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Fluidized Bed Quenching Performance and Its Application for Heat TreatingAluminum Alloys

J. KeistArizotah, LLC, Plymouth, MN, 55447 USA

S. Chaudhury and D. ApelianMetals Processing Institute, WPI, Worcester, MA, 01609 USA

Abstract

In the heat treatment of aluminum alloys, fluidized bedquenching is an attractive alternative to liquid quenching

processes since the part does not develop a vapor barrierduring quenching. This lack of a vapor barrier significantlyreduces residual stresses and part distortion that often plaguesliquid based quenching techniques. The heat transfer rate of

the quenching process, however, is lower than can be obtainedby liquid based quenchants. The lower heat transfer rate mayrule out fluidized bed quenching for some applications due topart geometry or alloy quench sensitivity. This paperproposes a method to determine the applicability of fluidized

bed quenching for a given part. In this research, two castingsof a given geometry and alloy were analyzed for the feasibilityof fluidized bed quenching. The heat transfer characteristicsof a fluidized bed quenching system were measured and thequench sensitivities of the alloys were approximated.

Computer modeling was then utilized to determine theapplicability of fluidized bed quenching for the heat treatmentof these castings.

Introduction

Fluidized bed technology has a history of over 100 years. An

American patent in 1879 first pointed out the excellenttemperature uniformity of roasting minerals under fluidizedconditions [1]. The fluidized bed consists of a medium of finehard particles (i.e. sand) that is partially suspended by a

fluidizing gas. The partial suspension of the medium allowsthe particles to easily slide past each other resulting in thefluidizing bed acting remarkably similar to a fluid. The fluid

like nature of the fluidized bed allows for easy insertion,conveyance, and extraction of parts for heat treating. Figure 1

shows two cast wheels partially submerged in a fluidized bed.Fluidized bed technology has found wide use in the heattreatment of steels, but its use for heat treating other metals

has been limited. Recently, however, there has beenconsiderable interest in utilizing fluidized bed technology for

the solution heat treatment of aluminum alloys [2,3].

Figure 1: Picture of two cast wheels taken during immersion

into the fluidized bed.

Solution heat treating (T6 and T7 tempers) of heat treatablealuminum alloy castings is conducted to enhance strength,

impact resistance, and toughness. Solution heat treatingconsists of three main steps: solution, quenching, and aging.During solution, the part is heated to a temperature just belowthe eutectic temperature for the alloy. At solutiontemperature, the strengthening phases are dissolved into the

aluminum matrix. After a sufficient soaking time at solutiontemperature, the part is quenched to lock in the

strengthening elements into solution within the aluminummatrix. Subsequent aging allows the strengthening elementsto precipitate out as small, fine phases that strengthen the

aluminum matrix.

A critical step in solution heat treating is the quenchingprocess. The rate of quenching determines the percentage of

the strengthening elements that remain in solution afterquenching. A rapid quench will force a higher percentage ofstrengthening elements to remain in solution. A slow quench,however, will allow the strengthening elements to precipitate

Proceedings of the 24th ASM Heat Treating Society Conference, September 17-19, 2007COBO Center, Detroit, Michigan, USA. Copyright 2007 ASM International. All rights reserved.

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out of solution as large and blocky precipitates and theseprecipitates contribute minimally to strengthening the alloy.Quenching a part too slowly will effectively reduce the peak

strength that can be achieved after aging.

To quench aluminum parts quickly, the industry hastraditionally used water based quenchants as a quenching

medium. Water based quenchants exhibit excellent heattransfer rates and are capable of quenching large parts (greaterthan 20 kg) within seconds. A drawback of water basedquenchants, however, is the possibility of developing largestresses during quenching that can result in high residual

stresses, part distortion, and cracking. Large stresses build upwithin the part because of large thermal gradients that developwithin the part during quenching. These large thermalgradients are the result of vapor barriers that form around thepart during the initial quenching stage as the part is first

immersed into the quenchant.

