Post weld heat treatment of aluminum casting? 5

[KirbyWan]

Howdy all,

I posted this in welding, bonding and fastener engineering forum a while ago and got no love. The issue has come up again with a different part, so I thought I would ask the question again in this forum. Could anyone help me out with this?

I have a cast aluminum part that is A356.0 which MMPDS-01 indicates is weldable. I'm trying to determine if a post weld stress relief is needed and if so what it should be. This is for a light to moderately stressed aircraft part that is secondary structure (a cast duct). If someone could help with this I would appreciate it, or if you could provide a source of information that explains what does and what does not need post weld stress relief that would be even better.

I seem to struggle everytime I need to design a repair for something using a welding process because I don't have a single source to go to for all the small yet critical details that go into a succesful weld.

Thanks,

-Kirby

Kirby Wilkerson

Remember, first define the problem, then solve it.

 

[WKTaylor]

KW: A356.0-T?????

Regards, Wil Taylor

 

[WKTaylor]

KW... and what welding rod alloy and weld spec???

Regards, Wil Taylor

 

[KirbyWan]

Wil,

I was working to AWS D17.1 I believe 4043 welding wire is acceptable. Thanks for your help.

-Kirby

Kirby Wilkerson

Remember, first define the problem, then solve it.

 

[KirbyWan]

The material ID in the CF6-80C2 thrust reverser CMM does not indicate a temper. (It does for everything else of course, but not what the part I care about at the moment.) It lists it in the cleaning section as AL-ALY-A-356.0 Class 2 Grade B. We want to weld up some double drilled holes in the actuator casting. I don't know what the Class 2 Grade B indicate either.

-Kirby

Kirby Wilkerson

Remember, first define the problem, then solve it.

 

[BobM3]

Put a note in the Materials Engineers: Metal and Metallurgy forum about this post. You may even want to search in that forum on this subject because I believe its been addressed before.

 

[KirbyWan]

Hello again,

I did a few searches for my answer but was unable to find one. If this topic dies I'll post to that forum next. Thanks for everyone helps. If anyone could suggest useful reading to better understand welding repairs on aircraft I would appreciate it. I feel this is a weak area for me and want to improve my knowledge. I'm pretty familiar with Heat treating wrought aluminum, and know not to weld 2000 or 7000 series aluminums (except 2219) but overall my knowledge of everything that goes into a successful weld is thin.

-Kirby

Kirby Wilkerson

Remember, first define the problem, then solve it.

 

[BobM3]

Here's a couple of threads that may be helpful:

thread330-166322
thread330-157151

 

[Neubaten]

For general info about welded repairs, check the specific chapters at:

http://tinyurl.com/8zrzn2

Not so much detail into metallurgical aspects though,like pre-treatments and PWHT.

Check AMS 2694 "REPAIR WELDING OF AEROSPACE CASTINGS", If you haven't done it before.

I have worked in repairs of metallic members by welding, and you are right in that there's nothing written about that specific questions.

I guess it is because for every single case and every material there are many variables, like how is the part stressed, how critical it is, how the material is affected by the weld, what happens at the HAZ (what transformations suffer the alloy from welding T to RT under slow air cooling), if the material is susceptible to cracking, etc, etc...

 

[TVP]

As WKT mentioned, it really depends on what temper treatment was used for the casting, and what are the desired mechanical properties. In general, normal precipitation heat treating temperatures (150-245 C) are too low to provide appreciable stress relief on quenched or welded parts. Annealing at 315-345 C for 2-4 hours (O temper) provides maximum stress relief of castings, but has significantly lower mechanical properties than a typical solution treated + artificially aged (T6) temper. The following is an excerpt from the article "Aluminum and Aluminum Alloy Castings" that appears in ASM HANDBOOK Volume 15 (2008) Casting:

Postweld Heat Treatment. Castings that require heat treatment should be heat treated after welding. Castings taht have been heat treated and then welded will suffer localized loss of tensile properties and an increase in localized residual stresses. However, postweld heat treatment will restore their properties if the proper filler alloy has been used in welding."

