metals
Article Effect of Cooling Rate on Microstructure and Properties of Twin-Roll Casting 6061 Aluminum Alloy Sheet
Zhen Xu 1,*, Sixue Wang 1, Hongbin Wang 1,*, Hua Song 2, Shengli Li 1 and Xingyu Chen 3
1 School of Materials Science and Metallurgy, University of Science and Technology Liaoning, No. 185, Qianshan Middle Road, Lishan District, Anshan 114051, China; [email protected] (S.W.); [email protected] (S.L.) 2 School of Mechanical Engineering and Automation, University of Science and Technology Liaoning, No. 185, Qianshan Middle Road, Lishan District, Anshan 114051, China; [email protected] 3 Anshan Zizhu Sci. & Tech.Profile Steel Co., Ltd., Anshan 114015, China; [email protected] * Correspondence: [email protected] (Z.X.); [email protected] (H.W.); Tel.: +86-041-2592-9535 (Z.X.); +86-041-2592-9563 (H.W.) Received: 2 July 2020; Accepted: 11 August 2020; Published: 30 August 2020
Abstract: In this study, a twin-roll casting sheet of 6061 aluminum alloy was cooled using furnace, asbestos, air, wind and water. The effect of cooling rate on the microstructure and properties of twin-roll casting 6061 aluminum alloy sheet were studied. Optical microscope, scanning electron microscope, X-ray diffraction, microhardness tester and universal tensile machine were used to observe the microstructure and properties of twin-roll casting sheet of 6061 aluminum alloy. The results show that the higher the cooling rate, the smaller the grain size of the alloy and the smaller the number of precipitated phases in the matrix. Uniform grain size of the alloy could be obtained at a stable cooling rate. The hardness, tensile strength and elongation of the twin-roll casting sheet increased with cooling rate. Under wind cooling condition, the twin-roll casting sheet demonstrated excellent comprehensive performance, i.e., 88 MPa of yield strength, 178 MPa of tensile strength and 15% of elongation, respectively. A quantitative Hall–Petch relation was established to predict the yield strength of 6061 twin-roll casting sheets with different grain sizes and cooling rate.
Keywords: cooling rate; aluminum alloy 6061; twin-roll casting; microstructure; mechanical properties
1. Introduction Over the last decades, increasing demand for lightweight materials from multiple industrial applications motivates researchers to focus on aluminum alloys [1–3]. The Al-Mg-Si series (6XXX series) alloy is one of the forged aluminum alloys in which Mg and Si can form main strengthening phase Mg2Si [4]. This series of alloys have the advantages of medium-high strength, good corrosion resistance, excellent welding ability, high plasticity, as well as good processing properties after heat treatment. As a result, it can be widely used in forging, rolling, stamping, and other processes [5–7]. Up to now, Al-Mg-Si aluminum alloy has been applied comprehensively in aerospace, automobile manufacturing, electronic home appliances, and other fields [8,9]. The twin-roll casting (TRC) process is that molten metal is poured, solidified, and plastic deformed in a short time through a pair of rotating casting rolls as the crystallizer, and then directly turns into a thin metal strip [10]. The twin-roll casting process combines two processes of sub-rapid solidification and hot-working, greatly shortening production process, and the corresponding products with fine microstructure could possess greater strength and hardness [11,12]. Therefore, the twin-roll casting (TRC) process is becoming
Metals 2020, 10, 1168; doi:10.3390/met10091168 www.mdpi.com/journal/metals Metals 2020, 10, 1168 2 of 11 a more and more efficient and vital part of aluminum alloy processing technology, especially for Al-Mg-Si series (6XXX series) alloy. During the solidification and cooling process, the cooling rate, one of the critical parameters, directly affects product performance. In recent years, numerous researchers have conducred a great amount of research in this field. S.G. Shabestari [13] reported that a higher cooling rate would significantly decrease the size of Dendrite Arm Spacing, the recalescence undercooling temperature, and total solidification time, and increase the liquidus temperature, nucleation undercooling temperature, and solidification range. B. Benjunior et al. [14] studied the effect of different cooling rate conditions on thermal profile and microstructure of aluminum 6061, indicating that different cooling rate conditions will alter the material phase change temperatures. Furthermore, cooling rate was performed to reduce the porosity by reducing the size of Dendrite Arm Spacing [15], which influences mechanical properties to some extent. In the roll-casting process, there are many factors that affect microstructure and properties of the cast strip [16]. Studies show that cooling rate affects the microstructure and mechanical properties of aluminum alloy twin-roll casting sheets, but few researchers studied the effect of post-rolling cooling rate on the microstructure and properties of cast-roll 6061 aluminum alloy sheets [17,18]. Based on this, this paper selects twin-roll casting and rolling 6061 aluminum alloy sheets as the research target, and introduces five different cooling methods, i.e., furnace cooling, asbestos covering, air cooling, wind cooling, and water cooling. The aims are to study the influence of cooling rate on the microstructure and mechanical properties of 6061 aluminum alloy twin-roll casting sheets, obtain a reasonable cooling method and related process parameters, and further optimize the microstructure and mechanical properties of twin-roll casting 6061 aluminum alloy. In the final, the Hall–Petch (H-P) relation between yield strength and grain size is investigated.
