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MINIMIZING MACHINING DISTORTION

IN

ALUMINUM ALLOYS

THRU SUCCESSFUL APPLICATION

OF UPHILL QUENCHING -

A PROCESS OVERVIEW


BY


Tom Croucher

Tom Croucher and Associates

Norco, CA, USA


June 2009


This is a preprint of an article accepted for publication in the Journal of ASTM International (JAI) Copyright @ 2009 by ASTM.



ABSTRACT


Recently, emphasis has been placed on achieving maximum dimensional stability in a wide range of critical aluminum components such as large complex aerospace machined parts, space mirrors, optical stages, lens mounts, and computer disks. In these cases, absolute stability is required before putting a part into service. In many cases, successful operation of a complete system hinges on the stability of a single part such as a space mirror in a satellite or a component of a laser guided missile. Proper application of the uphill quenching process has enabled the successful production of parts to provide minimum stresses resulting in the optimum component at the lowest cost. Basic principles for achieving dimensional stability and considerations for proper application of unique tools such as polyalkylene glycol polymer (glycol) quenching and uphill quenching to achieve maximum dimensional stability are discussed. Properly applied, and combined with an understanding of all factors contributing to part instability, these tools can be used as effective elements in a systems approach, which when effectively employed by experienced technicians, will achieve results not obtainable by other means. In addition, examples are provided illustrating successful use of the process to produce low stress parts.


KEYWORDS: uphill quenching, glycol quenching, water quenching, stress relief, aluminum, residual stresses, heat treatment, cryogenics, liquid nitrogen


The author would appreciate any comments made on this material and welcomes a dialogue with others in the industry who have had unique experiences in this field. He may be reached by email at <tom@croucher.us>, or by phone at his office at 909-502-0200.


INTRODUCTION


Stress Relieving Heat Treated Aluminum Alloys


Most stresses are introduced in heat treated aluminum alloys during the quenching process. In many instances, in spite of applying the best conventional quenching process after solution heat treating, residual stresses still remain that often cause problems during subsequent machining of the parts or when the parts are placed in service. To reduce these remaining quenching stresses to an acceptable level, an understanding of the various processes used to relieve and minimize these residual stresses is necessary.


How are Stresses Relieved?


For a part to exhibit a high residual stresses, it must contain both tensile and compressive stresses. A tensile stress means that some portion of the part is being elastically stretched and held in that stretched position by the remaining mass of the part which then assumes a level of compressive stress. A compressively stressed area is that portion of the part which is being elastically compressed and is held in that position by the remaining mass of the part which then must assume a net tensile stress.


The residual tensile and compressive stresses in the total part must balance each other. If one area of the part contains tensile stresses, another area of the part must exhibit equal compressive stresses for the net stresses to be equal to zero. Parts distort during machining when an area of the part that is in one residual stress mode is removed causing an imbalance of the tensile/compressive layers. Thus, the part moves or distorts to maintain a balance of compressive and tensile stresses. If the resulting residual stresses in the part are zero or very low, no movement occurs.

 

Residual stresses are normally relieved by allowing that area of the part that is elastically stressed to progress to a plastic state. In this state, differential strains that produce high residual stress in the elastic condition are relieved by allowing one area to plastically stretch or compress placing the balancing area in a much lower stress level. Stress relief occurs when the level of residual stress is greater than the yield strength of the material in its current plastic state. Thus, portions of the part actually yield (on a microscopic scale) to accommodate the excessive stress and strains introduced.


Historically, for aluminum alloys, the plastic stress relieved condition is achieved by one of the following methods:


 THERMAL - Involving the use of heat by raising the part to an elevated temperature.


 MECHANICAL - Accomplished by mechanically stretching or compressing the material.


THERMO-MECHANICAL - Commonly called Uphill Quenching which is achieved using cryogenics and heated steam.


