How to Weld thinwall tickness Magnesium

Date 09-28-2005 13:17

Mag welds almost like aluminum, just needs a bit better arc and heat control. I build intake heads for alcohol burning pulling tractors and they are all welded magnesium construction (not sure why it's not aluminum), but that's what they want. The blower runs about 45-70psi of boost into the intake and as you can imagine they do form cracks after 3-4 runs. We pre-heat the entire part to 300'F long enough to burn out any foreign material, then the part is brushed clean with a stainless brush just like aluminum. The part is then welded using the matching rod. One word of caution: magnesium is combustible if overexposed to extreme heat. This material can generate it's own O2 during combustion and fuel it's own fire. We have never seen this in our shop, but everyone has their own horror story(true or not) about welding magnesium. Shoot me an e-mail if you want to talk about it more: mdgwelding@yahoo.com

Mark

By swnorris (****)

Date 09-28-2005 20:09

For materials in the thickness range you mentioned, you should be running between 50 and 100 A, with argon shielding. Your electrode and rod diameter should be 0.094. Helium shielding will reduce the welding current about 20-30 A. Thorium bearing alloys will require about 20% higher current. If this is a casting, the area should be stripped of paint and degreased before welding. Conversion coatings should be removed from around the defective areas with stainless steel wool or wire brush.
When you say "Magnesium part started to form small holes", do you mean in the weld? A low welding current can cause oxide contamination and porous welds. Did the weld crack again after the part was repaired?

By ebbb1092 Date 09-28-2005 23:49

Thanks for your tricks (Mark also!) .

I should have wrote to you weeks ago. Actualy, Parts has been ''glued'' or bond usind Metal Epoxy. I decided to go this way before the machine shop guy scrap my 350$ part !

The damage was in the axis of the Fork (vertical) and lenght of the ''crack'' was around .5'' to .75''. We machined an Aluminium Rod to the inner diameter and stick it inside the Outer Cylinder (lower Fork). By this, the protrusion or the bump popped out. After this, still with the Rod inside the Fork, we bonded it using Metal Epoxy.

Feel free to answer, do you think that repair will remain? Important fact, normal pressure operation of this section are only residual (around 0 psig).

By MDG Custom Weld (***)

Date 09-29-2005 15:09

Just like anything else, time will tell. With the low residule stresses that you claim, it should last. The olny question I would have is will the inner cylinder rub or ware away the epoxy from the inside.

By OSUtigger (**)

Date 09-29-2005 17:47

Just as a caution, anytime something has a 0 psi residual stress, this is usually because it has an extremely high instantaneous stress (in your problem, I am visualizing an outer cylinder sliding vertically over an inner cylinder, and this acts as a damper system?--if it were the spring system, the residual stresses would not be 0...). Yes, in such a system, the pressure inside the sliding cylinders is normally 0 as long as nothing is moving, but once it moves, the pressure spikes, but only for an instant. All is well, until you realize that this spike potentially has much more damaging effects to the structure than a constant load. Under constant load, stuff bends rather controllably. Under impact loads, stuff flies apart violently.

Just a thought.

G. L.