Local Reinforcement of 6061 Aluminum Alloy BarUsing High-Frequency Induction Heating Apparatus

Fang-ni Shang1,+, Eiji Sekiya2 and Yoshihiro Nakayama3

1Department of Education, Interdisciplinary Graduate School of Medicine and Engineering,University of Yamanashi, Kofu 400-8511, Japan2Departments of Research and Design, YS Electrics Co. Ltd., Kofu 400-0043, Japan3Department of Research, Interdisciplinary Graduate School of Medicine and Engineering,University of Yamanashi, Kofu 400-8511, Japan

A commercial 6061 aluminum alloy bar was partially reinforced using a high-frequency induction heating apparatus. One end of the barwas locally heated for solution treatment (local solution treatment). The power supplied to a heating coil was adjusted based on the surfacetemperature of the bar, which was measured using a radiation thermometer. The temperature profiles and hardness distributions in thelongitudinal direction were investigated after the local solution treatment. The results of this study are as follows. The local solution treatmentachieved a rapid heating rate of 80K/s, overheating of about 2K at the temperature transition stage from the heating to holding process, and asmall-scale temperature fluctuation (below «8K) in the holding process. The reproducibility of the temperature profile during the local solutiontreatment was also sufficient. An increase in the hardness was achieved from solution treatment at 723K or more. A holding time of 5min ormore was not very important for increasing in the hardness. The partial reinforcement of a 6016 aluminum alloy bar can be realized by applyingan induction heating apparatus. [doi:10.2320/matertrans.L-M2012820]

(Received March 7, 2012; Accepted May 25, 2012; Published August 1, 2012)

Keywords: induction heating, 6061 aluminum alloy, local reinforcement, solution treatment

1. Introduction

The hardness and strength of many machine parts suchas gears, bearings, and turbine blades may be increased byheat treatment. However, partial strengthening may be moredesirable to maintain the toughness, rather than overallstrengthening of the products. For heat treatable aluminumalloys, local heat treatment at solution temperatures has beenconsidered to be a useful technique for partial reinforcement.Such local heat treatments are also expected to reduce thequenching distortion, energy consumption and productioncost, as well as improve the wear resistance and fatigueproperties. Induction heating, a laser, or a flame is commonlyused for this local heating. In the laser method, theapplication of a CO2 laser to the local softening of 1070aluminum alloy has been reported.1,2) An improvementin the formability as a result of local heating by the CO2

laser was also observed for steel sheets.3) Moreover, thewear resistance of Al­Si alloy products was improved by asurface melting treatment using a CO2 laser.4,5) In the caseof the flame method, the surface hardness of 12%Cr steelwas increased from 250HV to 420­550HV by flamehardening.6,7) After flame hardening treatment, cast iron alsoshowed a superior wear resistance.8) For the inductionheating method, the ductility of a 1070 aluminum alloy sheetwas improved as a result of local heat treatment usinginduction heating.9) The effect of partial heat treatment usinginduction heating on the distribution of the residual stress hasalso been reported for S45C steel bars.10) The dieless formingof pipe materials by local induction heating has been appliedfor cost reduction.11) The fatigue strength of a low carbonsteel pipe was also improved by applying local induction

heating.12) As mentioned above, there have been manystudies on the local heat treatment of steel, as compared toaluminum alloys. In particular, studies on the heat treatmentof aluminum alloys using induction heating are limited. Thismay be because the heating efficiency of aluminum alloyproducts using conventional induction heating apparatus isinsufficient. In addition, because a conventional thermo-couple cannot be used because of the magnetic fieldsgenerated by the induction heating coil and the emissivity ofaluminum alloys is unstable for temperature measurementusing a radiation thermometer, it is difficult to measurethe temperature of an aluminum alloy during heating byinduction heating apparatus. Our previous study13) showedthat aluminum alloy bars with a 10-mm diameter can beheated efficiently using the induction heating apparatus witha higher frequency (2MHz) than a conventional inductionheating apparatus (less than 400 kHz). Furthermore, surfacecoating of the aluminum alloy with a black spray also leadsto precision temperature measurement using a radiationthermometer. These experimental results suggest that com-bining a radiation thermometer and the induction heatingapparatus with a high frequency of 2MHz will enable theaccurate and efficient heat treatment of aluminum alloys.In particular, precise local heating using an induction coilmay be expected.

