American Welding Society Forum
Hi all, i am new to welding as well as new to this forum. I have a basic question regarding preheating of aluminum. How can i calculate preheat temperature for aluminum 2219 alloy? Also how would i know that is there a need of preheating like carbon equivalent in carbon steels?
Thanx in advance
many decades ago
I was taught to preheat for smaw
but Lawrence is much wiser
and no one smaw aluminum any more
not even oldtimers like me
there is those new things gtaw and gmaw
Welding Process - Thickness?
Both heat treatable and nonheat treatable aluminum alloys can be damaged when exposed to elevated temperatures during the welding operation.
Generally, minimizing the interpass temperature is beneficial when it is necessary to maximize the mechanical properties of the weld and HAZ. Interpass cooling can be used to minimize the time at temperature. Blowers can be used to hasten the cooling to minimize the time at temperature. AWS D1.2 has a table that can provide some insight on the maximum time at temperature.
While it is a common practice to preheat aluminum to enhance the fusion between the weld and the base metal, it is general a poor practice. Generally, increasing the heat input will improve the results, i.e., reduce incomplete fusion. GMAW, using spray mode transfer, typically provides the best mechanical properties. Pulsing with GMAW reduces the heat input while maintaining good fusion if the parameters are set properly. Unless one is welding very thin material, short circuit transfer is best avoided.
Best regards - Al
1)Process used is manual GTAW using square wave precision TIG 375 Lincoln Electric.
2)Base metals have dissimilar thickness, one is a sheet of 6 mm and other a flange of 15mm wall thickness. This is a round welding. 6 mm thickness rolled sheet is welded to 15 mm thickness flange. Joint design is butt with single v groove weld. I can upload a snapshot of a joint design. Length of the weldment is 600 mm. Outer diameter of both rolled sheet and flange is 524mm
3) Base metals are same, 2219 in annealed form. Filler rod is ER2319.
4)Zirconiated Tungsten electrode, Argon shielding gas, AC current.
5) Cleaning method is a heated( 70 degrees centigrade) solution of NaOH(10%) and water 90 % followed by Nitric acid solution in water. Stainless steel brushing treatment in the end prior to welding.
6) Use of oxyacetylene torch to preheat. Currently, we are preheating to 130 degree centigrade. ( Self assumption)
If you have a lot of these to make I would suggest GMAW.
Your base metal thicknesses roughly are equal to 1/4" welded to 1/2"
Your GTAW travels at about 170mm or 7 inches per minute (More than 3 minutes of weld time per each assembly) Meaning 3 minutes of heat soaking into your base metal in a best case scenario with the part on a rotating positioner and the welder never stopping to adjust their body or replenish filler wire .
With GMAW you will make that same weld in about 30-40 seconds per assembly (750 mm per minute travel speed). Much less overall heat input into your base metal, meaning better mechanical properties and less distortion. Also if you have a large quantity of these to make, there is a production "takt time" benefit.
Also, what is the fillet size required by engineering? GTAW will probably produce a 5/16" fillet... GMAW can easily produce a 1/4" fillet and GMAWP may be able to produce a 3/16" fillet. Again, depending on how many of these you have to make... a process change might pay for itself very quickly.
Remember the difference between a 3/16" fillet and a 1/4" fillet weld is 78% in volume and arc-time.
Remember the difference between a 4.7mm and a 6.3mm fillet weld is 78% in volume and arc-time.
The difference between a 6.3mm fillet and an 8mm fillet weld is 56% in volume and arc-time
The difference between a 1/4" fillet and a 5/16" fillet weld is 56% in volume and arc-time
If you must GTAW:
Zirconium is a good choice for GTAW electrode, 1.5% Lanthanum might be able to carry a little more current before degrading... (meaning hotter arc starts and elimination of pre-heat) Your Lincoln Precision Tig 375 has all the power required... Balance set at 55-65% DCEN or about 7-8 out of the 1-10 scale on Cleaning/Penetration. A water cooled torch is absolutely key for this kind of operation.
Cleaning process is outstanding provided you weld directly after solution cleaning. Oxide returns to surface of the aluminum whether or not it is bagged after cleaning.
Preheat is a waste of time, materials and can only lead to a reduction in mechanical properties even in light of post weld heat treatment and aging.
7) Thermal stress releiving and heat treating to convert annealed to T6 condition i,e solution treatment, quenching and artificial aging.
Huge bundle of thanx Lawrence.You have provided me very fruitful information. I have one more question regarding preheating.
Why prehaeting of aluminum is prohibited even in its most weak form i, e annealed, Al2219-O? I have heard that welding in solution treated condition will cause loss of strength in HAZ due to overaging.Hence preheat temperature is kept to maximum of 93 degrees centigrade if it is to be done. But how this preheating affects annealed base materail?
- Lincoln Electric welding guide snapshot (308k)
One quick and important example.
Even in annealed condition... Molten aluminum is soluble to hydrogen that will be picked up during welding yes?
The opportunity is greater for hydrogen pick-up as heat soaks and builds during the course of a weld. A grainy surface appearance or porosity on RT exam is often visible on an aluminum GTAW weld that was done with a slow travel speed or welded to the end of a joint that had no heat sink or a slow termination... So much hydrogen is picked-up that as the molten pool freezes it is not all ejected or squeezed back out...
