A New Infrared Heat Treatment on Hot Forging 7075 Aluminum Alloy: Microstructure and Mechanical Properties

materials

Article A New Infrared Heat Treatment on Hot Forging 7075 Aluminum Alloy: Microstructure and Mechanical Properties

Yi-Ling Chang , Fei-Yi Hung * and Truan-Sheng Lui

Department of Materials Science and Engineering, National Cheng Kung University, Tainan 701, Taiwan; [email protected] (Y.-L.C.); [email protected] (T.-S.L.) * Correspondence: [email protected]; Tel.: +886-6-2757575-62950

Received: 31 January 2020; Accepted: 3 March 2020; Published: 6 March 2020

Abstract: When hot forging 7075 aluminum alloy, as a military material durable enough for most of its applications, it needs to be heat-treated to ensure the target material property achieves the application requirements. However, the material properties change because of heat throughout usage. In this study, a new approach was devised to heat treat the alloy to prevent material property changes. The study further clarified the effect of rapid heat treatment on the high-temperature resistance of a hot forging 7075 aluminum alloy. Infrared (IR) heat treatment was used as a rapid heating technique to effectively replace the conventional resistance heat (RH) treatment method. Our experimental result showed that IR heat treatment resulted in better age hardening at the initial aging stage, where its tensile strength and elongation appeared like that of a resistance heat treatment. More so, based on hardness and tensile test results, the IR-heated treatment process inhibited the phase transformation of precipitations at a higher temperature, improving high-temperature softening resistance and enhancing the thermal stability of the hot forging 7075 aluminum alloy.

Keywords: hot forging 7075 aluminum alloy; rapid heating; infrared heating; long time high-temperature

1. Introduction The 7075. (Al-Zn-Mg-Cu) aluminum alloy is extensively used in the aerospace industry and military because of its high strength and lightweight [1,2]. The 7075 aluminum alloy is a heat treatable aluminum alloy. Heat treatment can enhance its hardness and strength through the precipitation hardening. The precipitation hardening sequence of 7075 aluminum alloy is a supersaturated solid solution (S.S.S.S) Guinier-Preston (GP) zones metastable phase (η’) equilibrium phase (η)[3–5]. → → → The major strengthening phases are GP zones, η’ and η (MgZn2) phases. At the peak age (T6) condition, the major strengthening phases are GP zones and η’ phase, and at the T7x aging condition, the major strengthening phase is the η phase [4]. In this study, IR heat treatment was used as a rapid heating method. An IR heat treatment furnace not only can conserve energy but it can also provide a faster heating effect [6]. Previous studies on 6082 aluminum alloy showed that rapid heating could shorten the heat treatment duration and produce a high solute concentration, which leads to more uniform and fine precipitates in the matrix under the T6 condition [7–9]. These precipitates can improve strength and hardness [8]. However, how the IR heated effect on the hot forging 7075 aluminum alloy has not been investigated. Thus, in this study, IR heat treatment was used to try to upgrade the concentration of solutes and change the precipitates conditions of hot forging 7075 aluminum alloy. Since 7075 aluminum alloy is used in the military, tensile strength after the high-temperature treatment was also investigated.

Materials 2020, 13, 1177; doi:10.3390/ma13051177 www.mdpi.com/journal/materials Materials 2020, 13, x FOR PEER REVIEW 2 of 12 Materials 2020, 13, x FOR PEER REVIEW 2 of 12 change the precipitates conditions of hot forging 7075 aluminum alloy. Since 7075 aluminum alloy is Materials 2020, 13, 1177 2 of 12 usedchange in thethe precipitatesmilitary, tensile conditions strength of afterhot forging the high-temperature 7075 aluminum treatment alloy. Since was 7075 also aluminum investigated. alloy is used Wein the expected military, that tensile IR heat strength treatment after couldthe high-temperature delay the occurrence treatment of high-temperature was also investigated. softening, effectivelyWe expectedexpected improving that the IR heathigh-temperature treatment could resistance delaydelay the of occurrenceT6 materials. of The high-temperature results can be softening, used as a ereferenceeffectivelyffectively in improving military and the aerospace high-temperature high-temperature industry resistanceapplications. resistance of of T6 T6 materials. materials. The The results results can can be be used used as as a areference reference in in military military and and aerospace aerospace industry industry applications. applications. 2. Experimental Procedures 2. Experimental We used commercial Procedures hot forging 7075 aluminum alloy. The hot forging 7075 forgings were made fromWe 52 mm used diameter commercial cylindrical hot forging extruded 7075 aluminum rods, where alloy. the Thechemical hot forging composition 7075 forgings is shown were in made Table from1. To 52make52 mmmm sure diameterdiameter the experiment cylindricalcylindrical results extrudedextruded can rods,berods, used where wh inere different the the chemical chemical shapes composition composition of hot forging is is shown 7075shown aluminum in in Table Table1. Toparts,1. To make make two sure suredifferent the the experiment experiment shapes of results resultsparts can werecan be be used used inin in di thisdifferentfferent study. shapes shapes Photographsof of hothot forgingforging of hot 70757075 forging aluminum 7075 parts,aluminum twotwo di partsdifferentfferent are shapes shownshapes of in partsof Figure parts were 1. were used used in this in study. this Photographsstudy. Photographs of hot forging of hot 7075 forging aluminum 7075 partsaluminum are shown parts inare Figure shown1. in Figure 1. Table 1. Chemical composition (wt.%) of 7075 aluminum alloy. Table 1. Chemical composition (wt.%) of 7075 aluminum alloy. Element Al Fe Si Cu Mg Mn Zn Cr Ti Element Al Fe Si Cu Mg Mn Zn Cr Ti Elementwt.% Bal.Al 0.15 Fe 0.08 Si 1.26 Cu 2.21 Mg 0.03 Mn 5.39 Zn 0.2 Cr 0.05 Ti wt.% Bal. 0.15 0.08 1.26 2.21 0.03 5.39 0.2 0.05 wt.% Bal. 0.15 0.08 1.26 2.21 0.03 5.39 0.2 0.05

