Preheating of aluminum using oxyfuel torch 3

[waqasmalik]

Hi all, please find the attached page. Its a snapshot from welding metallurgy book of sindo kou. It shows the chemical reactions and products in different envelopes.

My question is can i use the outer envelope which releases moisture/water as a product for preheating of aluminum? If not then what should be the preferred technique for preheating of aluminum?

Thanx all in advance.

 

[Tmoose]

I don't see an attachment.

 

[waqasmalik]

oh, so sorry, just a minute plz

 

[waqasmalik]

Here is it.

 

[CoryPad]

Why are you preheating the aluminium?

 

[metengr]

See below

http://www.lincolnelectric.com/en-us/support/welding-how-to/Pages/guide-aluminum-welding-detail.aspx

 

[waqasmalik]

Preheating is done to avoid porosity by giving hydrogen sufficient time to leave

 

[waqasmalik]

currently i am investigating the reasons for porosity in aluminum 2219 O weldment, preheating is done by oxyfuel torch. I suspect that moisture released by oxyfuel torch on the weld area being preheated leaves a mositure on the joint,so it causes lots of hydrogen to dissolve in molten aluminum during TIG welding.

 

[MikeHalloran]

Could you use electric heat, or electrically heated air?


Mike Halloran
Pembroke Pines, FL, USA

 

[CoryPad]

GMAW and GTAW of aluminium parts without preheat commonly produces porosity, it is not necessarily due to the preheating method. Having said that, Mike's suggestions of electric heating are worth pursuing. Review this document regarding porosity:

http://www.esabna.com/us/en/education/blog/porosity-problems-in-gmaw-welds.cfm

 

[waqasmalik]

Thanx friends, i know all these, i wanted to know that is oxyfuel heating is prohibited? Because it will surely cause porosity. Of course not the only reason for porosity.

Kind suggestion on its use is what i sought

 

[CoryPad]

I think oxyfuel preheating is not likely to be the primary source of porosity, other sources are more likely.

 

[Tmoose]

If the aluminum is preheated above the dew point the water won't condense on the surface, so by the time the preheat torch is turned off, will there be anything to worry about?

 

[gtaw]

There is no reason to preheat aluminum if the appropriate welding current is used. Hydrogen sources are many. Hydrocarbons and surface hydroxides are the most likely source. One source often over looked is finger feeding the filler rod without wearing clean gloves. Bare skin is a great source of oils. Oil free skin is cracked skin!

Heat treatable aluminum alloys overage if they are held at temperature while welding. Preheating only makes the problem worse.

The heat affected zone of the nonheat treatable aluminum alloys are softened and extended by preheating. Like the heat treatable alloys, the mechanical properties are degraded in the HAZ.

Preheat aluminum, not a good idea. Instead, crack up the welding current.


Best regards - Al

 

[CoryPad]

Great answer from gtaw, I agree with all of that.

 

[metengr]

I do not agree with blanket statements. I generally agree that preheat is not needed for relatively thin sections. The only time I have seen preheat used in welding aluminum was at a Power Plant where we needed to reduce the rate of cooling because of the material thickness (2"). We simply could not crank up enough current to avoid cracking, not porosity. I remember we used a rosebud torch and locally heated to 200 deg F and no cracking. I just believe the thermal stresses caused by cooling were so severe that we had a form of hot cracking in otherwise a crack free filler metal.

 

[waqasmalik]

Preheating in my case is inevitable, i would like to discuss the issue in more detail

 

[waqasmalik]

i weld parts of dissimilar thickness. Assembly consists of two parts. A skin and a flange. Skin of 6 mm thickness, flange of 15 mm wall thickness( 524 outer dia,494 inner dia). Parts are etched before welding using 10% NAOH solution in water followed by concentrated nitric acid solution in water 50/50. Then stainless steel brushing is used to remove the oxide layer from the joint surface. ER 2319 filler wire is used ( Base material 2219 annealed , it is cleaned with acetone. Preheating is done using oxyfuel torch, more heating is done on thicker flange, flame also moves over the joint surface. Then manual tig welding is done with 99.997% pure argon gas. After this, all depends on welder technique.

 

[waqasmalik]

NaOH solution is heated to 70 degrees before immersing the parts in that tank for one minute.It turns black when we take it out. Then after washing with water, it is immersed in nitric acid solution for 15 sec.

 

[waqasmalik]

i am removing all sources of hydrogen ( not atmospheric, it depends on welder technique, gas flow rate,wind drafts etc) like cleaning the base material, cleaning the filler wire, pure argon gas. why i thought that perhaps oxyfuel torch might have moisture in its discharge even if it is in small amount, it is a source of hydrogen. No, i am definitely not saying that efficiency of my cleaning process and welding technique is not faulty. There might be errors but these are right procedures, if done in wrong way could cause problem. I was investigating that is oxyacetylene heating is a right procedure? If wrong then i shall straight away prohibit its use and look into improving the efficiency of my right/recommended procedures so that the quality could be improved