Preheat and slow travel speeds increase the possibility/vulnerability of this type of occurrence.
Edit: Your wise Lincoln literature says 250F max for preheat right ? Let's meditate on that for a moment....
What does 250 preheat accomplish or benefit that a slightly hotter arc-start cannot? Zero ! The only possible reason for that preheat is to enhance the wetting of the puddle at the initiation of the weld... But it is being done at the expense of heating the whole part (high thermal conductivity).
The preheat is not in this case being applied for the same reason preheats are applied to steels (controlled cooling rates for an assembly)..... So again... If your equipment allows you to get your puddle started and fluid qucickly, there is no motivation to preheat... GTAW and even more so GMAW can apply the needed weld current instantly, thus eliminating any benefit of preheat.
Most modern GMAW equipment have a "hot-start" function that adds increased current for a specific time, this function helps provide the extra energy needed to get good fusion on a cold aluminum surface... once the weld puddle is established (about 3/4 of a second) the hot-start function ends and the weld procedure currents take over.... "Crater-fill" GMAW functions are the reverse of the steps. This can all be done manually with GTAW when a foot pedal control is used. (although at a much slower pace)
Have a look at the snapshot attached.Its the graph of solubility of hydrogen in aluminum. Preheating keeps the base metal in solid form which has very less affinity for hydrogen.Then how preheatinh increases chances of porosity?
Reasons of porosity are
1) Hydrated aluminum oxide layer
4)Dew point crossing .
Preheating brings liquid form wider and longer at the ends of the weld where heat builds up :)
The soluble state I agree is liquid.
Dat is the very case we are facing in our workshop. In RT examination, despite of cleaning we have high levels of porosity, often not lying in the ranges as specified by the inspection standard Mil 2035 invoked by the designer in engineering drawing. We preheat the base material.At times,we stop welding and preheat again. One more important point which you mentioned was that set the AC Balance at 60-70% penetration. I remember when i asked the TIG welder to use high current instead of preheating (Just for an experiment, I was not aware of the problems which preheat could cause) he said he could not do so, too much of current will be required. I notice now that he set AC balance at AUTO mode. Though i dont know what this auto mode do with AC balance but i do came to know today that penetration must be increased to 60,70% which this auto mode is not providing i guess.
Am i right in my understanding?
Auto-set balence control is for amateurs, who don't know how to control their equipment, and your work is clearly not amateur level .
Will post more tips later that should help.
Ok... Let's look at balance control and electrodes. These are the two things that are going to allow your operator to increase the current at GTAW weld starts.
Balance control is defined for the purposes of this conversation as: "The amount of dwell time on each side of the AC half-cycle.
Your Lincoln is a "transformer" type power supply. This means that when balance control is set at *MAX Penetration* the machine will deliver 68% EN. Inverter style machines can deliver more EN, but a machine like yours provides a maximum of 68. Meaning Max Pen, or 10 of 10 on the balance control dial delivers 68% EN.
EP is what provides cleaning action... EN provides penetration. Your excellent surface prep should allow you to get near the maximum value of EN, but you still need to be very careful that you are getting sufficient etch.. I would set your balance control on your Lincoln to 7-8 out of ten to begin with... Look at the toes of the weld and make sure you are seeing "etch" along them.
Tungsten electrodes.... While Zirconium is a good choice... Lanthanum 1.5 or Cerium electrodes are an even better choice because they will carry at least 20% more current... Which for your project is important. You will want a 3/32" or a 1/8" diameter electrode with your water cooled torch... The smaller size is preferred as long as the tip does not degrade.
The electrode tips should be prepared with a blunted point. Not a long slim taper, but a short radical taper.. less than 1 electrode diameter in length.
Now all this change in set-up will mean nothing to your RT results if the GTAW welding operator moves too slowly. Heat build-up is your enemy... Which is why GMAW was originally suggested for the fillets you are doing. But if GMAW is not an option, than the operator must practice moving quickly, especially toward the terminations, ends and parts of the work where the heat no longer has any place to run. Using the largest filler wire possible will also help cool the weld puddle and allow the operator to move at an easy quick pace from point A to point B.
Thanx lawrence. I am so grateful to you. You helped me a lot.
when I was working aluminum
and still do with old equipment
use pure tungsten
with ball tip
I will be wrong, but what's new
Here is a post where I go into a detailed rant against pure tungsten electrodes.https://app.aws.org/forum/topic_show.pl?pid=258932;hl=pure%20tungsten
The game changed 30 years ago my friend :)
Oh and shame on you... You asked this same question on *THIS* link exactly one year ago.
You just go on and keep balling up your pure tungstens and paying a kings ransom for your helium :)
You have been led to the water sir !
Many years go I acquired a Heli-Welder II that was an asset at the time. It dimmed the lights and used pure tungsten but got the job done. I also cut a lot of stainless with a zip wheel. As soon as finances aloud I had a square wave tig and a plasma cutter . I guess it doesn't hurt to remember the old technology just in case its needed, but I have become fond of 2% and square wave.
Also, i have heard that oxide layer grows rapidly at elevated temperatures. Preheating can also grow it rapidly which might create some problem during welding. Am i right in this?
Has the process and heat input you're generating been validated by a PQR, in which acceptable bends and tensiles were performed?
Good question Tim
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