For castings that are susceptible to distortion and cracking,quenching is often conducted by forced air. Utilizing forced

air assures that minimal thermal gradients will develop duringthe quenching process thus reducing the susceptibility todistortion and cracking. The drawback, however, is the lowheat transfer rate of forced air which results in a slow coolingrate for the part. Forced air quenching may not be a feasible

option for many quench sensitive alloys.

As an alternative, fluidized bed quenching offers an attractivemiddle ground between forced air and water based

quenchants. As shown in Fig. 2, the heat transfer coefficientthat can be obtained by the fluidized bed lies between forcedair convection and water [1]. Secondly, in contrast to waterbased quenchants, the particles of the fluidized bed remains in

direct contact with the surface of the part throughout the entirequenching process. Since a vapor barrier does not form,minimal thermal gradients develop within the part minimizingstresses. Compared to quenching in water, it was shown that

quenching in the fluidized bed reduced residual stresses bynearly 70% in an A356.2 PM casting [4].

Figure 2: Comparative heat transfer coefficients in W/mK for

water, fluidized bed, and forced air quenching.

The main drawback of fluidized bed quenching is the lowerheat transfer rates as compared to water based quenchants. Aswith forced air quenching, fluidized bed quenching may not be

feasible for some quench sensitive alloys. For example,Chaudhury and Apelian observed a significant decrease intensile properties for Al-Si-Mg type aluminum alloy D357sample quenched in the fluidized bed as compared to samples

quenched in water. In contrast, Al-Si-Mg-Cu alloys are lessquench sensitive than Al-Si-Mg alloys and the authors did notobserve a decrease in tensile properties for aluminum alloy354 or Sr modified aluminum alloy 319 with fluidized bedquenching as compared to water quenching [5].

Utilizing the fluidized bed technology for quenchingaluminum alloys is relatively new and the applicability forvarious alloy systems or size of castings is largely unknown.This paper helps lays out how the applicability of fluidized

bed quenching can be easily determined for quenchingcastings depending on the quench sensitivity of the alloy andthe dimensions of the casting. The heat transfer characteristicswere measured for a fluidized bed system that consisted of

staurolite sand fluidized by ambient air. The quenchsensitivity of aluminum alloy 319 and 356 type alloys wasapproximated by simulated TTT diagrams for these alloys.Finally, finite element analysis (FEA) was utilized todetermine the applicability of two castings (356 PM cast nail

gun housing and a 319 sand cast engine block).

Heat Transfer Characteristics

The heat transfer characteristics of the fluidized bed can varywidely and is dependent on various factors includingcharacteristics of the solid particles, properties of the

fluidizing gas, and bed temperature. For example, utilizing a

higher heat conductive fluidizing gas such as helium instead ofambient air can result in doubling the effective heat transfer

rate from the casting during quenching. Depending on thecharacteristics of the fluidized bed, the heat transfer rate for

the quenching system can range from 120 to 1200 W/mK [1].

An empirical model for determining the heat transfer

coefficient of the fluidized bed was developed by Saxena [6]where the heat transfer coefficient, h, was defined as follows:

))(()1( 22 wbwbtbr

pseo

b TTTTfck

fh +++=

where:

bf is the volume fraction of the air pockets within the

fluidized bed

eok is the effective thermal conductivity of fluidized bed

is the density of the fluidized bed

psc is the specific heat of the bed particles

is the Stefan-Boltzmann constant

t is the total emissivity

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r is the residence time

bT is the absolute temperature of the air pockets within the

fluidized bed

wT is the absolute temperature of the part

From the above model, one can note the importance of variousfactors in relation to the heat transfer characteristics of thefluidized bed system. For example, increasing the overall heattransfer coefficient of the fluidized bed may be achieved bychoosing a higher thermally conductive fluidizing gas, a

denser medium, or a medium with a higher specific heat.Using this model to predict the heat transfer coefficient for aparticular system, however, may prove cumbersome. Many ofthe values required for this model are not readily available andrequire means to experimentally determine them.