2. Experimental Procedures The commercial 6061 aluminum alloy was selected as the experimental material, and its chemical composition is shown in Table1. Experiment was carried out on a horizontal twin roll caster with a roll diameter of ϕ 130 mm. The 6061 aluminum alloy ingot was heated and smelted in a box type SX2-10-12 resistance furnace (Shenyang GE Furnace Co. LTD, Shenyang, China), and the heating temperature was 710 5 C. After the alloy was completely melted and stirred uniformly, it was kept ± ◦ in a heating furnace for 10 min. Then, the temperature was lowered to 690 5 C, and the molten ± ◦ metal was transferred to a twin roll caster for twin-roll casting test, and it was cast and rolled into sheets with a thickness of 1.5 mm. The twin-roll casting sheets were cooled at different cooling rates. The cooling rate was achieved using five methods: furnace cooling, asbestos covering, air cooling, air cooling, and water cooling.
Table 1. Chemical composition of 6061 aluminum alloy.
Composition Mg Si Cu Cr Fe Mn Zn Ti Al wt % 0.834 0.594 0.194 0.051 0.182 0.028 0.023 0.016 98.078
The twin-roll casting sheet was cut into small pieces by a electric discharge wire cutting machine. The specimens were ground and polished, and then etched with Keller reagent (2.5 mL HNO3 + 1.5 mL HCL + 1 mL HF + 95 mL H2O in volume) for 15 s, and the microstructure was observed using a ZEISS optical microscope (Jena, Germany). The sample was subjected to X-ray diffraction analysis using an X-ray diffraction (XRD, X’Pert Powder). The mechanical properties were measured at room temperature (20 ◦C) using an electrohydraulic servo material testing machine (UTM5305, HST, Jinan, China) with a tensile speed of 1 mm/min based on Chinese standards (GB/T 228-2002). The dimensional drawing of tensile specimens, which were cut by electric discharge machine from different positions of the cladding billet, is show in Figure1. Field emission high-resolution scanning electron microscopy Metals 2020, 10, 1168 3 of 11 Metals 2020, 10, x FOR PEER REVIEW 3 of 12
(SEM) was used to observe the fracture morphology and analyze the size and distribution of the determinedphases. using The a phasecompanion composition spectrometer. was determined The using surf aace companion of the sample spectrometer. was The ground surface to of a the bright light, and the hardnesssample was was ground measured to a bright using light, anda Vickers the hardness hardness was measured tester (Qness using a Vickers Q10M) hardness with testera test load of 3 N and a (Qnessdwell Q10M)time withof 15 a tests. In load addition, of 3 N and all a dwell the timetest of results 15 s. In addition,were averaged all the test resultsfrom werethree effective averaged from three effective values. values.