To understand the principles of uphill quenching, and why and when this process is both technically and economically advantageous, one must have an understanding (including advantages and disadvantages) of the thermal and mechanical stress relieving techniques


Thermal Stress Relieving


The plastic condition can be attained by heating a metal to a elevated temperature which is sufficiently high to effectively reduce the yield strength to a level below the locked-in residual stress. This procedure is not practical for heat treated aluminum alloys because temperatures high enough to relieve stresses cause further precipitation and aging to occur and thus seriously reduce strength. Prolonged exposure to elevated temperatures, which are in excess of the aging temperature, will significantly reduce room temperature tensile properties after exposure. The aging temperatures are not sufficiently high to effectively reduce the yield strength to achieve a high degree of stress relief. This effect is illustrated in Figure 1 which shows that a 250 ̊F (121 C) exposure for periods as long as 50 hours will not effectively reduce the yield strength of a 7000 series aluminum alloy [1]. Even for prolonged exposures up to temperatures of 300 ̊F (149 ̊C) and 350 ̊F (177 ̊C), the yield strength is only slightly reduced.


Figure 1 -  Effect of Aging Temperature on Reduction of Yield Strength For a 7000 Series Alloy. [1]


To effectively achieve significant reduction of the yield strength which could then be used as a stress relieving technique, a 7000 series aluminum part must be heated to a temperature in excess of 450 ̊F (232 ̊C). However, parts that have been heat treated and aged to the T6 or T7 tempers cannot be subsequently heated to these high temperatures because the strength level of these parts would be significantly reduced and the parts would not meet the specification requirements. In some instances, thermal techniques in the range of the aging temperature can be used to relieve severe fabrication stresses, but this practice is only recommended on a case by case basis.

 

Stress Relief by Mechanical Deformation

The desired plastic state can also be attained by mechanically deforming a part at room temperature immediately after quenching when the yield strength of the part is relatively low. This technique is the basis for the commercially successful stress relieving methods of stretching and compressing. In stretching, the workpiece is stretched 1.5 - 3.0 percent, which is sufficient to stress local areas into the plastic region and thus relieve the differential strain condition by yielding. Stretching is regularly applied to products of uniform cross-section such as sheet, plate and extrusions. Compression is applied to products that are not suited to stretching, such as hand- forgings and some die-forgings.

Removing heat treating stresses by stretching can be an extremely effective technique for achieving low-stress plate and extruded products. Special tempers are assigned to these products. Stretched products are given a T51 or T51X designation to indicate that the product has been stretched after solution heat treating and quenching to reduce the quenching stresses. However, many problems have been encountered with this technique. The technique is limited to uniform cross-sections such as sheet, plate and extrusions. Also, as the parts become thicker in cross-section, the equipment needed to uniformly stretch the material becomes much larger and effective stretching is more difficult and complicated in practice. The problem of slipping jaws during the stretching leads to inadequate movement of the material and ineffective stress relieving. Differential cross sections in extrusions can also lead to problems in obtaining a uniform stretch.

While mechanical deformation methods using compression have been used successfully on some forged rings and hand- and die-forgings, this technique is not generally effective for relieving residual quenching stresses in parts of an irregular shape. Mechanical deformation is also limited because it is only partially effective on extremely large parts because of the difficulty of yielding the center portion of the part. This technique is also not applicable to castings, cannot be applied once the part has been machined, and cannot be used in the commercial heat treating industry because of the necessity of having available compressing dies for each configuration.

 

Uphill Quenching - An Introduction

The bridge between the invention and successful application of useful technology has often been a mystery that is sometimes difficult to understand. Extremely effective and proven processes, which often have the potential to provide both technical and economic advantages often go unnoticed for years (sometimes forever) before some situation occurs that dictates that their application must be taken seriously.

Such is the case of "Uphill Quenching", a sub-zero process developed for reducing or eliminating residual quenching stresses in heat treated aluminum alloys. Developed about 40 years ago by Alcoa engineers [2], this process has remained relatively unrecognized as an "interesting phenomena" in most engineering design and manufacturing operations. Also, because of the similarity of the process to the sub-zero, "austenite ➔ martensite" transformation process used by steel heat treaters, uphill quenching is often misunderstood and mis-applied by those who have not properly understood the process or implemented it correctly.