Point Load Quenching ModelA simpler method is proposed to obtain the global heattransfer rate of a fluidized bed by utilizing the heat transfer

and energy balance equations and assuming a point load. Theheat transfer and energy balance equations are as follows:

))()( tTThAtQ p= (Eq. 1)

and

dt

dTmctQ

p

p=)( (Eq. 2)

where:

)(tQ is the heat flow rateh is the global heat transfer coefficient

A is the surface area of the part

T is the bulk fluidized bed temperature

)(tTp is the part temperature as a function of time, t

m is the mass of the part

pc is the specific heat of the part

Combing the two energy balance equations and solving for thedifferential equation yields

( ) pmchAt

pp eTTTtT

+= )0()( (Eq. 3)

where )0(pT is the initial temperature of the part.

To simplify the exponent in Eq. 3, the terms of the exponentwere grouped together into a time constant, . The definitionof is:

hA

mcp= (Eq. 4)

From the cooling curve obtained by quenching a sample in thefluidized bed, , can be obtained by finding the best fit curve

for the temperature data using the following equation:

( ) t

pp eTTTtT

+= )0()( (Eq. 5)

The global heat transfer coefficient, h, can then be determinedby rearranging Eq. 4 where

A

mch

p

= (Eq. 6)

Measurement of Heat TransferTo obtain the global heat transfer coefficient for the fluidized

bed, temperature data from a cast flat plate was obtainedduring quenching. The flat plate was a permanent mold castof aluminum alloy A356.2. The plate dimensions were 28 cm

in length, 20 cm in width, and 2.5 cm in thickness. Heattreating was conducted in a batch fluidized bed line shown inFig. 3. The batch line consisted of a solution, quenching, andaging beds. The dimension of the work chamber for the beds

was 70 by 90 cm with a depth of 120 cm. The fluidized bedsconsisted of staurolite sand that was fluidized by ambient airat room temperature.

Figure 3: Batch fluidized bed line consisting of three fluidizedbeds for solution, quenching, and aging.

The casting was heated to a solution soaking temperature of548C (1020F) and allowed to soak for 30 minutes. The

casting was then transferred to the fluidizing bed quenchingsystem and immersed within 10 seconds. The temperature ofthe quenching system was monitored by a K-typethermocouple and was maintained at 21C (70F). Thecastings were orientated vertically as shown in Fig. 4 to allow

for optimum contact of the quenching medium on both sidesof the plate casting. Temperature within the casting was

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monitored with five K-type thermocouples that wereembedded along with width of the casting . The K-typethermocouple measured 1.5 mm in diameter (0.062 in); data

was acquired every second from each thermocouple.

Figure 4: Plate orientation and five thermocouple locations

for measurement of the global heat transfer coefficient.

Figure 5: Average cooling curve of the flat plate casting in

the fluidized bed along with the calculated best fit curve and

the difference between best fit data and lab data.

The average quenching curve was obtained for the 1-inch flat

plate by averaging all the temperature data from thethermocouples. A best fit curve utilizing Eq. 5 was fitted tothe laboratory data. Fig. 5 shows the resulting average coolingcurve with the best fit curve along with the difference betweenthe two curves. The best fit curve followed the temperature

data within +/- 6C (+/- 11F).

The calculated time constant, , from the best fit curve was79.7 seconds. At the calculated time constant, the averagetemperature of the part was at 190C (374F) corresponding to

63.2% of the temperature drop from 540C (1000F) to 30C(86F). Assuming a density of 2.65 g/cm and a specific heatof 963 J/kgK for the aluminum alloy, the global heat transfercoefficient for the system was calculated (Eq. 6) to be 340

W/mK (60 Btu/fthF).

Alloy Quench Sensitivity

It is proposed that a critical time constant can be used toapproximate the quench sensitivity for an aluminum alloy.The time constant for a cooling curve is defined by Eq. 4. Thetime constant, , corresponds to the time when the temperature

of the part has cooled 63.2% of the total temperature range(initial temperature of the part minus the temperature of thequenching media). A lower time constant, , implies a fasteroverall quenching rate and vice versa. If the quench

sensitivity of a particular alloy requires that it needs to be

quenched at a certain rate, a critical time constant,

crit, can becalculated for that alloy. crit is defined as the critical timeconstant; values equal to or below crit will achieve anadequate quenching rate. The critical time constant can be

obtained from either laboratory experiments to determine thequench sensitivity i.e., a Jominey quench test, or viasimulated TTT diagrams for the alloy.