Figure 1. Photographs of 7075 aluminum hot forging parts. Figure 1. Photographs of 7075 aluminum hot forging parts. Figure 1. Photographs of 7075 aluminum hot forging parts. The heat treatment was performed in an infrared (IR) heating furnace and a resistance heating The heat treatment was performed in an infrared (IR) heating furnace and a resistance heating (RH) furnace (RH, CM30S, CHENG SANG, Changhua, Republic of China (Taiwan)), respectively. (RH)The furnace heat (RH,treatment CM30S, was CHENG performed SANG, in an Changhua,infrared (IR) Republic heating offurnace China and (Taiwan)), a resistance respectively. heating The IR heating furnace was developed at our laboratory. Figure 2 shows photographs of the IR The(RH) IR furnace heating (RH, furnace CM30S, was developedCHENG SANG, at ourlaboratory. Changhua, Figure Republic2 shows of China photographs (Taiwan)), of the respectively. IR heating heating furnace. The heating rate of the IR heating furnace has been published in our previous furnace.The IR heating The heating furnace rate was of the developed IR heating at furnace our labo hasratory. been publishedFigure 2 shows in our photographs previous research of the [7 ].IR researchheating furnace.[7]. The heating rate of the IR heating furnace has been published in our previous research [7].

Figure 2. Photographs of infrared (IR) heat treatment apparatus: (a) IR heating apparatus, (b) heating

space of the IR heating apparatus.

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Conventional artiﬁcial aging parameters [10,11] were utilized to establish baseline information (i.e., solution heat treatment conditions: RH/480 ◦C_60 min, aging conditions: RH/120 ◦C_24 h). The heat treatment conditions are shown in Tables2 and3. To evaluate high-temperature softening, post heat treatment (200 ◦C for 240 min) was performed in the RH furnace after T6 heat treatment.

Table 2. Experimental parameters and specimen codes (only solution heat treatment).

Experiment Condition Heating Method SHT Code RH 480 C, 30, 60, 120 min RA30, RA60, RA120 a (grip) ◦ IR 480 ◦C, 30, 60, 120 min IA30, IA60, IA120 RH 480 C, 30, 60, 120 min RB30, RB60, RB120 B (upper receiver) ◦ IR 480 ◦C, 30, 60, 120 min IB30, IB60, IB120 (SHT: solution heat treatment; RH: resistance heating; IR: infrared)

Table 3. Experimental parameters and specimen codes.

Experiment Condition Heating Method SHT Artiﬁcial Aging Code 480 C, 60 min 120 C, 0–24 h RA60-120_XH RH ◦ ◦ A 480 ◦C, 60 min 140 ◦C, 0–24 h RA60-140_XH (grip) 480 C, 60 min 120 C, 0–24 h IA60-120_XH IR ◦ ◦ 480 ◦C, 60 min 140 ◦C, 0–24 h IA60-140_XH 480 C, 60 min 120 C, 0–24 h RB60-120_XH RH ◦ ◦ B 480 ◦C, 60 min 140 ◦C, 0–24 h RB60-140_XH (upper receiver) 480 C, 60 min 120 C, 0–24 h IB60-120_XH IR ◦ ◦ 480 ◦C, 60 min 140 ◦C, 0–24 h IB60-140_XH X: heating duration (hours); SHT: solution heat treatment; RH: resistance heating; IR: infrared.

Metallographic investigations of the microstructural features and grain size were conducted using optical microscopy (OM, OLYMPUS BX41M-LED, OLYMPUS, Tokyo, Japan). All specimens (with the top surface perpendicular to the extrusion direction) were polished using SiC papers from #120 to #4000 grit in an Al2O3 aqueous suspension (1.0 and 0.3 µm) and a SiO2 polishing suspension and etched using Keller’s reagent. The secondary phases of the alloy after heat treatment were observed using scanning electron microscopy (SEM, SU-5000, HITACHI, Tokyo, Japan), with energy-dispersive X-ray spectroscopy (EDS, EDAX, Singapore) and X-ray diﬀraction (XRD, Bruker AXS Gmbh, Karlsruhe, Germany). XRD analysis with Cu Kα radiation was employed in the 2θ range of 30◦–55◦ to identify the secondary compounds. Hardness measurements were performed on heat-treated samples using the Rockwell hardness (HR) test (Mitutoyo, Kawasaki-shi, Japan). The measurement conditions for the HR test followed the B-scale. The mean value of ﬁve impressions was taken as the hardness of the corresponding condition. All tensile tests were performed using a universal testing machine at room temperature with a stretching rate of 5 mm/min. The dimensions of the tensile test specimen are shown in Figure3. Young’s modulus values were calculated from Young’s region of the tensile test graph, based on the Materials 2020, 13, x FOR PEER REVIEW 4 of 12 formula as follows: E = ∆σ/∆ε.

