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Residual Stress

Residual Stress

Residual Stress

(OP)
Hello,

How can I eliminate residual stress in 2219-T851 aluminum alloy material?  Does machining (milling, turning) contribute to additional stress in the material after the residual stress has been eliminated in the forging?  Any feedback would be appreciated.

Thanks...

RE: Residual Stress

The only way to eleminate stresses in the section is to normalize and reheat treat.

Forgings are produced using the open-die forging process through the controlled application of compressive stresses while the metal is heated in the plastic regime. The metal, once subjected to the compressive stress, will expand in two other directions unless constrained in either direction. The expanding metal will stretch the existing grains and, if the temperature is within the forging temperature region, will recrystalize and form new strain free grains. The formation of the new grains is not random, however. The new crystal structure is oriented along the direction of the metal flow and can be used to enhance the properties of the forged component by producing a forging that closely follows the outline of the component resulting in even better resistance to fatigue and stress corrosion than a forging that does not contour the component. Other contributors to grain flow are the expansion of microsegregated regions and/or inclusions in the direction of the metal flow.

Surface grinding tends to produce a resultant tensile stress within the surface layer.  The magnitude and depth of the residual stress distribution is determined by grinding wheel grade, wheel speed, depth per pass, grinding fluid, and the sharpness of the grinding wheel.  By judicious selection of grinding parameters, it is also possible to produce residual compressive stress in grinding.                                                                        
In milling, the predominant residual stress tends to be compressive. The sharpness of the cutter appears to be the most important factor in determining the magnitude and depth of the residual stress in milling and other  
 chip removal operations.  EDM resulted in residual tensile stress, while ECM and ELP produced essentially zero stress in the workpiece.

The distortion in the workpiece resulting from all the machining operations was found to be proportional to the integrated residual stress distribution in the surface layer.

See link for more info:
http://www.physiqueindustrie.com/_residual_stress.htm

RE: Residual Stress

The magnitude of the residual stress can be reduced by heating because the maximum residual stree that can be locked in a body is the yield strength of the material.  As you heat the body, the yield strength decrease and so will the residual stress.

There have been reports that vibrations at room temperature help, too.

RE: Residual Stress

From what I understand, your material is in the solution heat-treated, cold worked and naturally aged condition?  This is then followed by a stress relieving operation (x51 signifies stretching but if it is a forging, you would have cold compressed?). I doubt that you would want to heat the material, as this may substantially reduce your mechanical properties? We have found that for 7xxx series alloys, residual stress reductions only occur for T73 type ageing treatments where the temperatures are quite high (180C) and the times are quite long (http://www.ul.ie/~mst/jrobinson.shtml ). As stress relief will occur as a result of thermally activated plastic flow at these temperatures (creep) the magnitude of residual stress prior to heating will effect the percentage stress relief that you obtain.

If the cold working operation is undertaken after solution heat treatment, then this will incur the major stress distribution.  Machining will incur residual stress in the material - there have been numerous papers written on this where researchers have tried to model it and have used x-ray diffraction and other techniques to measure it (see http://www.lambda-research.com/publica.htm for example).

Why do you want to completely remove the residual stress? If you did something like a shot peening operation on the surface and introduced compressive residual stresses, would that solve the problem?

Regards,
David.

RE: Residual Stress

Re machining-induced residual stress, it can occur, but is *usually* (not always!) overwhelmed by whatever was in the billet/plate/forging before machining.

Note that an as-forged part will almost certainly not be free of residual stress, even if it is a simple forged block (as is likely for x51 - it's hard to do a controlled stretch on more complicated geometry). Many SCC failures have occurred in the past for forgings subsequently undergoing significant machining.

Your block should be solution treated after forging, which involves quenching from a soak at 1000 F. The quench itself is likely to result in residual stress. Having said that, 2219T851 gets an SCC A rating (good!) even in the ST grain direction, and the T851 heat treatment seems to be a rather long 18 hours at 350 F after a cold stretch of 1.5-3%. Thus the cold work and aging treatment look like a pretty good stress relief op.

If your worry is tolerances being spoiled by spring after machining, then try to take the part from the middle of the block, or at least symmetrically distributed about the middle; a part with a half its thickness from near the surface and half its thickness from near the center will have a greater distribution of residual stress in it, increasing deformation. Also, avoid heavy cuts, especially when machining near the final profile (duh!).

You could also consider aging after machining, possibly with the part held "flat" in a jig (EXTREME care must be taken to establish limits for flattening in such circumstances, to avoid exceeding dangerous stresses), but there may be an issue with delay after the cold work. I would have to seek further advice on that point. Anybody?

Finally, peen forming can be a very effective and safe way to correct final part deviations from tolerance, provided strict limits are set on the amount of deviation which can be so corrected.

RE: Residual Stress

Machining can put extra, residual stress in the outer layer of a part. In addition, you may have a stress such as residual compression on a forged part. What can happen is you machine the conmpressive layer from one face and the compressive layer from an opposing face may warp the part (stress relief). You should post anneal the part and slow cool it to toom temperature.

RE: Residual Stress

You must re-temper the alloy after anneal to bring the alloy strenght up.  In the T851 condition the yeild strength is 51 ksi annealed it is 11 ksi.  See Kaiser:
http://www.tennalum.com/td2219.htm

The shot penning introduces additional surface stresses, often increasing the fatigue resistance.

RE: Residual Stress

(OP)
Hi,
I would like to thank everyone for their feedback.  

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