In this work, a commercial 6061 aluminum bar was locallyheated using the induction heating apparatus having afrequency of 2MHz, with the objective of confirming thepossibility of partial reinforcement. The heating stateswere examined in terms of the temperature profiles andhardness distributions in the longitudinal direction of thebars. The effects of the holding times and heating ratesduring local heating on the hardness distribution were alsoinvestigated.+Corresponding author, E-mail: g09dm002@yamanashi.ac.jp

Materials Transactions, Vol. 53, No. 9 (2012) pp. 1604 to 1610©2012 The Japan Institute of Light Metals

2. Experimental

The material used in this study was an extruded bar ofcommercial A6061-T6 aluminum alloy (10mm in diameter,150mm in length). Before the local heating using theinduction heating apparatus, the samples were subjected toover-aging treatment (688K for 2 h) using an electricalfurnace to adjust them to almost the same states ofprecipitation (as-annealed). Figure 1 is a schematic diagramshowing the arrangement of the specimen, induction heatingcoil, radiation thermometer, thermograph and cooling waterbath. The frequency and power of the induction heatingapparatus were 2MHz and 10 kW, respectively. The surfaceof the sample was sprayed with a black coating to ensureprecise temperature measurement using a radiation thermom-eter with a measurable temperature range of more than 523K.Based on a previous study,13) the emissivity value of theradiation thermometer was set to 0.78 to accurately measurethe temperature. The radiation thermometer measured thetemperature at the tip of the bar. The tip of the bar was locallyheated for the solution treatment (local solution treatment). Inthis case, the power supply to the heating coil was regulatedby the temperature of the tip. The local solution treatmentconditions are summarized in Fig. 2. The temperaturedistribution in the longitudinal direction of the bar was alsomeasured simultaneously at the side surface of the bar using athermograph. After the local solution treatment, the bars werequenched in water at room temperature, which was locatedbelow the bar. After quenching, the samples were naturallyaged for 48 h at room temperature (T4). One of them wasartificially aged at 453K for 8 h using an electric furnace(T6). After the natural/artificial aging, a Vickers microhard-ness test, electrical conductivity measurement, and micro-

structure observation were performed. The Vickers micro-hardness was measured at a load of 100 gf and a holding timeof 15 s. The hardness distribution in the longitudinal directionwas measured at the central part of a sample. The electricalconductivity was measured using a digital electrical con-ductivity meter (Autosigma 2000). The samples for opticalmicroscopy observations were prepared using the anodicoxidation method. These measurements and observationswere also conducted at the center plane of the bar.