5×1.5
Rolling direction
Figure 1. Dimensional drawing of tensile specimens (all dimensions are in mm). Figure 1. Dimensional drawing of tensile specimens (all dimensions are in mm). 3. Result and Discussion
3. Result and3.1. Determination Discussion of Cooling Rate The 6061 aluminum alloy twin-roll casting sheet was cooled by five different cooling methods, 3.1. Determinationand the cooling of Cooling rate was Rate measured using a thermocouple [19]. A plot of slab temperature versus time for different cooling modes was plotted, and the first derivative plot was calculated from the obtained The 6061cooling aluminum curves, as shown alloy in twin-roll Figure2. casting sheet was cooled by five different cooling methods, and the coolingThe rate 6061 aluminumwas measured alloy twin-roll using casting a thermoco sheet wasuple placed [19]. in a furnaceA plot to achieveof slab slow temperature cooling, versus time for differentand the average cooling cooling modes rate waswas 0.006 plotted,◦C/s. Figureand 2thea shows first thederivative temperature plot change was curve calculated under from the slow cooling and the derivative curve of temperature changing, i.e., the cooling rate curve. The furnace obtained coolingcooling rate curves, appeared as shown to be the in slowest Figure and 2. gradually decreased when the furnace temperature The 6061approached aluminum room temperature. alloy twin-roll casting sheet was placed in a furnace to achieve slow cooling, and theRelatively average slow cooling cooling was rate achieved was 0.006 by covering °C /s. the Figure 6061 aluminum 2a shows alloy the twin-roll temperature casting sheet change curve under slowwith cooling asbestos, and with the an average derivative cooling curve rate of of 0.2 temperature◦C/s. The temperature changing,/time relationshipi.e., the cooling is shown rate curve. in Figure2b. It can be seen that the d T/dt curve exhibits a straight line parallel to the horizontal axis. The furnaceThe cooling overall cooling rate rateappeared in this cooling to be modethe tendsslowest to be and uniform, gradually and the coolingdecreased rate appears when tothe furnace temperaturebe slower. approached room temperature. RelativelyFigure slow2c showscooling the was temperature achieved change by curvecovering and cooling the 6061 rate curvealuminum for air coolingalloy twin-roll of the casting sheet withtwin-roll asbestos, casting with 6061 an aluminum average alloy cooling sheet. rate It took of about0.2 °C/s. 275 s The for the temperature/time twin-roll casting sheet relationship to is naturally cool to room temperature (25 1 C) in air, and the average cooling rate was 2.4 C/s. ± ◦ ◦ shown in FigureThe twin-roll 2b. It castingcan be 6061 seen aluminum that the alloy d sheetT/dt wascurve cooled exhibits relatively a quicklystraight by aline blower parallel to to the horizontalaccelerate axis. The the overall air flow, cooling and the average rate in cooling this cooling rate was 3mode◦C/s. Figuretends2 dto shows be uniform, the temperature and the cooling rate appearsprofile to andbe slower. cooling rate curve for this cooling condition. The trend of cooling rate changed during wind Figurecooling 2c shows in a similar the way temperature to air cooling. However,change thecurve cooling and rate cooling at high temperatures rate curve for for wind air cooling cooling of the was greater than that of air cooling, and the cooling rate was more stable. The maximum cooling rate twin-roll castingcondition 6061 was achieved aluminum by the methodalloy sheet. of water It cooling. took about The twin-roll 275 s casting for the sheet twin-roll was immediately casting sheet to naturally coolplaced to in room water, temperature and the twin-roll (25 casting ± 1 °C) sheet in was air, taken and out the from average the water cooling after the rate temperature was 2.4 °C /s. The twin-rollcooled to room casting temperature. 6061 aluminum alloy sheet was cooled relatively quickly by a blower to accelerate theThe air temperatureflow, and changethe average curve and cooling the cooling rate rate was curve 3 °C/s. obtained Figure under 2d the shows water coolingthe temperature condition are shown in Figure2e. As can be seen from the figure, when the slab was immersed in water, profile andthe cooling cooling raterate was curve largest for and this could cooling be maintained condition. for about The 12 trend s. When of the cooling sheet was rate cooled changed to during wind coolingabout in 50 ◦aC, similar the cooling way rate to remained air cooling. substantially However, unchanged, the andcooling the average rate coolingat high rate temperatures of the for wind coolingwater coolingwas greater process wasthan 21.3 that◦C/s. of air cooling, and the cooling rate was more stable. The maximum cooling rate condition was achieved by the method of water cooling. The twin-roll casting sheet was immediately placed in water, and the twin-roll casting sheet was taken out from the water after the temperature cooled to room temperature. The temperature change curve and the cooling rate curve obtained under the water cooling condition are shown in Figure 2e. As can be seen from the figure, when the slab was immersed in water, the cooling rate was largest and could be maintained for about 12 s. When the sheet was cooled to about 50 °C, the cooling rate remained substantially unchanged, and the average cooling rate of the water cooling process was 21.3 °C/s.
