A SYSTEMS APPROACH

FOR ACHIEVING STRESS FREE PARTS

IN

HIGH STRENGTH ALUMINUM ALLOYS

 

BY

TOM CROUCHER


MARCH 2011


 P.O. Box 6437 ● Norco, CA 92860 ● (888) 502-8488
 Email: Tom@Croucher.us
 
Click HERE to View and/or Print This Entire Article on the “Systems Approach” in pdf format.


SUMMARY:

Residual Stresses that can cause problems with dimensional stability in high strength aluminum alloys have been a constant problem in the aerospace industry for many years.  This problem reached its peak in the 1960's and early 1970's.  The results were: (1) premature failure of parts in service, including both stress corrosion cracking and early fatigue failures and (2) unwanted part movement -  both in service and during final machining to meet required dimensional tolerances.   Unfortunately, the problem continues today as (1) the aerospace and aluminum industries push the envelope toward bigger aircraft requiring larger and larger high strength aluminum components and (2) extremely tight dimensional stability and tolerances are required in optical components such as space mirrors and telescope parts.

This article presents, in detail, a systems approach which we have  developed over the past 50 years.   We have consistently proven that, when applied correctly, any high strength aluminum alloy part can be produced with minimum residual stresses while at the same time achieving all the structural properties desired.  It is based on my 50 years experience in combating these problems first as a senior lead metallurgist at a major aerospace prime, then later directing a commercial aluminum heat treating company whose primary mission was the production of distortion free parts, and finally, during an extensive consulting career, in assisting many who were faced with these same types of distortion problems in high strength aluminum alloys.  Through the years, I have had numerous articles published on the subject, but to date have never presented in specific detail, the final systems procedure that we use to accomplish the results that we do.  This is the purpose of this paper - to outline in detail the step by step procedure that is necessary to produce stress free, dimensionally stable, high strength aluminum alloy parts. 

In order to understand the basis for our approach, it is necessary to grasp a few fundamental concepts. 

1) It must be understood that all residual stresses in aluminum alloys are the result of unequal expansion and contraction in a part during processing.  This is true whether these stresses result from mechanical or thermal means. 

2) It must be realized that 90% of existing dimensional instability problems in high strength aluminum alloys result from residual stresses that are imparted during the quenching operation while being solution heat treated. 

3 That the quenching process is a freezing process that attempts to freeze the hardening atoms that have been positioned during the solution heat treating process, in place.  To accomplish this freezing process, the cooling rates must be fast enough to prevent significant diffusion during cooling.  

4) During the cooling process, the aluminum is not smart enough to know what it is being cooled by.  It does not understand the difference between cold water, hot water, glycol or a spray.  It only understands that it is being cooled at some rate which may or may not allow diffusion to take place. 

5) Over the past 50 years, the emergence of less quench sensitive alloys and of newer quenching fluids and methods, mainly the polyalkylene glycols, allow for the first time a truly engineered approach to solving problems of troublesome residual stress.

6) The proper application of a cryogenic stress relief method of stress relieving “as quenched” parts by a thermo-mechanical technique (commonly called uphill quenching - developed by Alcoa in the late 1950's), allows for an effective stress relief technique to be employed to relieve or reduce these high quenching stresses on finished complex parts.

As I near retirement, it is hoped that my 50 year experience may help those newcomers who are now starting to face their upcoming problems.  This current article outlines in detail the current process that we have used to produce stress free parts and can be viewed and/or printed in pdf format by clicking on the link (1) below.  A previous article which summarizes my experience and provides a technical synopsis of the problem can be viewed and/or printed in pdf format by clicking on the link (2) below.  There also appears on my website, at http://www.croucher.us  a number of other articles on the subject.

(1) Click HERE to View and/or Print This Entire Article on the “Systems Approach” in pdf format.

(2) Click HERE to View and/or Print Entire Article regarding my previous experience in pdf format.


Key Words:

aluminum; heat treating; cryogenic; distortion; glycol; quenching; heat treatment; heat treating; liquid nitrogen; polyalkylene glycol; residual stress; solution treating; solution heat treating; stresses; stress relief; stress relieving; machining stresses, uphill quenching; warpage; water quenching; compression; dimensional stability; Ucon Quenchant;