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A SYSTEMS APPROACH

FOR ACHIEVING STRESS FREE PARTS

IN

HIGH STRENGTH ALUMINUM ALLOYS ©

BY

TOM CROUCHER

MARCH 2011

P.O. Box 6437 M Norco, CA 92860 M (888) 502-8488

Email: Tom@Croucher.us

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©COPYRIGHT 2011

byTom Croucher

All rights reserved

The use of this document is limited to the party to which it was directly sent. No partof this document may be reproduced, or transmitted in any form or by any means,electronic, mechanical, photocopying, recording or otherwise for any other purposewithout the prior written permission of the copyright owner. However, if this article isto be used for solving residual stress or quenching problems or for training purposes,permission will be readily granted by calling the author at 888-502-8488 or by email at“Tom@Croucher.us”.

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BACKGROUND AND INTRODUCTION

Residual Stresses that can cause problems with dimensional stabilityin high strength aluminum alloys have been a constant problem in theaerospace industry for many years. This problem reached its peak inthe 1960's and early 1970's. The results were: (1) premature failure ofparts in service, including both stress corrosion cracking and earlyfatigue failures and (2) unwanted part movement - both in serviceand during final machining to meet required dimensional tolerances. Unfortunately, the problem continues today as (1) the aerospace andaluminum industries push the envelope toward bigger aircraftrequiring larger and larger high strength aluminum components and(2) extremely tight dimensional stability and tolerances are required inoptical 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 beproduced with minimum residual stresses while at the same timeachieving all the structural properties desired. It is based on my 50years experience in combating these problems first as a senior leadmetallurgist at a major aerospace prime, then later directing acommercial aluminum heat treating company whose primary missionwas the production of distortion free parts, and finally, during anextensive consulting career, in assisting many who were faced withthese same types of distortion problems in high strength aluminumalloys. Through the years, I have had numerous articles published onthe subject, but to date have never presented in specific detail, thefinal systems procedure that we use to accomplish the results that wedo. This is the purpose of this paper - to outline in detail the step bystep procedure that is necessary to produce stress free, dimensionallystable, high strength aluminum alloy parts.

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

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

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2) It must be realized that 90% of our existing dimensionalinstability problems in high strength aluminum alloys result fromresidual stresses that are imparted during the quenchingoperation while being solution heat treated.

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

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

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

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

As I near retirement, it is hoped that my 50 year experience may helpthose newcomers who are now starting to face their upcomingproblems. This current article outlines in detail the current processthat we use to produce stress free parts. A previous article whichsummarizes my experience and provides a technical synopsis of theproblem can be viewed and/or printed in pdf format by clicking on thelink below to my website. There also appears on my website, athttp://www.croucher.us number of other articles on the subject.

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Key Words:

aluminum; heat treating; cryogenic; distortion; glycol; quenching; heattreatment; heat treating; liquid nitrogen; polyalkylene glycol; residualstress; solution treating; solution heat treating; stresses; stress relief;stress relieving; machining stresses, uphill quenching; warpage;water quenching; compression; dimensional stability; UconQuenchant;

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The following is a step by step outline of our process for producingstress free parts.

1) Before starting any solution heat treatment, the part must be asfree as possible from any prior residual stress, either from priorheat treatment, forming, casting or machining operations. Ifresidual stresses are suspect, the part should be annealed orfully stress relieved adequately, and then check and straightenedto bring the part back into dimensional tolerance. Residualstresses present in a part that is loaded into a solution heattreating furnace can be relieved during the heating process, andcause the part to distort in the furnace. This results in adistorted part after quenching, but it needs to be recognized thatthe distortion was not caused by the quenching process butrather by prior induced stresses. In some instances, this typedistortion can scrap a fully machined part. If the part is to besalvaged, it will probably need additional check andstraightening operations which will also induce additional stressto the part. If any straightening process to achieve dimensionaltolerance has been excessive, the part should be given anadditional stress relief and checked a second time.

2) Have a part produced to as close as possible to final dimensionbefore solution heat treatment. The thicker the part, the higherthe stress that will be imparted upon quenching. As residualstresses are almost exponentially a function of thickness duringthe quench, the lowest stress levels are obtained if the sectionthickness is reduced. However, with parts of large sectiontransitions, sometimes it is better to leave some excess materialon the thinner sections to attempt to achieve equal cooling in allareas of the part and machine if off later.

3) Avoid imparting machining stresses into the part. Be sure thatmachining is performed in a manner to eliminate inducedstresses. Adequate cooling and proper selection of cutters,feeds and speeds is necessary to avoid any heat build up in thepart while machining. High speed machining can remove a lot ofmaterial but if not done properly, can impart huge stresses intothe material.