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600 20 600 (a) (b) 250 Average cooling rate: 0.006℃/s 500 500 Average cooling rate: 0.2℃/s 15 200 Cooling curve 400 Cooling curve
C) 400 C) ° ° 150 300 10 300
100 dT/dt 200 dT/dt 200
5 Temperature ( Temperature ( 50 100 dT/dt 100 0 dT/dt 0 0 0 0 1000 2000 3000 0 5 10 15 20 25 Time (s) Time (h) 600 50 600 30 (c) (d) Average cooling rate: 3℃/s 500 500 Average cooling rate: 2.4℃/s 40 Cooling curve
400 400 20
30 C) C) ° ° 300 Cooling curve 300 dT/dt
20 dT/dt 10 200 200
10 Temperature ( Temperature ( 100 100 0 dT/dt 0 dT/dt 0 0
0 50 100 150 200 250 300 0 50 100 150 200 250 300 Time (s) Time (s)
600 100 (e) Average cooling rate: 21.3℃/s 500 80 Cooling curve 400
C) 60 ° 300
40 dT/dt 200
Temperature ( 20 100 dT/dt
0 0
0 10203040 Time (s) FigureFigure 2. 2. CoolingCooling curves curves for for the the different different sheets: sheets: (a) ( acooled) cooled in infurnace, furnace, (b) ( bcoated) coated with with asbestos, asbestos, (c) cooled(c) cooled in air, in ( air,d) cooled (d) cooled by wind, by wind, and and (e) (coolede) cooled by bywater. water. 3.2. Influence of Different Cooling Rates on the Properties of Twin-Roll Casting 6061 Aluminum Alloy 3.2. Influence of Different Cooling Rates on the Properties of Twin-Roll Casting 6061 Aluminum Alloy Figure3 shows the mechanical properties of 6061 twin-roll casting sheets at room temperature Figure 3 shows the mechanical properties of 6061 twin-roll casting sheets at room temperature under different cooling conditions. It can be seen from the figure that the cooling rate has no significant undereffect different on the yield cooling strength, conditions. but does onIt can elongation be seen and from tensile the strength. figure that Furthermore, the cooling higher rate cooling has no significantrate can increase effect on elongation the yield and strength, tensile strength but does of theon alloy,elongation which and is consistent tensile strength. with the literatureFurthermore, [20]. higherWith thecooling increase rate can in cooling increase rate, elongation the overall and mechanical tensile strength performance of the alloy, shows which an increasingis consistent trend. with theHighest literature yield [20]. strength, With tensilethe increase strength, in andcooling elongation rate, the of theoverall sheet mechanical are found under performance wind-cooling shows and an increasingwater-cooling trend. conditions, Highest whichyield strength, are 88 MPa, tensile 178 MPa, strength, 15% and and 95 elongation MPa, 177MPa, of the 14%, sheet respectively. are found under Thewind-cooling hardness curveand water-cooling of twin-roll casting conditions, sheet which under diareff erent88 MPa, cooling 178 conditionsMPa, 15% isand shown 95 MPa, in 177MPa,Figure4 .14%, As can respectively. be seen from the figure, the hardness value of the twin-roll casting sheet shows an upward trend as the cooling rate increases. The hardness of the alloy is minimal with furnace cooling, about 52 HV, and the highest hardness of the alloy was obtained for the water-cooled condition, at about 95 HV. It can be inferred that the hardness of the 6061 twin-roll casting sheet can be increased with its cooling rate.
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250 20
Yield strength
200 Ultimate tensile strength Elongation 15
150 Metals 10 2020, , 1168 10 5 of 11 Metals 2020, 10, x FOR PEER REVIEWMPa 5 of 12 σ/ 100
250 20 Elongation/% 5 50 Yield strength
200 Ultimate tensile strength Elongation 15 0 0 Furnace coolingAsbestos cover Air cooling Wind cooling Water cooling 150 Different cooling method 10 MPa σ/ Figure 3. Mechanical100 properties of twin-roll casting sheets at different cooling rates. Elongation/% The hardness curve of twin-roll casting sheet under different cooling5 conditions is shown in 50 Figure 4. As can be seen from the figure, the hardness value of the twin-roll casting sheet shows an upward trend as the cooling0 rate increases. The hardness of the alloy0 is minimal with furnace cooling, about 52 HV, and theFurnace highest coolingAsbestos hardness cover Air cooling of the Wind coolingalloy Water was cooli ngobtained for the water-cooled Different cooling method condition, at about 95 HV. It can be inferred that the hardness of the 6061 twin-roll casting sheet can be increased FigurewithFigure its 3. 3.coolingMechanicalMechanical rate. properties properties of of twin-roll twin-roll cast castinging sheets atat didifferentfferent cooling cooling rates. rates.