3. Results

3.1 Accuracy and reproducibility of heating stateBecause it was difficult to maintain the thermal conduction

in the bar and the heat radiation into the air constant, it wasnecessary to investigate the heating state during the localsolution treatment in detail. Therefore, the accuracy andreproducibility of the heating state during the local solutiontreatment were examined, that is, the temperature profile atthe tip and the temperature distribution in the longitudinaldirection of the bar were measured repeatedly. First, theheating rate dependence of the temperature profile during thelocal solution treatment was investigated because temperaturecontrol appeared to be difficult at a rapid heating rate. Thetemperature profiles at heating rates of 80 and 0.8K/swere measured at the tip of the bar using the radiationthermometer. Figure 3(a) shows the heating rate dependenceof the temperature profile. The enlargements of the temper-ature profiles immediately after the completion of thetemperature rise are shown in Figs. 3(b) and 3(c). Becausethe measurable lower limit temperature of the radiationthermometer was 523K, temperatures above 523K areshown in Fig. 3(a). A linear rise in the temperature up tothe solution treatment temperature during the heating processwas confirmed for each heating rate. As the enlargementsshow, a small-scale overheating (2K) was achieved duringthe temperature transition stage from the heating to holdingprocesses. In addition, there was no significant temperaturefluctuation (below «8K) in the holding process. From theseresults, precision temperature control during the local solu-tion treatment can be achieved using an induction heatingapparatus for each heating rate. To check the heat conductionstate during the local solution treatment, the temperaturedistribution in the longitudinal direction of the bar wasmeasured using a thermograph. Figures 4(a) and 4(b) showthe temperature distributions at each holding time for theheating rates of 0.8 and 80K/s, respectively. The temperaturedistributions of the 0.8K/s rate were almost the same atevery holding time because a sufficient time for thermalconduction was maintained by the very slow heating rate.However, for the heating rate of 80K/s, heat at the tip of thebar conducted gradually in the longitudinal direction with thelapse of holding time. In particular, significant heat wasconducted from 0 to 1min. These temperature distributionssuggest that an effective local reinforcement at the tip of thebar may be realized when the tip is heated rapidly in a shorttime. To examine the reproducibility of the heating stateduring the local solution treatment, the temperature distribu-tions in the longitudinal direction were measured five timesfor the local solution treatment at a holding time of 5min for

Fig. 1 Schematic diagram of experimental apparatus.

Fig. 2 Summary of local solution treatment conditions.

Local Reinforcement of 6061 Aluminum Alloy Bar Using High-Frequency Induction Heating Apparatus 1605

the heating rates of 80 and 0.8K/s. Figure 5 shows thetemperature difference ¦T between the highest and lowesttemperatures, at each position for the five measurements.That is, the values of ¦T show the reproducibility of theheating state. For the heating rate of 0.8K/s, ¦T increasedrapidly from 0 to 10mm and then saturated at about 7K. Asimilar tendency was obtained for the 80K/s heating rate.However, the saturated value was around 10K higher thanthat of 0.8K/s. The reason for the higher ¦T value withthe 80K/s rate is that the state of the heat conductions inthe bar and into the air became unstable with an increasein the heating rate. As the maximum value of ¦T, 17K,is equivalent to 2% of the solution temperature of 803K,a comparatively good reproducibility for the heating statecan be secured for each heating rate, especially within 10mmof the tip.

3.2 Holding time dependence of hardness distributionTo examine the effect of the holding time on the partial

reinforcement, the hardness distribution was measured forspecimens that were locally solution treated at the rapid

Fig. 3 Temperature profiles for various heating rates (a) and enlargements of temperature profiles immediately after completion oftemperature rise for heating rates of 80K/s (b) and 0.8K/s (c).

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Fig. 5 Temperature difference, ¦T, between highest and lowest temper-ature at each position. Temperature distributions were measured five timesfor local solution treatment at holding time of 5min for heating rates of 80and 0.8K/s.

F. Shang, E. Sekiya and Y. Nakayama1606

heating rate of 80K/s. The local solution treatment wasconducted at 803K for various holding times, and then thesamples were artificially aged at 453K for 8 h. Figure 6shows the Vickers hardness distribution in the longitudinaldirection. The hardness near the tip of the bar clearlyincreased until a holding time of 5min, but was saturated atabout 100HV with a further increase in time. An approx-imately similar tendency was also observed in the hardenedarea. These results suggest that various hardness distributionscan be realized by adjusting the holding time. The hardnessshown in Fig. 6 was measured at the center of the bar. If thetemperature distribution in the radius direction of the bar issteep as a result of the skin effect14) of the induction heating,the hardness may vary in the radial direction. Figure 7 showsthe Vickers hardness distribution in the radial direction. Thehardness distribution in the radial direction was measured for

a sample that was locally solution treated at 803K for 1minand then aged at 453K for 8 h. As shown in Fig. 7, there wasno significant difference in the hardness distribution in theradial direction at any distance from the tip (Y-position).These hardness distributions suggest that a steep temperaturedistribution in the radial direction of the bar does not occurbecause of the good heat conduction of the aluminum alloy,as well as the small sample size.