 

THE NEED FOR UPHILL QUENCHING

High strength aluminum alloys attain their strength through heat treatment. While the specific treatment depends upon the particular alloy being treated, most parts are quenched in water from a solution heat treating temperature, usually in the range of 870-1000 ̊F (427-538 ̊C). During the quench, the part surfaces cool faster than the interior of the part. Thermal gradients are created causing different areas of a part to contract at different rates. During the later stages of cooling, these gradients disappear, but their effect introduces an uneven distribution of residual stresses. These residual quenching stresses are the major cause of part instability during subsequent machining operations and can often cause stability problems later while the part is in service, e.g. dimensional change.

Ideally, residual stresses are compressive on the part surface, which cools first, and tensile in the slower cooling interior. The magnitude and distribution of the final stress varies with the particular alloy, the thickness of material and especially the severity of the quench. However, when quenching actual production parts, the surface stresses are not always in compression. Stresses at the surface can be both compressive or tensile depending upon the configuration of the part, how the part was racked and the effectiveness of the quenching fluid to extract heat at each location on the part.

After the quench, the tensile and compressive stresses present in the part are completely balanced resulting in a total net stress = 0 for the entire part. The compressive stresses and the tensile stresses must balance each other. If the net stress is not zero, the part will move in the direction of the larger stress until a complete balance is obtained and the part achieves equilibrium.

It is not readily apparent by just viewing a part whether or not it may contain a high level of residual stress. Larger parts, because of their total mass, may not have experienced distortion during the quench. Thus heat treated parts, although visibly acceptable, may contain an unacceptable level of residual stress and the part may be prone to twisting or warping to relieve internal stresses upon subsequent material during machining. During machining, metal removal disturbs the original stress distribution and a new distribution is established. The resulting re-orientation of the stress pattern normally results in part distortion which allows for the net stress on the part to remain at a zero value. This distortion can significantly increase the machining costs and often leads to high rejection rates due to a failure to meet dimensional tolerances.

 

THE PRINCIPLE OF "UPHILL QUENCH"

The technique of "Uphill Quenching" has seen limited use over the past fifty years. In its initial form, it was known as "deep freezing" or "tri-cycle stress relieving". In its early use, parts were usually immersed in dry ice followed by an immersion in boiling water. Reports on the success or failure of the technique to achieve effective stress relief varied until the late 1950's when Alcoa metallurgists studied the process in detail and further developed it using a liquid nitrogen/steam approach [2]. They reasoned that since the residual quenching stresses result from thermal gradients which are induced when the part is being cooled, it should be possible to develop residual stresses of an opposite nature by subjecting a cold piece to rapid heating; i.e, by an "uphill quench." For maximum effectiveness, the uphill quench would have to develop more severe temperature gradients (ΔT) than were obtained by the conventional deep freeze approach. Furthermore, such a treatment could not involve temperatures sufficient to affect tensile properties.

An extensive investigation was then conducted to evaluate and compare different methods of uphill quenching and to develop the most effective method for use in stress relieving complex heat treated aluminum components [2]. The results showed that optimum stress relief involved the use of liquid nitrogen and high pressure steam.

Other techniques showed various degrees of effectiveness as shown in Table 1. These other techniques, particularly when boiling water was used instead of high-velocity steam, all resulted in much lower stress relief than the liquid nitrogen/high-velocity steam technique. Because the treatment was thermal in application while the residual stress relief was accomplished by mechanical plastic deformation, the process was termed "thermo-mechanical."

Table 1: Comparison of Different Uphill Quenching Methods [2]


The Uphill Quenching Process

The steps involved in an uphill quenching process include:

Step 1       Effectively quench the parts after solution heat treating. Select the quenchant which will provide the optimum quenching power to achieve the desired mechanical properties.

Step 2       Cool the parts in liquid nitrogen. If the parts cannot be immediately processed, with the nitrogen, they must be stored in a ice-box at (< 0 ̊F, -17.8 ̊C) to retard any room temperature aging which may reduce the effectiveness of the process.

Step 3       Let the parts stabilize at temperature in the liquid nitrogen.