110 The hardness curve of twin-roll casting sheet under different cooling conditions is shown in Figure 4. As can be seen from100 the figure, the hardness value of theWater twin-roll cooling casting sheet shows an upward trend as the cooling rate increases. The hardnessWind coolingof the alloy is minimal with furnace 90 cooling, about 52 HV, and the highest hardnessAir ofcooling the alloy was obtained for the water-cooled condition, at about 95 HV. It80 can be inferred that the hardness of the 6061 twin-roll casting sheet can be increased with its cooling rate. 70
Hardness (HV) 110 Asbestos cover
60 100 Furnace cooling Water cooling
50 Wind cooling 90 Air cooling Different cooling methods 80 Figure 4. Hardness curves of twin-roll casting sheets using different cooling rates. Figure 4. Hardness70 curves of twin-roll casting sheets using different cooling rates. 3.3. Influence of Different CoolingHardness (HV) Rates onAsbestos the Micro-Structure cover of Twin-Roll Casting 6061 Aluminum Alloy 3.3. Influence of Different Cooling60 Rates on the Micro-Structure of Twin-Roll Casting 6061 Aluminum Alloy Some studies show thatFurnace the largercooling the crystallization interval, the more likely it is to cause composition segregation and composition inhomogeneity in the cast aluminum alloy [21,22]. The liquid Some studies show that50 the larger the crystallization interval, the more likely it is to cause compositiontemperature segregation of 6061 aluminum and composition alloy is 652inhomogeneity◦C, and solidus in the temperature cast aluminum was 582 alloy◦C. [21,22]. The wide The liquidtwo-phase temperature region of would 6061 causealuminum tissue alloy unevennessDifferent is 652 °C,cooling and and composition methods solidus temperature segregation was during 582 casting°C. Theand wide two-phaserolling. During region the would rolling cause process tissue of a unevenness thin sheet, the and inside composition keeps a high segregation temperature during since thecasting cooling and rolling.rate at During the outerFigure the surface 4.rolling Hardness of theprocess sheetcurves isof of faster. a twin-roll thin This sh causescastingeet, the coarsesheets inside equiaxedusing keeps different graina high cooling at thetemperature center rates. of thickness since the coolingdirection rate andat the fine outer grains surface at the of outer the sheet edge, is so fast thater. the This interlayer causes appearscoarse equiaxed and the overallgrain at grain the center size 3.3.becomes Influence uneven. of Different Samples Cooling were Rates performed on the Micro- in the rollingStructure direction, of Twin-Roll and the Cast samplinging 6061 and Aluminum observation Alloy ofMetals thickness 2020, 10 ,direction x FOR PEER and REVIEW fine grains at the outer edge, so that the interlayer appears and the overall6 of 12 sites are shown in Figure5. grainSome size becomes studies showuneven. that Samples the larger were the performed crystalliz ination the rollinginterval, direction, the more and likely the samplingit is to cause and observationcomposition sites segregation are shown and in compositionFigure 5. inhomogeneity in the cast aluminum alloy [21,22]. The liquid temperature of 6061 aluminum alloy is 652 °C, and solidus temperature was 582 °C. The wide two-phase region would cause tissue unevenness and composition segregation during casting and rolling. During the rolling process of a thin sheet, the inside keeps a high temperature since the cooling rate at the outer surface of the sheet is faster. This causes coarse equiaxed grain at the center of thickness direction and fine grains at the outer edge, so that the interlayer appears and the overall grain size becomes uneven. Samples were performed in the rolling direction, and the sampling and observation sites are shown in Figure 5.
Figure 5. Schematic diagram of the sampling site. Figure 5. Schematic diagram of the sampling site.
Figure 6 shows microstructures and grain sizes of 6061 aluminum alloy twin-roll casting sheets under different cooling conditions. As seen in Figure 6a, the grain size of twin-roll casting sheets is large with furnace cooling conditions, because the alloy remains at a high temperature for a long time during furnace cooling [23]. Hence, recrystallization followed by grain growth occurs. Moreover, a large number of dispersed precipitates appear in the crystal, and the crystal grains are mostly elliptical or irregularly shaped polygons. The petal-like grains are found in the dendrite when cooled by asbestos-covering (shown in Figure 6b), and the grain size is smaller than that of the grain structure cooled by furnace. Figure 6c shows the microstructure obtained in an air-cooled state. The results indicate that the fine, equiaxed grains with uneven distribution were found in the air-cooled state. Small grains in the form of dendrites with larger petal-like grains interspersed, therefore, are observed for wind-cooled state. Compared to the three cooling methods described above, the grains under wind-cooling conditions are significantly coarse, as shown in Figure 6d. Figure 6e shows the twin-roll casting structure under water-cooled conditions. It was seen that fine grains were surrounded by equiaxed grains, the grain sizes were large and uneven, and the grain boundaries were coarse. It can be seen from the above description that there are big differences in the size and morphology of crystal grains in the 6061 twin-roll casting sheet under different cooling conditions. With an increase of cooling rate, finer crystal grains and less precipitates in the crystal were found in the twin-roll casting sheet.