4) Do not put stress in the parts during the quench in the firstplace. This process involves an understanding of the heat

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treating process and the effects of different quenchingprocedures on developing adequate properties in alloys of givenchemical compositions, and different thickness. Most of thisinformation is already available for common alloys, although forsome of the newer alloys, particularly with those of zirconiumadditions, quench sensitivity data needs to be developed. If it isnecessary to develop a new quench sensitivity curve, use thefive or seven bar tensile test procedure shown in References [1]and [2].

5) Define the alloy, i.e. 7075 or 7050.

6) Determine the minimum tensile strength that is required in thepart.

7) Determine the maximum thickness of the part during solutionheat treating and quenching. If possible, follow step (2) andmachine the part to as close to final dimension as possible.

8) Obtain the alloy’s quench sensitivity curve. These have beenpublished for most alloys and many of them that we use areshown in Appendix #1 which is attached. For newer alloys, asimple method shown in References [1] and [2] involving thetesting of as little as 5-7 tensile test bars can be used to developan adequate curve.

9) From the quench sensitivity curve, determine the quenching ratethat is necessary to achieve the desired strength properties fromthe quench sensitivity curve.

10) Before solution heat treating, a detailed racking plan andimmersion procedure needs to be developed and put into placethat will ensure even cooling in all areas of the part duringquenching. How a part is racked and supported is critical toachieving a uniform distribution of stress throughout the partduring cooling. The part must be allowed to expand and contractfreely and be quenched effectively without any restrictions. Aninadequate racking procedure, an improper orientation or anincorrect immersion rate of the part into the fluid both can resultin unequal stresses developed in different areas of the part. Immersing the part at an incorrect angle or immersion rate canresult in differential stresses from the first area that hit the fluid

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from the last area being immersed, particularly when using fasterquenching rates.

11) When solution heat treating, select a quenchant that will achievelow residual stresses. Ideally, you want to use a product that willcool as slow as possible to achieve low stresses, but fastenough to achieve all required properties. Using availablecharts, find a quenchant that will repeatably produce that quench rate for the part thickness determined in Step (7). Avoidwater, prefer glycol or spray if possible. Boiling water quench issometimes acceptable if desired mechanical properties can bemet. Some of the charts we use are shown in Appendix B.

12) Solution heat treat and quench the parts.

13) If there is to be any delay after the quenching operation beforestress analysis or check and straightening operations, the partmust be refrigerated immediately after quenching at -10°F orlower. Allowing the part to naturally age for any length of timecan reduce the affect of any stress relieving operation and willinduce additional stresses if a later straightening operation isrequired.

14) After quenching, measure the residual stresses by an effectivetechnique (we use x-rays) to determine the level of stressesdeveloped by the quenching process are acceptable and ifadditional stress relief is warranted.

15) If necessary stress relieve after quenching either by a. mechanical if effective on raw materialb. uphill quench if necessary on parts

16) Apply the stress relieving process if required. In manyinstances, if a slow enough quenchant is used, low residualstresses will result and additional stress relieving will not benecessary. In fact, applying an arbitrary mechanical or cryogenicstress relieving process to a part that has developed a low stresslevel as a result of a proper quenching process can actuallyinduce unwanted stresses. If stress relieving is required,consider uphilling with high velocity steam. Avoid using hotwater as the uphill media, as it is only partially effective. If it hasbeen proven that hot water will reduce the stresses sufficient for

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an individual case, the process might be acceptable.

17) Measure the resulting stresses after the stress relieving process. If the level of stress is acceptable, go directly to aging the part. Ifthe stress level is unacceptable, uphill one more time, againmeasure stresses and then go to age. Lately we have found, thatin contrast to earlier work, that sometimes more than one uphillprocedure may be necessary. This is especially true withextremely thick parts.

18) Lock in the process in detail with a fixed plan, specification ordrawing note. The plan must be extremely detailed. Justspecifying a normal heat treating or quenching requirements isnot adequate for achieving repeatable low stress parts. Quenchants types, temperature tolerances, concentration level,racking method and quench orientation, possible immersionrates must be specified.

19) Once the process is working and is locked into place, do notallow anyone to make even the slightest change without acomplete review by knowledgeable personnel especiallypurchasing agents who are trying save a little money.

Summary:

Understanding the primary causes for residual stress in high strengthaluminum alloys is paramount to achieving success in producingstress free parts. This includes an understanding of the quenchingprocess and the means by which parts can be effectively quenched toproduce low stress levels. It also includes an understanding of thedifferent methods of stress relieving, mechanical, cryogenic, and theircharacteristics, advantages and disadvantages. With thisunderstanding and a proper application of the principles advocatedhere, any aluminum part can be easily produced essentially stress freewhile at the same time meeting all required properties.