3.3 Heating rate dependence of hardness distributionAs shown in Fig. 4, the temperature distributions varied

with the heating rate, and a sharp distribution of thetemperature occurred with rapid heating. Therefore, theheating rate is expected to have a significant influence on thepartial reinforcement. To investigate the effect of the heatingrate on the partial reinforcement, the hardness distributionsfor slow and rapid heating rates were measured. Figure 8shows a comparison of the hardness distributions between theheating rates of 0.8 and 80K/s. The samples were subjectedto local solution treatment at 803K for 0min and artificialaging treatment at 453K for 8 h. The hardness with the0.8K/s heating rate was higher than that for 80K/s, and thehardened area was also extended. The sufficient heat conduc-tion and extension of the time in the high-temperature rangewith a slow heating rate of 0.8K/s may result in a higherhardness value, as well as an extension of the hardened area.Although the increase in the hardness at 80K/s was small,this result also suggests that an increase in the heating ratecould produce reinforcement in a narrow area.

4. Discussion

As mentioned above, the tip of the bar could be partiallyreinforced using an induction heating apparatus. This partialreinforcement may result from precipitation hardening causedby the local solution treatment followed by the artificial agingtreatment. Thus, on the basis of the temperature profiles andelectrical conductivities, the state of the solid solution during

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Fig. 6 Vickers hardness distributions in longitudinal direction for variousholding times. Hardness measurement was conducted after T6 treatment.

Fig. 7 Vickers hardness distribution in radial direction.

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Fig. 8 Comparison of hardness distribution between heating rates of 80and 0.8K/s.

Local Reinforcement of 6061 Aluminum Alloy Bar Using High-Frequency Induction Heating Apparatus 1607

the local solution treatment and the precipitation behavior inthe artificial aging treatment were examined. The temperatureprofiles at each measurement position (Y-position) in thelongitudinal direction were obtained from the thermographdata and are shown in Fig. 9. The sample was locally solutiontreated at 803K for 10min at a heating rate of 80K/s. Thetemperature increased rapidly in a short time and saturatedafter additional time. As the position approached the tip ofthe bar, the increase in temperature became faster and thetime required for saturation also became shorter. Thesaturated temperature at each position decreased with anincrease in the distance from the tip. The electricalconductivity at each measurement position was also meas-ured to investigate the relationship between the temperatureprofile during the local solution treatment and the amount ofsolid solution. This electrical conductivity measurement wasperformed for the samples under naturally aged (T4) andartificially aged (T6) conditions. Figure 10(a) shows theelectrical conductivity in the T4 condition. For less than40mm, the electrical conductivity decreased with an increase

in the holding time. Particularly, there was a significantdecrease in electrical conductivity within 5min. In addition,this decrease in electrical conductivity was markedly at aposition close to the tip. This change in electrical con-ductivity suggests that the amounts of solid solute atoms inthe matrix increased within 5min at a position near the tip.Figure 10(b) shows the difference in the electrical conductiv-ities between the T4 condition and T6 condition, ¦IACS% =IACS%T6 ¹ IACS%T4. The increase in ¦IACS% suggeststhat the quantity of the solid solute atoms in the matrixdecreased as a result of the formation of precipitates duringthe artificial aging treatment. Although a slightly differentchange was seen in the 10min period, ¦IACS% generallyincreased with an increase in the holding time. For a positionwithin 30mm from the tip, a significant increase in ¦IACS%was recognized. These changes are in contrast to those for theelectrical conductivity during the local solution treatment, asshown in Fig. 10(a). Therefore, a precipitation hardening canbe expected to occur for a position within 30mm from the tip.It is well known that the temperature and holding timeduring solution treatment have a significant influence on themechanical properties. To investigate the relationshipsbetween the hardness, temperature, and holding time, thehardness was plotted against the temperature, as shown inFig. 11. The hardness and temperature values at eachmeasurement position for various holding times wereobtained from Figs. 6 and 9. In Fig. 9, distances of 3, 10,15, 20 and 30mm were selected as typical positions in thelongitudinal direction. Similarly, 1, 5, 6, 8 and 10min inFig. 6 were chosen as the holding times. Figure 11 showsthat a temperature of 723K or more is necessary to increasethe hardness, and a rise in temperature brings about a higherhardness. The hardness increase at each position resultedfrom the extension of the holding time, while the quantity ofhardness increase became lower for a holding time of 5minor more. A previous report13) on heat treatment with rapidheating pointed out that a sample must be heated to more than713K to increase the mechanical properties. The findingsshown in Figs. 6, 9, 10 and 11 are in good agreement withthose of the previous study. Namely, temperatures above723K made the quantity of solid solution atoms increase,