Step 4       Immediately transfer the parts to the steam chamber. The transfer time should be kept to a minimum avoiding any ice build-up on the parts and the part must be steamed on all surfaces.

Step 5       Uphill Quench with high-velocity steam. The duration of the steam application is dependent upon the thickness of the part, but should not be excessive.

Step 6       Measure the part temperature to verify that it has reached the proper temperature during the steaming operation.

Step 7       Measure the residual stress of the part, preferentially with the x-ray diffraction. The Sach's hole drill method in a proper location may be used if approved by the customer.

Step 8       Age the part in a normal manner.



Effectiveness of Uphill Quenching

To effectively determine the need for stress relieving parts after quenching, it is necessary to measure the residual stress levels that result from water quenching [16] different thickness materials. Table 2 illustrates the different stresses that result when 1/4 in. (6.35 mm) to 2 in (50.8 mm) thickness material is water quenched using different water temperatures [2]. A high residual stress of approximately 33,500 psi (231 MPa) was measured for all thicknesses of materials above 1/2 in (12.7 mm) when a cold water quench was used. The thinner 1/4 inch (6.35 mm) material exhibited a lower stress level because a smaller temperature differential (ΔT) was established between the surface and center. When the 1/4 inch material was quenched in 150 F (66̊C) water, the residual stresses measured were lower. This data illustrates that the greatest dimensional stability problems are encountered when thicker materials are water  quenched, especially when cold water is used. The necessity of creating the greatest (ΔT) during the uphill process is shown in Figure 2.


As previously shown in Table 1, various uphill quenching practices are currently used in the aluminum heat treating industry. The effectiveness of a particular technique is a direct function

of the ΔT that is developed during the uphill quench. The Alcoa data [2] showed that the largest ΔT achieved, (which resulted in the greatest stress relief), was only achieved with the liquid nitrogen and the high-velocity steam. (See Table 1). The stress relief achieved with this procedure reached a level of 82%. The low-velocity steam was not nearly as effective as the high-velocity steam. Thus, for maximum stress relief, the mere presence of steam is not sufficient to insure a good stress relief. The steam velocity must be extremely rapid to obtain the maximum effect. In practice, the specific technique for each part must be verified by measuring the part temperature after the part has been steamed and the effectiveness of the technique verified by measuring the final stress level of the part by x-ray diffraction or a Sach's material removal method.

Figure 2 - Effect of ΔT During Uphill Quenching On The Reduction of Residual Stress [2]

When boiling water was used rather than high velocity steam, (with either LN2 or dry ice), the maximum residual stress relief achieved was a low 19%, so the use of boiling water with uphill quenching should only be considered when a minimum degree of stress relief is acceptable. It should not be used when maximum stress relief is desired as in the case of space mirrors. This was confirmed by this author in recent test programs conducted when only 15% stress relief was achieved by the boiling water technique while a 91% stress relief was realized by the high velocity steam approach using ½ -inch plate samples of 7075 alloy. [3] [4]

The uphill process is extremely critical as to the timing of application. Since aluminum alloys naturally age after quenching, resulting in an increase in their yield strength at room temperature, the process loses it's effectiveness. The Alcoa data in Table 3 [2] shows that when the uphill quench was performed within an hour after the initial quench, the stress relief was extremely effective as the stresses were reduced from 24,000 psi (165 MPa) to 4,000 psi (27.5 MPa). However, if a part was allowed to remain at room temperature for eight hours, the stress relief was not as effective with a resulting value of 10,000 psi (69 MPa). After one day exposure, the final residual stresses were 14,000 psi (96.5 MPa).

When uphill quenching was performed after the material had been aged to the T6 temper, the parts exhibited no stress relief. Thus it is imperative that the stress relief be conducted as soon as possible after the quenching operation and that using the process in any form to relieve quenching stresses in the T6 temper is, in most cases, ineffective. In spite of this fact, many companies wrongly assume that they can apply the process to fully aged parts and achieve significant stress relief.