TTT diagrams are used to understand the role of cooling rateon the heat treatment characteristics of the alloy. In order toassure that a minimal percentage of strengthening elementscomes out of solution during quenching, one would like toquench a part at a fast enough rate so that the quenching path

falls below the nose of the TTT diagram. TTT diagrams

simulated for 319 and 356 alloys are shown in Figures 6 and 7,respectively. The simulations were carried out by considering

that the alloys were cooled from the solutionizing temperaturesuch that only 0.2 wt% phase transformation took place.

If the quenching rate for the alloy was slow enough that thecurve was above the nose of the TTT diagram then the CuAl2

phase will start to precipitate out of the aluminum matrix. Tomaintain the Cu and Al in solution, one needs to assure thatthe quenching curve of a part quenched in the fluidized bedwill fall below the nose for the TTT diagram. For aluminum

alloy 319, the nose of the ' - CuAl2 (THETA_PRIME at 0.2wt%) curve was approximated at a temperature of 250C(480F) for a time of 180 seconds. The critical time constant,crit, for the 319 TTT curve of' - CuAl2 was calculated to be240 seconds for an alloy quenched from 500C to 30C

(932F to 86F).

Aluminum alloy A356.2 is a quench sensitive alloy and it isevident from the TTT diagram that the nose of the curves isfurther left than those of the curves simulated for 319. The

nose of the ' - Mg2Si (BETA_PRIME at 0.2 wt%) curve wasapproximated at a temperature of 340C (640F) for a time of20 seconds. The critical time constant, crit, for this alloy was

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calculated to be 49 seconds quenching from 540C to 30C(1000F to 86F).

Figure 6: TTT diagram of aluminum alloy 319 (Al-3.4Cu-

1.0Fe-0.1Mg-6.0Si-0.25Ti) for 0.2 wt% of phases transformed.

Figure 7: TTT diagram of aluminum alloy 356 (Al-0.1Cu-

0.12Fe-0.35Mg-7.0Si-0.2Ti) for 0.2 wt% of phases

transformed.

Finite Element Analysis

Finite element analysis can be used to determine the coolingrates within various sections of the part. Analyzing thesimulated quenching path at various locations and comparing

the results to the critical time constant, crit, a prediction wasmade whether the part will undergo an adequate quenchingrate.

Two castings were analyzed to determine the applicability of

these castings for fluidized bed quenching. This first castingwas a permanent mold cast housing for a nail gun. Thealuminum alloy for this casting was Al-Si-Mg alloy (A356.2).The casting exhibited a complicated geometry and had boththick and thin sections. A second part analyzed was an 8

cylinder engine block with a simplified geometry cast from anon-quench sensitive Al-Si-Mg-Cu alloy (319). For thisanalysis, the global heat transfer coefficient (340 W/mK)measured from the flat plate was used on all surfaces of the

parts.

Figure 8 shows the temperature profile from a cut away of theA356.2 nail gun casting near 49 seconds which corresponds tothe crit that was calculated from the TTT diagram. At crit the

temperature of the casting should be at or below 220C(430F) to assure an adequate quench (63.2% of the totaltemperature range). The temperature profile of most of thecasting was below 220C, however, the thicker regions were

still above 220C which mean that a loss of strength propertiesmay result in these regions.

Figure 8: Finite element result showing the temperature

distribution of a nail gun housing after 50 seconds ofquenching in the fluidized bed. Temperatures are in degrees

Celsius.

Figure 9 shows the temperature profile of a simplified 8

cylinder engine block after 240 seconds quenched in thefluidized bed. This time corresponds to the crit that wascalculated from the TTT diagram for 319. At crit, thetemperature should be at or below 170C (338F) to assure an

adequate quench. In this case, the entire casting was

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sufficiently below 170C and therefore should be expected toachieve full tensile properties after aging.