 

[tbuelna]

KirbyWan,

A356.0-T(?) is definitely weldable, and even new parts cast from this material are commonly weld repaired at the foundry to fix casting porosity, defects, or to plug cored holes. Ideally, the part would always undergo a post weld stress relief and heat treatment to whatever condition is specified by the engineering.

But you should be careful about welding a part that is already finish machined. If the welding is extensive, or in a section susceptible to causing thermal distortions, any close tolerance part features may be affected.

Also, the class 2, grade B specification you mention is likely referring to MIL-STD-2175. Class 2, grade B is a very high quality casting. Class 1 is highest criticality and class 4 is lowest criticality. Grade A requires the fewest casting defects and grade D allows the most. Making weld repairs of a casting that has an overall classification of class 2, grade B would likely be frowned upon. But regardless, you should refer to the original casting engineering drawing to verify where, or even if, weld repairs are permitted with this cast part.

Good luck.
Terry

 

[WKTaylor]

KW...

sorry for delay... been very busy...

NOTE. AWS D17.1 indicates that a stress analysis may be required for every significant weld repair. D17.1 repeats/implies several times that the ONLY reliable method to determine mechanical allowables across a weld is by welding-then-testing using various samples and equivalent methods and weld-materials... especially since there are soooo many variables involved.

NOTE. in-process welding of NEW castings [post cast, pre or post heat treat... probably before finish machining] is described in AMS2694 "In-Process Welding of Castings".

WARNINGs.

a. Old [weather-exposed] and poorer quality castings [porous and/or corroded... all magnesium and certain aluminum alloys] have serious issues when repair welding. The material is likely contaminated with oxides, MOISTURE, cleaning solvents, penetrant dyes, etc... that may catastrophically affect the weld-heat affect areas [blow-outs, running cracks, mini-explosions, etc] that would not occur on virgin casting materials being repair-welded for casting or machining defects. There is a parallel to this thinking for welders: welding rods and wire must be carefully packaged, stored and handled to insure they are used in pristine condition... and do not induce defects in welds due to unintended contamination [dust, moisture, oils, sweat, etc].

b. Complete defect [crack, corrosion, pitting, etc] grind-out and cleaning is mandatory to positively eliminate oxides and other contaminates in the crack faces, sharp crevices, pits, etc that WILL induce weld failures.

c. Also, PRE-weld baking [180--210F] for +24-hours (or more) to drive-out moisture and other volatile contaminates has proven to be very useful.

d. PRE-heating, and warm application of inert gas in the oven... over the entire part before welding... followed by still-air [blanket-covered] slow-cool-down will also minimize weld-induced stresses.

Now...

Lets assume You have a casting per AMS4218 "Aluminum Alloy Castings 7.0Si - 0.35Mg (A356.0-T6P) (Formerly T61P Temper)
Solution and Precipitation Heat Treated".

The following Minimum Tensile Properties apply.
Property Value Tensile Strength 32.0 ksi (221 MPa)
Yield Strength at 0.2% Offset 22.0 ksi (152 MPa)
Elongation in 4D 2%.

If welded with the same weld alloy [AMS4181 "Aluminum Alloy, Welding Wire 7.0Si - 0.38Mg - 0.10Ti (4008 or 356)" and fully solution heat treated and re-aged per AMS2771 [or per AMS4181 requirements] using good welding AWS d17.1 methods/techniques, then attaining close to original material properties is highly possible [assuming good/clear NDI after welding... and restoration of geometry].

IF welding with ER4043 [AWS A5.10] then a very good quality weld is possible... however the weld will likely be under-strength and non heat treatable [SR only]. In some cases, baking at the precipitation heat treat temperature for the base-alloy for a short period will stress-relieve the base alloy gran away from the weld bead and HAZ.

The question now becomes: what values for mechanical allowables are appropriate??? This is where testing is required... unless Your company already has tested values. A good starting point for guesstimating allowables is the "as-cast" minimums for the base alloy and for the the weld filler-wire: whichever is lower will probably prevail... the integrate a "casting factor" [such as X/1.25] into the equation. Obviously weld quality is a factor that cannot be ignored.

Regards, Wil Taylor

 

[Neubaten]

And what about stress concentration factors at weld beads?

Appart from the properties reduction, do you also take into account stress concentrations at weld beads?

Based in typical literature (Roark's) Kc can be very high sometimes (somewhere about 3).