PRODUCTION RESULTS 

There are numerous production examples that illustrate the ability of uphill quenching to increase dimensional stability of aluminum parts by reducing machining distortion and by achieving improved part stability in service applications. Recent applications are thick die- forgings, electronic components, machined parts taken from thick hand-forgings or plate products, optical mirrors and some premium castings. Specific examples illustrating the benefit of using this technique to solve production machining problems are discussed below. Recent research has also shown the benefit of combining uphill quenching with controlled quenching techniques [4,5].

Production Examples

Uphill quenching using the high-velocity steam approach is used effectively with conventional water quenching techniques. This process is optimized when coupled with glycol quenching methods in a system approach which provides further tools for minimizing the stresses. [7-15] In most cases, the lowest stresses are achieved when both glycol quenching and the high-velocity steam approach are used. However, depending upon the part configuration, thickness, and quench sensitivity of the alloy being process, low stresses can be achieved by the used of glycol quenching alone [7-15] or uphill quenching alone.

Uphill quenching practices must be performed by experienced personnel who are familiar with the practices for applying the high-velocity steam, design and use of tooling and proper quality control aspects, particularly the final measurement of the stresses.


EXAMPLES OF REDUCING STRESSES THROUGH UPHILL QUENCHING USING NORMAL WATER QUENCHING METHODS

Example -1- 7049 Cut Plates [5]

This example involved a number of 7049 plates approximately 10 x 10 in (25.4 x 25.4 cm) in three different thicknesses. A major aluminum mill had produced the 7049-T7351 stretcher/stress relieved plates to allow an airframe manufacturer to machine them with no distortion. The aluminum producer was experiencing a problem with their stretching procedure, and the plates were not adequately stretched to achieve the desired stress relief. When the production plates were cut into 10 x 10 in (25.4 x 25.4 cm) squares and subsequently machined, there was significant distortion noted during the machining operation. The cut-up plates could not be used in their current condition. Re-heat treating and re-stretching was not an option because all the production plates had already been cut into the 10 x 10 in (25.4 x 25.4 cm) sections. A test program was conducted to illustrate the effect of reducing stresses by re-heat treating and uphill quenching the previously heat treated cut plates.

The residual stresses in the original test plates were measured by X-ray diffraction and showed stress levels as high as 24,000 psi (165.5 MPa). The plates were re-solution heat treated, quenched in cold water per the specification requirement and then uphill quenched using the liquid nitrogen, high velocity steam approach. Immediately after quenching, the residual stresses were measured and the quenching stresses measure from 19,000-23,000 psi (131 - 158.5 MPa), which was comparable to the stresses in the original stretcher/stress relieved plates confirming the fact that the stretching done by the mill on these plates was totally ineffective. After uphill quenching and final aging, the residual quenching stresses were reduced to a range of 4000 - 8000 psi (27.5 - 55.1 MPa). A summary of results is shown in Table 4.


Example 2 - 2014 Dynamic Gyro Forging


A typical large component that was successfully processed is a large 2014 hand forging with a 7-in (17.78 cm) cross-section shown in Figure 3. The part had been originally heat treated by a competing vendor. To insure against possible machining distortion, the customer had the residual stresses measured by x-ray diffraction and the results showed stress levels as high as 42,000 psi (289.5 MPa), which would have led to significant machining distortion during final machining. Because of its heavy section thickness and because premium properties were required in the part, glycol quenching could not be effectively used to reduce the stresses. The customer required cold water quenching.

The part was re-heat treated and uphill quenched using a liquid nitrogen, high-velocity steam approach immediately after the cold water quench. Residual stress measurements made after the aging operation showed that the highest surface stress present after aging was 6000 psi (41.4 MPa). The part was subsequently machined with no noticeable distortion.