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References:

[1] T.R. Croucher; "Critical Parameters for Evaluating PolymerQuenching of Aluminum," Heat Treating, Vol XIX, No. 12, December1987; pp 21-25.

[2] Ed Blalock and Tom Croucher; A Proposed Method ForSelecting Polymer Quenchant for New Aluminum Alloy Applications,Tom Croucher and Associates; Report AMR 87-202. October 1987.

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Appendix A

Compilation of Quench Sensitivity Curves Collected From the Literature

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Figure A1 - Quench Sensitivity Curves According to Hunsicker [1]

Figure A2 - Three Point Quench Sensitivity Curves From Vruggink, Alcoa Data; UnknownPublication. [2]

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Figure A3 - Quench Sensitivity Curves Published by Mackenzie [3]

Figure A4 - Quench Sensitivity Curves For Different Alloys and Tempers [4]

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Figure A5 - 7075-T6 Quench Sensitivity Curve From Croucher Using Different GlycolConcentrations [5]

Figure A6 - Croucher Schematic Showing the Relationship of Quench Sensitivity in Two Areas.

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FigureA7 - Quench Sensitivity Curves for 2014 and 7075 Alloys Showing Effects on TensileStrength and Corrosion Resistance According to Hunsicker [1]

References

[1] H.Y. Hunsicker, "The Metallurgy of Heat Treatment"; Aluminum, Vol I, p135; Published byAmerican Society for Metals, 1967.

[2] J.E. Vruggink, "Quenching Rate Effects on Mechanical Properties of Heat Treatable AluminumAlloys," Aluminum Company of America Report, New Kensington, Pa.

[3] Scott MacKenzie; “Design of Quench Systems for Aluminum Heat Treating Part I, QuenchantSelection;” Industrial Heating; June 14, 2006.

[4] P. Kavalco, L Canale and G Totten; Quenching of Aluminum Alloys: Property Prediction byQuench Factor Analysis; Heat Treating Progress; May/June 2009.

[5] Unpublished data by Tom Croucher

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Appendix B

Compilation of Cooling Rate Charts of Different Quenchants

Collected From the Literature

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Figure B1- Cooling Rate Chart According to Hunsicker [1]

Figure B2 - Effect of Different Quenchants on the Cooling Rates of .040-Sheet - After Lauderdale [2]

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Figure B3 - Compilation of Lauchner’s Cooling Rate Data For Glycol Quenching of Sheet Metal [3].

Figure B4 - Croucher Re-Plot of Lauchner’s Data From Figure B-3.

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Figure B5 - Croucher Plots of Cooling Rate Data For AMS 3025 Required Thicknesses.

Figure B6 - Original Croucher Plots - Effect of Glycol on Cooling Rates of Thin Plate Material.

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Figure B7 - Original Croucher Cooling Rate Data For Plates Plotted According to Hunsicker Method. [4]

Figure B8 - Original Croucher Glycol CoolingRate Data For Plates Up to 3-inches Thick. [5]

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Figure B9 - Cooling Performance of Water at Four Different Temperatures Compared to Two GlycolConcentrations Plotted According to Hunsicker Method [6].

Figure B10 - Croucher Semi-Log Plots of Glycol Cooling Rate Data Up to 6-inch Thick.

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References:

[1] H. Y. Hunsicker, "Chapter 5, The Metallurgy of Heat Treatment" Aluminum Vol 1; American Societyfor Metals; 1967, pp 136-137.

[2] Lauderdale, R.H., "A New Quenchant for Thin Gage Aluminum;" Metal Progress, December 1967.

[3] Lauchner, E. A. and Smith, B. O., "Evaluation of Ucon® Quenching," NOR 69-65, NorthropCorporation, May 1969.

[4] Tom Croucher; "Applying Synthetic Quenchants to High Strength Alloy Heat Treatment" Presented October 1970 at the ASM National Metal Congress, Cleveland, Ohio; ASM, MetalsEngineering Quarterly May 1971.

[5] Tom Croucher; “Polymer Quenching, Their Advantage For Quenching Aluminum Alloys, HeatTreating”; November 1982. Volume XIV, No.11, pages 18-19

[6] Tom Croucher; “Effectively Quenching Thick Sections of High Strength Aluminum Alloys UsingPolyalkylene Glycol Quenchants” Presented to SAE Amec, October 2009. Available at www.croucher.us.