Figure 9: Finite element result showing the temperature

distribution in a cut-away of a simplified 8-cylinder engine

block after 240 seconds of quenching in the fluidized bed.

Temperatures are in degrees Celsius.

Discussion

In this paper, a simple method is proposed to determine the

applicability of fluidized bed quenching for a particular partthat depends on part geometry and alloy. This method,however, should only be used as a first cut for determining the

feasibility of fluidized bed quenching for a particular process.The global heat transfer coefficient calculated by assuming apoint load is only applicable for parts of simple geometry. For

complex parts with internal passages, it would be expectedthat heat transfer rates would be significantly lower frominternal surfaces. With FEA analysis, however, one canchange the heat transfer coefficient at various surfaces aroundthe part. The local heat transfer coefficient can still becalculated using the method described in this paper by

designing test pieces that more closely represent actual parts.

Conclusions

A point load may be used to obtain a simple model forthe quenching of aluminum parts in the fluidized bed.From this model, the global heat transfer coefficient for

the fluidized bed system can be easily calculated byapplying a best fit curve of the model to the coolingcurve of a sample.

From the cooling analysis of an aluminum samplecasting, the global heat transfer coefficient for afluidized bed system using ambient air as the fluidizinggas and staurolite sand as the solid particles was 340

W/mK (60 Btu/fthF).

A critical time constant, crit, was proposed as a methodto approximate the quench sensitivity of an alloy.Using the point load quenching model, a part will

undergo an adequate quenching rate if the time constantis at or below the critical time constant.

From the finite element analysis, fluidized bedquenching a nail gun housing of aluminum alloyA356.2 exhibited excessively slow cooling rates insome sections. Aluminum alloy A356.2 (Al-Mg-Sialloy) is a quench sensitive alloy and the applicability

of fluidized bed quenching for castings of these alloysshould be carefully analyzed.

In contrast to the nail gun housing, the finite elementanalysis predicted more than adequate cooling rates forall sections of a large 8-cylinder engine block ofaluminum alloy 319 (Al-Cu-Mg-Si alloy). For low

quench sensitivity alloys such as 319, the fluidized bedquenching process may offer a viable alternative for

castings of various sizes and shapes.

Acknowledgements

The authors would like to acknowledge the contributions ofDave Dingmann (formerly with Arizotah, LLC) and for his

work with the finite element modeling that was conducted forthis research.

In addition, Arizotah, LLC, would like to acknowledge the

NIST Advanced Technology Program for its support in theadvancement of the fluidized bed technology. Special thanksto Jean-Louis Staudenmann, NIST-ATP program manager, forhis support and guidance.

References

1. R. Reynoldson, Heat Treatment in Fluidized BedFurnaces, pp 33-52, ASM International, MaterialsPark, Ohio (1993)

2. S. Chaudhury and D. Apelian, Fluidized Bed HeatTreatment of Cast Al Alloys, in the proceedings of

the John Campbell Symposium, TMS Annual

Meeting, California, USA, p 283 (2005)3. S. Chaudhury and D. Apelian, Effect of Rapid

Heating on Solutionizing Characteristics of Al-Si-MgAlloy Using a Fluidized Bed, Metallurgical and

Materials Trans. A, 37A, 763-778 (2006)4. J. Keist, D. Dingmann, and C. Bergman, FluidizedBed Quenching: Reducing Residual Stresses andDistortion, In the Proceedings of the 23

rdHeat

Treating Society Conference, Pittsburgh, PA, 263-270 (2005)

5. S. Chaudhury and D. Apelian, Effects of FluidizedBed Quenching on Heat Treating Characteristics ofCast Al-Si-Mg and Al-Si-Mg-Cu Alloys,Inter. J. ofCast Metals Research, 19(6), 361-369 (2006)

6. S. Saxena,Advanced Heat Transfer, p 97 (1989)

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