Figure 3 – AA 2014-T6 Dynamic Gyro Forging

EXAMPLE 3 - C355 CAST HOUSING

A unique illustration of the cooperation between a foundry and an independent heat treater is a cast housing manufactured from C-355 alloy. This part illustrates that solution heat treating and quenching can be performed at one facility and the uphill quenching portion of the systems process performed in a different facility. In this instance, the castings are solution heat treated and quenched in a normal manner at the foundry and then placed on dry ice to achieve a temperature below -10 F (-23.3 ̊C). The parts were transported to the heat treater who placed them in a controlled temperature freezer at -10 ̊F (-23.3 ̊C) or lower. Parts and required test bars were then uphill quenched using the high-pressure steam approach. Process control required that one part of each lot be tested for residual stresses by x-ray diffraction and must routinely meet the requirement of 6 ksi (41.4 MPa) or lower. The results of typical production loads showed that the normal residual stresses constantly fell within the range of 0-6 ksi (0-41.4 MPa). After uphill quenching, the parts and test bars were then returned to the foundry for final aging.

 

EXAMPLES ILLUSTRATING STRESS REDUCTION THROUGH COMBINATION PROCESSING


The following examples illustrate the successful application of the systems approach utilizing a controlled quenching practice incorporating glycol quenching [6-15] and an uphill quench following the solution heat treating operation.

 

Example 4 - Cast Electronic Chassis Housing

A large (approximately 3 ft. x 4 ft (0.91 m x 1.22 m) electronic chassis housing, cast from A-356 alloy, housed electronic components in a nuclear submarine. When the production contract to the foundries producing the part came up for renewal, it was let to a new foundry and a new heat treat company. While the new order of castings were being machined at the prime contractor, severe distortion and cracking occurred in the castings due to the high residual stresses in the part. Cracks and part movement up to 1.0 in (25.4 mm) occurred in some areas as shown in Figures 4 and 5. An audit of the procedures revealed that the problem occurred from the lack of control while purchasing the parts. The original vendor, realizing the problem, had been solution heat treating the parts, quenching them in 22% glycol, and then applying an uphill quench procedure to relieve the residual quenching stresses. When the parts went out for re-bid, to save money, the purchasing department eliminated the requirement for the glycol quench/uphill quench procedure and agreed to a contract without these provisions. Thus, the new foundry and heat treater needed only to follow normal heat treat specification requirements that called for a hot water quench and no uphill stress relieving because the provisions for uphill quenching were not on the drawing and were not part of the contract. Eliminating these requirements had left extremely high residual stresses in the parts that led to the cracking during the machining operation. The problem was easily resolved by returning to the systems approach of glycol quenching followed by the uphill quench of the production castings.
 

Figure 4 - Cast Electronic Chassis Housing



Figure 5 - Two Cracks Showing Movement Caused by Residual Stresses

Example 5 - Optical Mirror

This example involved an optical mirror machined from a 6061-T6 forging shown in Figure 6. The parts were procured in the T6 condition by the customer, rough machined, and then reprocessed using the systems approach. At the time of re-heat treatment, the parts measured 4 1/4 x 2 1/8 x 1.0 in (10.8 x 5.40 x 2.54 cm) high and weighed only 0.27 lb (0.122 kg). Systems analysis showed that the optimum processing involved the use of glycol quenching plus an uphill quench after the re-solution heat treat and quenching operation. After the liquid nitrogen soak, parts were steam blasted for 10 seconds. Residual stresses measured on the final parts after final aging showed a low level of 1.75 ksi (12.06 MPa). The parts were then final machined to a tight tolerance of +0.010 in (+0.254 mm)with no distortion during the machining process.

This part is an example of parts being procured in a fully heat treated T6 condition in order to facilitate machining operations, and then being re-heat treated back to the T6 condition to achieve the final optimum part with high mechanical properties and low stress levels.

Figure 6 - Optical Mirror Successfully Uphill Quenched

Example 6 - Computer Support Rail

Figure 7 shows a rail manufactured from the A356 casting alloy. This part is used for supporting computer components needing a high level of dimensional stability. Parts are heat treated in the rough cast condition using a 22% glycol concentration for the quenchant, ice boxed, and then uphill quenched for 30 seconds using the high velocity steam approach. These parts have been continuously processed for over 20 years, and have repeatedly demonstrated adequate mechanical properties and low residual stresses. All machining is performed in the final heat treat/stress relieved condition and abnormal machining distortion has never been observed.

