a little extra cleaning action from the flame helps get the fusion and purity factor increased.

One of the biggest problems though as I see it is those who don't know the limits and overheat the product and either ruin it with the heat of at least the minute they strike an arc because they are so close to the melting point that everything just falls away. It isn't steel. People need to be aware of the limitations of the product they are using.

Now, there is also a big difference between code work or some around the farm (OOPPSS, Farm Code) repairs. Some allowable applications doing equipment repair may be totally unacceptable when working on an aerospace project.

Just my two tin pennies worth.

He Is In Control, Have a Great Day, Brent

By waqasmalik (**)

Date 07-12-2015 07:28

Brent

I shall also like to take up issue of oxy-fuel preheating of aluminum into the notice of experts here in this forum later.

By waqasmalik (**)

Date 07-12-2015 07:22

803056

I have emailed that publication to you. I was not able to attach it here. Publication is pretty old, published in 1974.

By ssbn727 (*****)

Date 07-12-2015 07:27

The March edition of the 1974 AWS Welding Journal...
Hi Al! :grin:

Btw Waqasmalik, "Weldcome" to the World's Greatest Welding Forum! :cool:

When one reads these many articles one must always take into account that the research being performed at any given time covering many different factors, or simultaneously more than one at the same time... One also must consider whether or not the research is, or was being performed in optimal environmental conditions with respect of exposure to possibly disruptive, or not conditions that could lead to less than optimal results that are proclaimed in any given research article... So really careful attention must be paid to every little detail no matter how insignificant it, or they may initially seem to be... Interesting thread indeed and as always, Al has succeeded again in describing the metallurgical differences between Al and CS which must be be fully understood in order to gain a thorough understanding how excess, non diffused or under diffused H affects each of these metals differently, and more importantly, not to treat the 2 very different metals with respect to welding and heat treatment as well as preheating vs not preheating Aluminium.

For me, this sentence says it all: "Aluminum does not form Martensitic microstructures, i.e., Body Centered Tetragonal crystalline structures, and we do not alloy aluminum with carbon. So, preheating is of no benefit in that regard." Sure we can use more layman terminology but, then we would have to use a sort of Vulcan Mind melting technique like Spock would (R.I.P. Leonard Nimoy) in order to give anyone interested an accelerated welding metallurgy fundamentals course or an excellent power point presentation so students could comprehend what is dynamically happening during the entire fusion cycle. Great explanation Al! Good to be back for the time being again. :eek:

I always miss the gang when I'm hospitalized again and again, and again... What a life!

Respectfully,
Henry

By waqasmalik (**)

Date 07-12-2015 08:32

Henry

Thanx sir for a comment.

I am new to welding but i have read some articles.What i have learnt is this.

I have some knowledge of metallurgy of aluminum and steel, welding differences, undesirable micro structure in steels , heat treatment differences, strength loss in HAZ of heat treated and cold worked aluminum alloys after welding, Cold worked aluminum alloys lose their strength after welding regardless of the initial temper and their HAZ are almost annealed as welded. Increasing heat input will not make them further weaker but will increase the width of HAZ. While for heat treated alloys there is no blanket statement . Strength loss do occur but it depends on initial temper and time and temperature dependent phenomenon. Keeping in view the last treatment which is done at 204 degree centigrade, we should make some provisions like increasing heat sink, increasing welding speed etc to qualify the minimum tensile strength value as specified by the code. Increasing the heat input will make their HAZ weaker and wider. So, heat input is closely monitored.

Gaining the lost strength in HAZ is often not practical and not recovered completely. Initial temper T4 will have more HAZ strength than T6. Aging after this can give the T4 HAZ strength somewhat close to the base metal but aging of T6 welded the HAZ strength will increase though but not close to that of base metal. If complete recovery of strength is desired then full heat treatment ( Solution treatment, quenching and artificial aging) should be done. Alloying elements should be in proper amount to regain the lost strength because actually they make the heat treatment effective and give strengthening.

Moreover, cold worked aluminum alloys have less strength loss as compared to heat treated alloys. Cold worked have higher welded strengths.