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Figure6 shows microstructures and grain sizes of 6061 aluminum alloy twin-roll casting sheets under different cooling conditions. As seen in Figure6a, the grain size of twin-roll casting sheets is large with furnace cooling conditions, because the alloy remains at a high temperature for a long time during furnace cooling [23]. Hence, recrystallization followed by grain growth occurs. Moreover, a large number of dispersed precipitates appear in the crystal, and the crystal grains are mostly elliptical or irregularly shaped polygons. The petal-like grains are found in the dendrite when cooled by asbestos-covering (shown in Figure6b), and the grain size is smaller than that of the grain structure cooled by furnace. Figure6c shows the microstructure obtained in an air-cooled state. The results indicate that the fine, equiaxed grains with uneven distribution were found in the air-cooled state. Small grains in the form of dendrites with larger petal-like grains interspersed, therefore, are observed for wind-cooled state. Compared to the three cooling methods described above, the grains under wind-cooling conditions are significantly coarse, as shown in Figure6d. Figure6e shows the twin-roll casting structure under water-cooled conditions. It was seen that fine grains were surrounded by equiaxed grains, the grain sizes were large and uneven, and the grain boundaries were coarse. It can be seen from the above description that there are big differences in the size and morphology of crystal grains in the 6061 twin-roll casting sheet under different cooling conditions. With an increase of cooling rate, finer crystal grains and less precipitates in the crystal were found in the twin-roll casting sheet. Metals 2020, 10, x FOR PEER REVIEW 7 of 12
(a) (b)
50μm 50μm
(c) (d)
50μm 50μm
(e) (f) 70 cooled in furnace 60 coated with asbestos cooled in air 50
40 cooled by water
cooled by wind 30 Grain sizes/μm 20
10 50μm cooling conditions FigureFigure 6. Microstructure 6. Microstructure of oftwin-roll twin-roll casting casting sheets sheets at atdifferent different cooling cooling rates: rates: (a ()a cooled) cooled in in furnace, furnace, (b) coated(b) coated with asbestos, with asbestos, (c) cooled (c) cooled in air, in (d air,) cooled (d) cooled by wind, by wind, (e) cooled (e) cooled by water by water and and (f) average (f) average grain sizes.grain sizes.
The distribution of precipitates in the sample can be observed by backscattering scanning electron microscopy, as shown in Figure 7 and Table 2. Figure 7a shows the microstructure of the alloy under furnace cooling conditions. It can be seen from the figure that the precipitation phases along the grain boundaries were distributed in an irregular polygon. Due to the high temperature in the furnace and lower cooling rate, the precipitation phase was redissolved, which is consistent with the results described in the literature [24].
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The distribution of precipitates in the sample can be observed by backscattering scanning electron microscopy, as shown in Figure7 and Table2. Figure7a shows the microstructure of the alloy under furnace cooling conditions. It can be seen from the figure that the precipitation phases along the grain boundaries were distributed in an irregular polygon. Due to the high temperature in the furnace and lower cooling rate, the precipitation phase was redissolved, which is consistent with the results Metalsdescribed 2020, 10 in, x the FOR literature PEER REVIEW [24]. 8 of 12
(a) (b)
b a
(c) (d)
e
c
d
(e)
f
Figure 7. SEMSEM morphology morphology of of twin-roll twin-roll casting casting sh sheetseets at at different different cooling rates: ( (aa)) cooled cooled in in furnace, furnace, (b) coated with asbestos, (c) cooled in air, ((dd)) cooledcooled byby wind,wind, andand ((ee)) cooledcooled byby water.water.