Figure 7 - A-356 Cast Computer Support Rail

Example 7 - Hub Assembly

Figure 8 shows a hub assembly, manufactured from 7075 aluminum bar. The customer requires that the parts be heat treated to the T73 temper and requires residual stress levels be kept to about 6000 psi or below. To facilitate the machining operation, the material is procured in the T7351 stretcher stress relieved bar and rough machined. A systems approach was recommended where the parts would be re-solution heat treated, quenched in 22-25% glycol solution and then uphill quenched (using the high-pressure steam approach) prior to final ageing. The customer required the heat treatment to be according to AMS 2770. Since AMS 2770 does not allow the quenching of 1.6 in. (4.06 cm) bar in glycol, let alone a higher concentration of 22-25%, special instructions had to be added to the drawing to allow the practice of glycol quenching after solution heat treatment. Preliminary tests and final tests on all subsequent production loads showed that mechanical properties, well in excess of the minimums required were achieved with this systems approach.

Initial residual stress measurements showed that the stresses in the parts received at the heat treater were ranging from 35 - 42 ksi (24.1 - 28.9 MPa). After applying the proper systems heat treat processing, the residual stress levels were reduced to about 6 ksi, a reduction on the average of 85%.


This part is another example of parts being procured in a fully heat treated condition to facilitate initial machining operations, and then being re-heat treated to achieve the final optimum part with high mechanical properties and low stress levels. It also illustrates that if some raw stock is received in a stress relieved condition by mechanical stressing after final machining, the stresses can still be unusually high. The high stress level can be attributed to be a combination of inadequate stretch in the original material plus the stresses imparted by the machining operation. In this case, the problem was not the machining distortion in the original T73, but instability and resulting part movement during the final cuts.

 

                                            Figure 8 - 7075-T73 Hub Assembly

Example 8 - Large 6061 Forging


Figure 9 shows a large 6061 forging being placed in the high velocity steam chamber after the part had reached the liquid nitrogen temperature. Prior to producing the part in this manner, attempts were made to machine the part directly from a hand forging in the T652 temper. All parts produced by machining in the T652 temper had to be scrapped because they could not meet the dimensional tolerances required. This part illustrates the fact that extremely large die or hand forgings can be uphilled successfully, particularly if the systems analysis is correctly applied. The rough forging in this case weighed over 2000 pounds. The forging was rough machined to a weight of 700 pounds at which point it was re-heat treated, quenched, uphill stress relieved and then aged to the T6 condition. The final machining operation further removed material to a final weight of the part of 70 pounds. No part movement was detected during the final machining operation and several parts have been successfully produced using this uphill/high velocity steam approach.

Figure 9 - Large 6061 Forging Being Placed Into The High Velocity Steam Chamber

Disadvantages of Uphill Quenching

In spite of the proven effectiveness of the uphill quenching technique for achieving dimensional stability in many parts, the process is not applicable for all situations and has some disadvantages. As the most common cause of dimensional instability in aluminum alloys is the quenching operation, careful selection of a boiling water or glycol quench can potentially achieve dimensionally stable parts without the need for an uphill quench. Also, test data has shown that uphill quenching may actually result in an increase in the stress level and part instability for parts that have been quenched extremely slowly. As uphill quenching is a thermo-mechanical process, applying it to parts that already exhibit an existing low level of residual stress, may cause a stress reversal.

Thermo-mechanical stress relieving only works when thermal gradients are introduced. As a result, the process is not particularly effective in relieving fabrication stresses that result from severe forming or welding operations. These stresses should be relieved by another suitable process, preferably before a solution heat treating operation.

Uphill quenching is a relatively expensive process and currently the size of parts that can be processed is somewhat limited. When steam is used as the heating medium, extensive fixturing is often required to achieve proper steam impingement and large volumes of liquid nitrogen may also be required. Typical steam fixtures are shown in Figures 10 and 11. As further successes are reported however, larger chambers will provided and thus larger parts will be able to be processed.