Preheating for aluminum is not recommended if we have high amperage welding plant.. Why is this so? Why not preheat? What damages could it cause? I have read in many articles preheating may increase the time at specific temperature therefore it might cause lose of strength and procedure will not be qualified wrt minimum tensile strength as specified by the code. This is the one disadvantage which i see . But, then if i weld in an annealed- O condition, then this strength loss problem is still a valid argument against preheat? What damage can happen to an annealed strength?

I was told this by Lawrence that preheating brings the molten puddle bigger in size towards the ends of the weld where heat sink has less effect, so it decreases the solidification rate, thereby the hydrogen has a bigger ground to attack.

When i read this paper it said that increasing the current also increases the size of the molten puddle which increases the porosity. Dats why i got confused and asked the question here.

I am a recent graduate mechanical engineer, its been just 3.5 months that i have been appointed into welding shop. I like to ask questions and learn from experts. Please give a response to this post and correct me wherever i am wrong. I have not experienced yet what i have learnt from reading.

Thank you all for spending your valuable time in reading and commenting.

By ssbn727 (*****)

Date 07-12-2015 10:57

REmember this... Nothing is always absolutely the same as it was one second ago so don't stress yourself too mush over some of the potentially contradicting statements which pop up in one paper or another and please don't fall into the trap of assuming that conditions remain static because they do not! Learn as much as you can when it comes to welding metallurgy and yet remember that physical metallurgy can be way way different in the use of terminology...

I must say that for someone just out learning, you're doing really well and I have confidence that you will learn even more about welding and it's varying dynamic behaviors as well as the variety of metals as well as other materials where the term welding is correctly applied as a manufacturing process...

I would like to suggest a couple of texts to read up on regarding the metallurgy of welding Aluminium (English spell it and pronounce it this way) in case you're interested and some other reference material related to welding ferrous and non ferrous metals... Stand By...

Respectfully,
Henry

By waqasmalik (**)

Date 07-12-2015 17:22

Sure sir, I would love to read all the texts which you share on this forum. I will make full use of it.

By ssbn727 (*****)

Date 07-13-2015 06:16

I'm back...

Please don't misunderstand what I meant by saying that I would refer you to some excellent books for you to review regarding welding metallurgy of carbon steels and Aluminium as well as the other metals used in a variety of industries today... I am not going to share any of my own books with you, but I will share links to the very same references which you may desire...

In fact, just to make it easier for everyone to search the forum for such information, the metallurgy section in this very forum is a repository of knowledge and data which would prove to be very useful for you to journey through... Here are a few:

Welding Design & Fabrication are offering a 5 modules Metallurgy course set in case anyone is interested...

Each module is priced separately but, I would imagine that they might offer a discount for taking all of the courses as a package deal... However, I cannot guarantee that they will do such a deal and the only way to find out for sure would be to contact their eduction department @ "Welding Design & Fabrication." These courses might be good for someone who needs more education in metallurgy in preparation for taking the AWS CWI Certification exam...

Here's the link to the course offerings:

http://weldingdesign.com/catalog/15602

https://app.aws.org/forum/topic_show.pl?tid=33794

Hydrogen Embritttlement: https://app.aws.org/forum/topic_show.pl?tid=33752

I ran across this and I thought that it would work well for anyone looking to answer some basic questions regarding metallurgy if it were included in the Metallurgy section...
So here it is!

http://habairon.org/History/Metal%20History.pdf

Enjoy the read and I hope this helps anyone understand better this subject of Material Science. :surprised:

https://app.aws.org/forum/topic_show.pl?tid=33291

https://app.aws.org/forum/topic_show.pl?tid=32016

https://app.aws.org/forum/topic_show.pl?tid=29594

So much more information, articles and links to all sorts of stuff pertaining to welding metallurgy can be found here in this link

https://app.aws.org/forum/board_show.pl?bid=43;pg=5

And more after those pages where you will find just about anything you want at Alice's restaurant! Have fun reading because there's so much in volume here to digest so use the search features to your advantage... Well that's enough for now and this should keep you busy for some time! :eek:

Respectfully,
Henry

By waqasmalik (**)

Date 07-12-2015 18:54

Henry

Also if you some useful links/ Texts for iron carbide diagram, please share them too.