Counting that and properties reduction, and still mantaining MS factors, welding repairs would be suscribed to very under-stressed components (i.e. ashtrays), which is not the actual case.

 

[tbuelna]

KirbyWan,

If your casting is qualified to MIL-A-21180, here's the relevant section on weld repairs:

"3.8.1 Weld repair of castings. Castings may be weld repaired only upon written permission
of the purchaser. The granting of such permission requires that the facility performing repairs
demonstrate that repaired castings will meet all applicable quality requirements of this
specification. Further, weld procedures for repair and welder qualifications shall be acceptable to
the purchaser. In any repaired casting, repaired areas shall conform to all quality requirements
specified for that area on the drawing or in other acquisition documents. Repaired castings shall
be subjected to the same inspections and meet the same acceptance criteria as a casting which is
sound in the same areas. Such areas shall be identified to facilitate inspection. Repairs shall be
made prior to final solution heat treatment and inspections specified herein, and prior to any
surface finishing treatment (see 6.5 for precautions for welding A357.0 alloy)."

Regards,
Terry

 

[WKTaylor]

Tbuelna..

The info You cite is for brand-new castings with repairable defects... probably not yet machined... assuming the purchaser will allow repair.

AMS2694 was developed to provide a logical/technical method for weld-repair of defective castings in-production. MIL-A-21180D [AMS-A-21180A-NC] carefully embedded this intent in the body of the spec. MIL-C-21180A [1958] had similar, but primitive verbiage, stating [roughly] repairs to castings have to be approved by the purchaser. This old spec appears to have implied all typical forms of casting repairs: welding; plugging; impregnating/sealing [porous, but adequate parts]; etc.

Weld-repairing [field or depot] aluminum or magnesium parts made from machined castings, that have seen many years of usage and environmental exposure, is an entirely different beast than repairing defects in brand-new raw castings. You haven't lived til You've seen/heard a casting crack or explode due to entrapped liquids in the weld bead or HAZ.

OK... time for a parallel war-story...

Similar/worse problems exist with hot-repairs on old aluminum-honeycomb/aluminum-skin or composite [skins] parts with entrapped fluids.

There was the time we were hot-bonding [450F] a graphite patch to boron-epoxy skin [F-15 rudder?]. As I was walking out of the shop, there was a dull but obvious explosion from the part. examining the ruined part we found an obvious [sooty] explosive event originating in one HC cell. I found out that the skin had been cleaned with copious amounts of IPA... and deduced that a small "whiff" of IPA found its way into the cell by capillary action through the porous skin matrix... a VERY explosive combination when the combined with rapid onset of high heat and subsequent trapped-air-pressure-rise (before pressure could be bled-off by vacuum]. An on-line report [DTIC?] confirmed the explosive problem when using flammable solvents [like 90% IPA] to clean porous composites... then hot bonding before re-drying the parts of the cleaning agent [typical @180--200F-bake for 24-hrs W light vacuum].


Regards, Wil Taylor

 

[MNLiaison]

Within the last year we have performed a weld repair to an A356-T6 casting. Took initial hardness and conductivity measurements at multiple areas, used a CMM to get critical points, took X-ray's at adjacent areas all prior to any annealing/welding operations. Used a scrap casting to anneal, weld and reheat treat to the -T6 to get the process down and captured on paper. Then performed weld repair on the actual casting, verified minimal distortion after heat treat via CMM, performed conductivity and hardness on areas where fastener holes would be drilled, took Xrays and compared all of it to the original.

Some info on castings that can be found on DTIC is below.

ADA108145 - Cast Aluminum Primary Structure
ADA087336 - Cast Aluminum Structure Technology (CAST) Structural Test and Evaluation Phase Part II
ADA245237 - Durability and Damage Tolerance of Aluminum Castings

Regards

 

[KirbyWan]

Thanks for everyone's input. Sorry I haven't responded in the last week, but I was on vacation. There is alot of information for me to work through and i will review it all. At the moment we are looking at doint an exchange with a unit in better condition which means I will have the time to review all this thouroughly before comeing up with a fix if we decide that is cost efficent

-Kirby

Kirby Wilkerson

Remember, first define the problem, then solve it.