Figure 10 - Clamshell Steaming Fixture (Courtesy of Newton Heat Treating)




Figure 11 - Steaming Fixture for Hollow  Cylinder     Application         (Courtesy         of Newton Heat Treating)

Because of the lack of interest over the past 30 years by major companies in the industrial community, adequate industry generated process specifications and technical data regarding the effect of different process parameters have not been provided. As higher strength alloys are more widely used and the need established for improved dimensional stability for extended service applications, interest in this process has and will increase.

 

SUMMARY

A unique capability for achieving heretofore unattainable results has been demonstrated. It has been shown that application of a systems analysis approach may potentially produce any high strength aluminum part which meets all drawing requirements and a low level of residual stress thereby avoiding severe manufacturing problems due to machining distortion. Residual stresses present in parts can effect their performance and high residual tensile stresses can lower fatigue life and cause severe stress corrosion cracking problems in service.

 

REFERENCES

  1.  G.M. Orner and S.A. Kulin, "Development of Stress Relief Treatments For High Strength Aluminum Alloys" NASA Contract 11091; Quarterly Progress Report; 1964, Man Laps Inc. Document Number: UR-65-0504.
  2.  H. N. Hill, R.S Barker and L.A Willey, "The Thermo-Mechanical Method For Relieving Residual Quenching Stresses in Aluminum Alloys" Transactions of the ASM, 1960, Vol 52, 1960, pp 657–674
  3. T. Croucher, "Using Uphill Quenching to Effectively Stabilize Machined Aluminum Parts", Internal Report, October 2006, Report Available Upon Request: Tom Croucher and Associates, PO Box 6437, Norco, CA, 92860, USA, Tel: 909-502-0200, E-Mail: tom@croucher.us
  4. T. Croucher; "The Effect of Different Uphill Quenching Processing Parameters on the Reduction of Residual Stress", Internal Report, February 2008, Report Available Upon Request: Tom Croucher and Associates, PO Box 6437, Norco, CA, 92860, USA, Tel: 909-502-0200, E-Mail: tom@croucher.us
  5. T. Croucher, "Uphill Quenching of Aluminum", Heat Treating, 1983, October, p. 30-34.
  6. SAE AMS 3025B, "Polyalkylene Glycol Heat Treat Quenchant", SAE International, 400 Commonwealth Drive, Warrendale, PA 15096-0001, July, 2000.
  7. SAE AMS 2770H, "Heat Treatment Of Wrought Aluminum Alloy Parts", SAE International, 400 Commonwealth Drive, Warrendale, PA 15096-0001, August, 2006.
  8. SAE AMS 2771C, "Heat Treatment Of Aluminum Alloy Castings", SAE International, 400 Commonwealth Drive, Warrendale, PA 15096-0001, July, 2004.
  9. T. Croucher, "Critical Parameters for Evaluating Polymer Quenching of Aluminum", Heat Treating, 1987, December, p. 21-25.
  10. T.R. Croucher and M.D. Schuler, "Applying Synthetic Quenchants to High- Strength Alloy Heat Treatment", Metals Engineering Quarterly, 1971, May, p. 6-11.
  11. T. Croucher, "Synthetic Quenchants Eliminate Distortion", Metal Progress, 1973, p. 52-55.
  12. T. Croucher and D. Butler, "Polymer Quenching of Aluminum Castings", 26th National SAMPE Symposium, April 26-28, 1981, p. 527-534;
  13. T. Croucher and D. Butler, "Proper Racking - The Key to Distortion Control for Aluminum Alloys", Heat Treating, 1983, Vol. 15, No. 3, p. 19-20.
  14. T. Croucher and D. Butler, "Racking for the Quench: Critical Factors for Controlling Distortion", Heat Treating, 1983, Vol. 15, No. 5, p. 16-17.
  15. T.R. Croucher and M.D. Schuler, "Distortion Control of Aluminum Products Using Glycol Quenchants", Metals Engineering Quarterly, 1970, August, p. 14-18.
  16. T. Croucher, "Water Quenching Procedure for Aluminum Alloys", Heat Treating, 1982, Vol. 14, No. 9, p. 18-19.

 

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