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Fig. 10 Changes in electrical conductivity after T4 treatment (a) and difference in electrical conductivities between T4 treatment and T6treatment (b).

F. Shang, E. Sekiya and Y. Nakayama1608

which resulted in precipitation hardening. In addition to theabove, it was also shown that a holding time of 5min ormore was less important. To investigate the microstructuralfeatures in detail, optical microscope images were observed.Figure 12 shows the cross-sectional microstructures of thesamples under the as-annealed and locally solution treatedconditions. The local solution treatment was conducted at803K for 10min with a heating rate of 80K/s, and themicrostructure was observed at the positions of 3, 10, 20 and30mm. The broken line inserted in the figure indicates theboundary of the grain-growth region and the fine-fibrousgrain region. The grain growth in the outer section, whichmay have been formed in the extrusion process or followingthe T6 heat treatment, was observed for the as-annealedsample. When the local solution treatment was conducted,the area of grain growth increased at the positions of 3 and10mm. However, the state of the grain growth at thepositions of 20 and 30mm was almost the same as the

as-annealed sample. From these microstructures and thetemperature profiles shown in Fig. 9, grain growth waspromoted in the outer section at about 753K or more. Incontrast, the fibrous grains in the inner section remained ina fine state regardless of the heat treatment. Because thehardness was measured at the center plane of the bar, thehardness distribution shown in Fig. 6 is considered to haveresulted from precipitation hardening.

5. Conclusions

The ability to partially reinforce a heat-treatable aluminumalloy bar was investigated through the application of aninduction heating apparatus. The temperature and hardnessdistributions after local solution treatment were examined.The results of this study are as follows:(1) Rapid and excellent local heating could be performed

for a 6061-aluminum alloy bar using an inductionheating apparatus. The local solution treatment achiev-ed a rapid heating rate of 80K/s, overheating of about2K at the temperature transition stage from the heatingto holding process, and a small-scale temperaturefluctuation (below «8K) in the holding process. Alocal solution treatment with good reproducibility couldbe performed using the induction heating apparatus.

(2) Good agreement was found between the temperatureprofile and hardness. An increase in hardness resultedfrom a solution treatment at 723K or more. The holdingtime of 5min or more was less important for theincrease in hardness. Applying an induction heatingapparatus made it possible to partially reinforce a 6016aluminum alloy bar.

Acknowledgements

This work was supported by Asakawa Netusyori Co., Ltd.and Marushin Heat Co., Ltd. for the technical analysis. Partof this study was funded by the Light Metal EducationalFoundation, Inc.

Fig. 12 Cross-sectional microstructures of samples under as-annealed and locally solution treated conditions. Local solution treatmentwas conducted at 803K for 10min with heating rate of 80K/s. Broken line in figure indicates boundary between grain-growth regionand fine-fibrous grain region.

Fig. 11 Hardness versus temperature plots for positions of 3 to 30mm.

Local Reinforcement of 6061 Aluminum Alloy Bar Using High-Frequency Induction Heating Apparatus 1609

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