So I was at the track last night and a customer of ours was racing a drag radial car (ran a 4.91 in the 1/8th, forth pass out) and had a head gasket failure. The head had been damaged last week in testing and had been repaired by welding. When the heads came off the car last night the damage was in the exact same spot as if the weld had vaporized. I was surrounded by some VERY knowledgeable people but when I posed this question everyone had a blank look on their face.

I stated that somewhere I had read when welding used dirty aluminum (like a combustion chamber or in this case the area between two combustion chambers) that the aluminum needs to be cleaned by heating to get rid of any oil residue / impurities that will stop the weld from sticking. No one argued the point but not much was said other than that is not how it was repaired the previous time which resulted in failure after 4 runs. Has anyone else heard what I have heard with regards to heating aluminum before welding?

Just wondering if I am going insane or not?
i
I think they are going to end up milling the head and switching to oringed copper gasket til they get a chance to pull the block and have everything machined back 100%, we'll see.

 

There is virtually no way to weld aluminum that heavy without a considerable preheat anyways. Yes, it does help to work some contaminants out of the material, but its more a matter of having cold lap or incomplete fusion due to a lack of preheat. You need to preheat the aluminum, use considerable amperage depending on the damaged area of the chamber, and almost always use a helium/argon mix to do a successful repair.

 

:withu:

Preheat the head to 400° to 500° F for up to two hours before welding '” unless the casting is a heat-treated alloy, in which case keep the temperature under 275° F. Limit the weld time to about five minutes or less to avoid overheating the casting.

Once the head is at temperature, it will retain heat for 15 to 20 minutes. Using shielding around the weld area to reduce drafts and retain heat will improve heat retention and extend your welding time. If the heat's temperature drops too low, put it back in the oven and reheat it back to your working temperature.

Your TIG welder should have at least 250-amp capacity and probably 350 amps is closer to what might be needed. Use 15-20 cfm of argon/helium.

 

I think most people welding alum heads just get ansy and start jabbin filler rod in. I had a good friend of my dads weld up a rough spot on the deck of one of my trickflows. 15,000 miles later the head still looks flawless!

 

Aren't all cylinder heads heat treated? Even if a 275* pre-heat max is used, will the welded area still be softer than the rest of the casting?

Does the Argon/Helium mix all for deeper penetration than argon alone?

Thanks.
Dave

 

I sent my damaged head here
http://www.brodix.com/repair/repair.html
These guys repair heads for a living...

 

Eh, yes and no. There are tradeoffs in property. It will be marginally softer, yield strength will decrease, ductility for that area increases (assuming a T6 Al, which are artifically aged). A lot depends on the properties of the filler metals. In all honesty, most of the heat treatments for cylinder heads are for refinement of the base material, and is done with the raw material. The properties of cylinder heads really don't need to be particularly demanding, and as long as the welding is done properly, the weld should be a non-issue.

And yes, helium creates a significantly hotter arc than argon, at the expense of arc stability.

 

Thank you. One more, if you don't mind [well, two]. What mix ratio for the gas? [sorry, i've always used 100% Argon]. Is there a filler alloy that would be good for most castings?

Dave

 

Gas ratio really depends on the situation. I've welded with straight argon, straight helium, 60/40 He to Ar mix, etc.

For most castings I use 5356, 4043 for billet, but it always helps to double check with the manufacturer to see what the material actually is.

 

Thank you.

Dave

 

I agree with the above repair techniques but would add the that the area welded needs to be peened with a hammer immediately after welding. It stress relieves the area and helps remove the porosity of the welded aluminum. If done properly the welded area is hardly noticeable after resurfacing. I also agree with the 75/25 gas.....It makes the torch burn hotter requiring less welding amperage.

 

I don't agree on the peening removing porosity, but it is a good idea to peen IF the head isn't slow cooled. It's a good idea to plunk the head into some sand or equivalent to let the head cool slowly, which will also minimize warpage.