Table 2. EDS analysis of the twin-roll casting sheets with different cooling rates. Table 2. EDS analysis of the twin-roll casting sheets with different cooling rates. Elements/Atomic Fraction Al Si Fe Mg Cu Ni O Elements/Atomic fraction Al Si Fe Mg Cu Ni O aa 87.3387.33 1.63 1.63 3.51 3.51 00 00 2.96 4.57 4.57 b 89.06 3.57 2.96 1.41 0 0 3.00 b 89.06 3.57 2.96 1.41 0 0 3.00 c 84.22 4.95 0.62 7.17 0 0 3.03 dc 70.7984.22 8.63 4.95 5.02 0.62 7.227.17 00 0 3.03 8.33 ed 72.7370.79 10.45 8.63 3.29 5.02 7.387.22 0.760 0 8.33 5.41 fe 74.65 72.73 9.75 10.45 4.37 3.29 5.807.38 1.480.76 0 5.41 3.95 f 74.65 9.75 4.37 5.80 1.48 0 3.95 The precipitated phase has a discontinuous dot shape at the grain boundary, and a dispersed precipitateThe precipitated phase appears phase in thehas crystal. a discontinuous Twin-roll castingdot shape sheet at withthe grain asbestos-covered boundary, and cooling a dispersed displays precipitatemore precipitated phase appears phases, whichin the arecrystal. distributed Twin-roll in ancasting irregular sheet network with asbestos-covered along grain boundaries cooling displays more precipitated phases, which are distributed in an irregular network along grain boundaries (Figure 7b). It was clearly observed that the second phase represented by the black dot was dispersed along the grain boundary, and a few bright white rod phases were also precipitated at the grain boundaries. There was almost no precipitate phase in the crystal, which is consistent with the results described in the literature [25]. Figure 7c shows that the grain size of the air-cooled sheet is inhomogeneous, and the precipitated phase is located along the grain boundary and is coarser than the previous two cooling methods (furnace cooling and asbestos covering). The precipitated phase mainly forms bright white rod phase and black dot as a second phase. The two phases are interspersed in the grain boundary,
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(Figure7b). It was clearly observed that the second phase represented by the black dot was dispersed and there is noalong obvious the grain precipitation boundary, and aphase few bright in the white crystal. rod phases Figure were also 7d precipitatedis the SEM at theof a grain twin-roll casting sheet cooled byboundaries. wind Therecooling. was almost The nofine precipitate grains phase were in therelatively crystal, which uniform, is consistent and with the the resultsprecipitated phase was distributeddescribed along in thethe literature grain [ 25boundaries,]. and a large-sized lamellar eutectic phase exists. The Figure7c shows that the grain size of the air-cooled sheet is inhomogeneous, and the precipitated precipitated phasephase is of located the alongwater-cooled the grain boundary sheet and unev is coarserenly thandistributes the previous along two cooling the grain methods boundary, and there is a coarse(furnace band-like cooling and eutectic asbestos covering). structure. The precipitatedNo precipitate phase mainly phase forms exists bright whitein the rod crystal phase (Figure 7e). Due to the higherand black cooling dot as a secondrate, the phase. precipitation The two phases arephase interspersed accumulated in the grain at boundary, the grain and there boundary, is so that no obvious precipitation phase in the crystal. Figure7d is the SEM of a twin-roll casting sheet cooled the fracture wasby wind more cooling. likely The to fine occu grainsr wereduring relatively stretching uniform, andprocess the precipitated [26,27]. phase was distributed The energy-dispersivealong the grain boundaries, spectrum and a large-sized(EDS) analysis lamellar eutectic (as shown phase exists. in TheTable precipitated 2) of the phase twin-roll of casting the water-cooled sheet unevenly distributes along the grain boundary, and there is a coarse band-like sheets with differenteutectic structure. cooling No rates precipitate shows phase that exists the in the main crystal components (Figure7e). Due toof the black higher dot-like cooling rate, precipitates are Al, Mg, and Si,the and precipitation the main phase components accumulated at of the the grain brig boundary,ht white so that rod-like the fracture precipitates was more likely are to Al, Fe, and Si, the lamellar andoccur coarse during stretching ribbon-like process [precipitates26,27]. consist of Al, Si, Fe, Mg, and Cu. Combining EDS The energy-dispersive spectrum (EDS) analysis (as shown in Table2) of the twin-roll casting analysis withsheets XRD with analysis different coolingresults rates (shown shows that in theFigure main components 8), it can of blackbe seen dot-like that precipitates the precipitates are in the sheet consist Al,mainly Mg, and of Si, andblack the maindot-shaped components Mg of the2Si bright and white a bright rod-like white precipitates rod-like are Al, Fe,or and lamellar Si, iron-rich phase. the amellar and coarse ribbon-like precipitates consist of Al, Si, Fe, Mg, and Cu. Combining EDS analysis with XRD analysis results (shown in Figure8), it can be seen that the precipitates in the sheet consist mainly of black dot-shaped Mg2Si and a bright white rod-like or lamellar iron-rich phase.
Figure 8. XRD analysis of twin-roll casting sheets with different cooling rates.
