Stress relieving of aluminium die casting

[gdodd]

Is it possible by metallographic means, hardness or other simple test to determine whether a pressure die cast aluminium alloy has been stress relieved or not?

The material is Japanese spec ADC12 (close to AA335).
Stress relieving is done at 210 degrees C for 2 hours after die casting and cooling to room temperature.

It is thought the reason for cracks found in an automotive casting may be caused by a batch that missed heat treatment

Thanks in advance for any help received.

 

[etch]

I would say that hardness checking would suffice. Typical quantative analysis of checking one un-heatreated, and then testing one treated will give you casting variance.

 

[gdodd]

Etch
Thanks for your input. We have tried hardness tests, however there is no trend in hardness variation I could hang my hat on.
There is no heat treatment conducted on the part apart from the low temperature stress relieve of 210C x 2 hrs.

 

[TVP]

I'm going to go out on a limb and say no, I don't don't believe there is a simple test that can be performed to detect the effects of this treatment. I will do some more investigation, but I am not aware of any microstructural changes that would be evident after an excursion to croygenic temperatures.

Since stress relief is the intended function, techniques used for measuring stress would be my suggestion. It may be possible to do something relatively simple like strain gauging, but x-ray diffraction is the standard method for measuring residual stress. TEC, Proto, and Stressolvers are all companies specializing in this area.

The following website has some interesting info on a recently developed technique that you may find useful:

http://www.lanl.gov/contour/

 

[etch]

another "out of the box solution would go on resonance,as surely a higher stressed part will resonate at a higher freqency

 

[kenvlach]

Besides hardness, the only ‘simple’ tests are electrical conductivity and eddy current (eddy current can be used for electrical conductivity if calibrated). Eddy current can detect changes in properties at an earlier stage than metallography. It can be used to monitor the heat treatment of age-hardenable aluminum alloys (nearly a linear increase in conductivity with hardness) as well as residual stress heating (increase in conductivity as stress decreases).
The Aluminum Association (

http://www.aluminum.org)

has info and plots in the quality control sections of both “Aluminum Standards and Data – 2003” and “Standards for Aluminum Sand and Permanent Mold Castings, 2000.” This is also covered in NDT books, e.g., ASM Handbook, Volume 17 “Nondestructive Evaluation and Quality Control.”

http://www.asminternational.org/

Another thing you might try are before and after measurements on a part that you slice and dice. Warping would demonstrate residual stress. Measure, take a thin slice through the center using lots of coolant on the saw blade, and remeasure. Then cut a big section out of the center and measure some more.

I have a question: Are these cracks that occurred in use (under load)? If not, i.e., if the cracks were found in virgin parts, then you are looking for the wrong thing. Cracking in diecast is mostly due to hot tearing after initial solidification, while the part is still in the die. The purpose of the stress relief is to allow a) finish machining w/o distortion and b) enable handling of design loads during end use.

 

[TVP]

I looked at the ASM HANDBOOKS for more information, and basically my answer stays the same-- microstructurally, there should be no changes evident after exposure to cryogenic temperatures. ASM HANDBOOK Volume 17 Nondestructive Evaluation and Quality Control has a section on Electromagnetic Techniques for Residual Stress Measurements. X-ray diffraction, ultrasonics, and electromagnetics are all discussed, but the electromagnetic techniques are only discussed relative to ferromagnetic materials (Barkhausen noise, non-linear harmonics, and magnetically induced velocity change). kenvlach's suggestion to examine the electrical conductivity appears to be a good candidate, as it is definitely a simpler test than any of the others mentioned. Good luck.

 

[gdodd]

Thanks guys
Kenvlach
The cracking occured during an engine durability test of a trial development part of unknown heat treatment.
We are currently trying to reproduce the failure by repeating the test.

I will try the eddy current option with known heat treated and as cast parts parts.

Thanks again

 

[kenvlach]

The following might be useful:
SAE Standard
Document Number: ARP891
Date Published: July 1976
Revision Number: A
Title: Determination of Aluminum Alloy Tempers Through Electrical Conductivity Measurements (Eddy Current)
Issuing Committee: G-3, Aerospace Couplings, Fittings, Hose, Tubing Assemblies

Scope:This recommended practice establishes a procedure for checking the tempers of aluminum alloys through eddy current conductivity measurements and defines criteria for acceptance or rejection.
Product Status: In Stock
File Size: 189K

http://www.sae.org/servlets/productDetail?PROD_TYP=STD&PROD_CD=ARP891A

Eddy current doesn't have a lot of penetrating power, but there should be a lot of residual stress in the skin layer.