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Thin Aluminum Sheet Welding Shrinking

11/15/2016 12:27 PM

We are having trouble with this design moving when welded. The picture below will shrink the back overall dimension and bow the sides out. We took out the cross weld and welded it along the sloped bend but now the top hole (by the narrow point) is moving down in the weld process. Will the direction of the weld or pattern of tacking help any? What about stress relieving or post welding heat treatment? It is powder coated after.

Drew K

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#1

Re: Thin Aluminum sheet welding shrinking

11/15/2016 1:33 PM

some type of heat sink would help as well as intermitted welding.

I have to say this, after its stitch/tack together and when you full weld, do you weld a section, stop and let that cool and then weld in another area, stop let then cool and weld elsewhere.

As far as shrinkage, Your never get over that.

The shipyard were I worked, before I worked there back in the 60's, they did a lot of research welding aluminum. It surprising how much a ship shrinks in size if the plates isn't compensated for welding.

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#9
In reply to #1

Re: Thin Aluminum sheet welding shrinking

12/04/2016 12:50 AM

i think the phrase you're looking for is backstep welding or sequenced backstep ( skip backstep )

the weld direction and sequence are shown below , these are proven methods that massively reduce distortion in the right application.

for the OP however vacuum brazing to Mil Std and changing the joint designs would likely be his most effective solution.

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#2

Re: Thin Aluminum sheet welding shrinking

11/15/2016 1:35 PM

Reducing the effects of weld distortion can be approached from different ways, and sometimes must be approached in more that one way to get satisfactory results.

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Design is the best place to start. This is an over simplification, but consider how the contraction of the joint going from just solidified down to the temperature of the rest of the piece will change the shape or induce stress.

If you are having trouble visualizing this, have a welder lay a thick hot continuous bead down the center of one side of a flat piece of plate scrap. It will become obvious.

It isn't always possible to achieve, but in general welds running parallel to critical dimensions will have less effect on those dimensions than those running perpendicular.

Thickness heat and continuity of a weld affects distortion. A weld without excess heat, without excess filler material and that is stitched will distort far less than a hot fat continuous weld.

The weld process and the welders skill also can have a significant effect. An experience tig or paw welder will be able to give you far better results than my dog on a mig machine.

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Utilizing a pulse, especially on sheet or thin material can be emensely helpful in reducing distortion. Getting the puddle to freeze and then adding just enough heat rapidly for complete penetration melting just enough for the next ripple, can allow very low distortion very strong welds with less internal stress.

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As to stress releaving. You may see some reduction of the distortion by strss releaving processes that involve surface plastic deformation, i.e. peaning or burnishing. I doubt you will get much straightening out of heat treatment alone.

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Oh, two more things:

You could always make the welds perpendicular to the critical dimensions and then work to make the welds consistent enough, such that the parts could be made longer by exactly the amount of shrinkage expected.

Also, while, as noted above, a heat sink can help in some cases (although aluminum already dissipates heat well, and sheet aluminum acts remarkably like giant radiator fins...) it is critical that any heat sink used not restrict the movement of the piece while it is being welded.

While it may seem like restricting motion of a joint during welding would help, it does just the opposite. Tack in a jig only. Weld without a jig once tacked.

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#3

Re: Thin Aluminum sheet welding shrinking

11/15/2016 1:53 PM

Depends on how critical the part is.

If it absolutely must be straight and perfect it will probably require dip brazing.

Dip Brazing | Metal Parts Fabrication | American Aluminum Company

When I worked on satellite electronics we had a captive dip braze shop. Those guys were very impressive.

You might also consider heating the entire assembly near the deflection temperature while welding.

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#5
In reply to #3

Re: Thin Aluminum sheet welding shrinking

11/15/2016 10:51 PM

For our aluminum chassis, we usually specify vacuum brazing. Here is a paper with a good overview of the process.

Understanding Key Process Parameters of Vacuum Aluminum Brazing

Ridge is the company in our local area that gets a lot of our business.

http://www.ridgeeng.com/aluminum-dip-vacuum-brazing.html

There are many others I am sure

http://www.thermacore.com/products/aluminum-vacuum-brazing.aspx

http://www.californiabrazing.com/portfolio/cold-plate-aluminum-dip-vacuum-braze/

Cheers!

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#4

Re: Thin Aluminum sheet welding shrinking

11/15/2016 3:20 PM

What welding process are you using?

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#6

Re: Thin Aluminum Sheet Welding Shrinking

11/16/2016 5:57 AM

The distortion you are experiencing is the result of the differential contraction of aluminum between melting and ambient temperature. Narrower welds reduce the contraction (e.g. electron beam or laser welds). Peening is another method of reducing the effects of contraction induced stress.

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#7

Re: Thin Aluminum Sheet Welding Shrinking

11/16/2016 10:11 AM

Welding aluminum is always a challenge. Thin sheet aluminum increases that challenge. If you need strength, then just stitch weld one inch stitches on the non-visible side and tig weld the outside using aluminum filler rod. The more heat you put on the material the more warping you're going to get so minimize the heat as much as possible. Also note that welds in aluminum makes the material weaker.

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#8

Re: Thin Aluminum Sheet Welding Shrinking

12/04/2016 12:24 AM

The first starting point is to change the design so that the longest sides are folded rather than welded.

welding the shorter sides will reduce distortion and result in faster weld times (cheaper )

using heavy copper backing bars clamped inside joint corners can also reduce distortion , in large production runs it is well worth it to custom make copper blocks for that purpose.

what is the material thickness ?

if over 2 mm I often use pulse or double pulse MIG for ultrafast welds , I have compared TIG and pulse MIG on the same delicate assemblies and on average the pulse MIG would be 85 - 90 % less distortion than TIG.

another option if the job function allows it aesthetically , is to change the joint design so you have a folded lip of around 1/4" in size lapped up to its mating part so instead of a corner to corner joint with flat sheet , you now have a reinforced lap joint which reduces distortion and eliminates burn through .

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#10

Re: Thin Aluminum Sheet Welding Shrinking

12/06/2016 4:56 PM

In order to help you it is imperative to know:

1. thickness

2. joint design

3. length

4. welding method (manual, automatic ) TIG; plasma, Mig and etc

5. is design require : vacuum joint , strength joint

6. allow deformation

7. welding parameters in order to calculate deformation or avoid it

8. are you utilized fixture

9. shielding and back up gases

your part after PWHT will require strengthen to correct distortion.

Vladimir

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#11

Re: Thin Aluminum Sheet Welding Shrinking

12/06/2016 5:57 PM

Resistance spot, or projection welding will dramatically reduce the stress. This assumes that there is a frame or structure behind the sheet metal to weld to. You might also consider pop-rivets or adhesives..

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