3.4.Figure Hall–Petch 8. XRD Relation analysis of Strength of and twin-roll Grain Size casting sheets with different cooling rates. As recognized, grain plays a significant role in the mechanical strength of metals, especially for 3.4. Hall–Petchthe Relation yield strength of Strength of Al alloys and based Grain on Hall–Petch Size (H–P) relation [28]. Figure9 shows the profile plot 1/2 of yield strength against the inverse square root of average grain size (d− ), where the slope represents As recognized,the H–P slope grain (K). plays The yield a significant strength can be role analyzed in the according mechanical to the H–P strength relation as belowof metals, [29]: especially for the yield strength of Al alloys based on Hall–Petch (H–P)1 relation [28]. Figure 9 shows the profile σs = σ0 + Kd− 2 (1) plot of yield strength against the inverse square root of average grain size (d−1/2), where the slope represents thewhere H–Pσs isslope the yield (K strength,). The σyield0 is the strength friction stress can when be dislocations analyzed glide according on the slip plane, to theK is theH–P relation as magnitude of boundary obstacle against deformation propagation, and d is the average grain size. below [29]: ଵ ି ߪ௦ ൌߪ ܭ݀ ଶ (1) where ߪ௦ is the yield strength, ߪ is the friction stress when dislocations glide on the slip plane, K is the magnitude of boundary obstacle against deformation propagation, and d is the average grain size. As shown in Figure 9, the fitted H–P curve is derived by two distinguished gradients for 6061 twin-roll casting sheet. The critical grain size is about 44 μm for the transition from twinning to slip dominated flow, at which a distinct change in the H–P relation occurs. Therefore, fine grains can be defined as those of sizes smaller than the critical size (< 44 mm), and coarse grains correspond to ones with larger grain sizes (> 44 mm), according to the distinguished deformation modes during ᇱ ᇱ tension. Based on the data in Figure 3, it can be calculated that ߪ= 344 MPa and ܭ = 2959 MPa −1/2 ᇱᇱ ᇱᇱ −1/2 μm have a grain size of 13–44 μm, while ߪ = 78 MPa and ܭ = 161 MPa μm have a grain size of 44–53 μm.
Metals 2020, 10, 1168 9 of 11 Metals 2020, 10, x FOR PEER REVIEW 10 of 12
160
140
-1/2 120 σs=78+161d
100
80 -1/2
Yield strength(MPa) σs=-344+2959d
60 44 μm
0.10 0.15 0.20 0.25 0.30 -1/2 -1/2 d (μm ) Figure 9. Hall–Petch relation of yield strength and the inverse square root of average grain size. Figure 9. Hall–Petch relation of yield strength and the inverse square root of average grain size. As shown in Figure9, the fitted H–P curve is derived by two distinguished gradients for 6061 3.5. Discussiontwin-roll casting sheet. The critical grain size is about 44 µm for the transition from twinning to slip dominated flow, at which a distinct change in the H–P relation occurs. Therefore, fine grains can be Fromdefined the asabove those of experimental sizes smaller than results, the critical it sizecan (
Metals 2020, 10, 1168 10 of 11
4. Conclusions (1) As the cooling rate increased, the tensile strength and elongation of the twin-roll casting sheet increased. The wind-cooled twin-roll casting sheet with an average cooling rate of 3 ◦C/s showed the best overall performance. The yield strength at this time was 88 MPa, and a tensile strength of 178 MPa and elongation of 15% were obtained for the wind-cooled twin-roll casting sheet. In addition, a quantitative H–P relation was established to predict the yield strength of 6061 twin-roll casting sheet with different grain sizes and cooling rates. (2) At higher cooling rates, more precipitates and second phase Mg2Si and iron-rich phase will appear. (3) The morphology of the second phase of Mg2Si changed from a continuous needle shape to an. intermittent particle shape at high temperatures. As the cooling rate increased, the iron-rich phase changed from a pellet in a high temperature states to a rod shape or even a thick lath.
Author Contributions: Conceptualization, Z.X., S.L. and H.W.; methodology, Z.X.; software, Z.X. and S.W.; validation, Z.X. and S.W.; investigation, Z.X. and H.W.; resources, Z.X., X.C. and H.W.; writing—original draft preparation, Z.X.; writing—review and editing, Z.X., H.S. and S.W.; funding acquisition, S.L. and H.W. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by National Natural Science Foundation of China (Grant number 51874172) and Research Fund for young teachers of University of Science and Technology Liaoning (Grant number 2019QN09). Conflicts of Interest: The authors declare no conflict of interest.
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