Figure 3. Diagram of the tensile test specimens. Figure 3. Diagram of the tensile test specimens.

3. Results and Discussion

3.1. Microstructure Observation Figure 4a,d shows the microstructure observation results of A and B forgings. Two kinds of intermetallic compounds could be distinguished for all specimens (gray and black second phases particles) that could be observed. These second phase particles were distributed evenly in the matrix in both A and B forgings. There was no second phase distribution difference between the A and B forgings.

Figure 4. Microstructure observation results of specimens (forging A: (a–c), forging B: (d–f)). (a) As- received A, (b) RA120, (c) IA120, (d) as-received B, (e) RB120, and (f) IB120.

Figure 4b,c shows the microstructure observation results of heat-treated (480 °C, 120 min) A forging; RH-heated (Figure 4b) and IR-heated (Figure 4c), comparing RH-heated and IR-heated specimens, where there was no difference between them. Figure 4e,f shows the microstructure observation results of heat-treated (480 °C, 120 min) B forging; RH-heated (Figure 4e) and IR-heated (Figure 4f), comparing RH-heated and IR-heated specimens, where there was no difference between them. Figure 5 and Tables 4 and 5 show the results of SEM observation and EDS analysis. According to the EDS analysis results, the gray compound (A, C, E, G, I and K compounds of Figure 5) was Al- Cu-Cr-Fe, and the black compound (B, D, F, H, J and L compounds of Figure 5) was Al-Mg-Si-Cr. According to previous research [12], Al-Cu-Cr-Fe and Al-Mg-Si-Cr compounds formed during casting and were rearranged during the hot forging process. Both Al-Cu-Cr-Fe and Al-Mg-Si

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Materials 2020, 13, 1177 4 of 12 Figure 3. Diagram of the tensile test specimens.

3. Results Results and and Discussion Discussion

3.1. Microstructure Microstructure Observation Figure 44a,da,d showsshows thethe microstructuremicrostructure observationobservation resultsresults ofof aA and and B B forgings. forgings. TwoTwo kindskinds ofof intermetallic compounds compounds could could be be distinguished fo forr all specimens (gray and black second phases particles) thatthat couldcould bebe observed.observed. TheseThese second second phase phase particles particles were were distributed distributed evenly evenly in in the the matrix matrix in inboth both a and A and B forgings. B forgings. There There was nowas second no second phase phase distribution distribution diﬀerence difference between between the a and the B A forgings. and B forgings.

Figure 4. 4. MicrostructureMicrostructure observation observation results results of specimens of specimens (forging (forging A: (a– A:c), forging (a–c), forging B: (d–f)). B: (a (d) –As-f)). (receiveda) As-received A, (b) RA120, A, (b) RA120, (c) IA120, (c) IA120,(d) as-received (d) as-received B, (e) RB120, B, (e) RB120, and (f) and IB120. (f) IB120.

Figure4 4b,cb,c shows shows the the microstructure microstructure observation observatio resultsn results of heat-treated of heat-treated (480 ◦ (480C, 120 °C, min) 120 a forging;min) A forging;RH-heated RH-heated (Figure4 b)(Figure and IR-heated 4b) and (FigureIR-heated4c), (F comparingigure 4c), RH-heatedcomparing andRH-heated IR-heated and specimens, IR-heated wherespecimens, there where was no there diﬀerence was no between difference them. between them. Figure4 4e,fe,f shows shows the the microstructure microstructure observation observatio resultsn results of heat-treated of heat-treated (480 ◦ C,(480 120 °C, min) 120 B forging;min) B forging;RH-heated RH-heated (Figure4 e)(Figure and IR-heated 4e) and (FigureIR-heated4f), (Figure comparing 4f), RH-heatedcomparing andRH-heated IR-heated and specimens, IR-heated wherespecimens, there where was no there diﬀerence was no between difference them. between them. Figure5 5 and and Tables Tables4 and4 and5 show 5 show the the results results of SEMof SEM observation observation and and EDS EDS analysis. analysis. According According to tothe the EDS EDS analysis analysis results, results, the the graygray compoundcompound (A, C, C, E, E, G, G, I and I and K Kcompounds compounds of Figure of Figure 5) was5) was Al- Al-Cu-Cr-Fe,Cu-Cr-Fe, and and the the black black compound compound (B, (B,D, D,F, F,H, H, J and J and L Lcompounds compounds of of Figure Figure 5)5) was Al-Mg-Si-Cr. According toto previous previous research research [12 ],[12], Al-Cu-Cr-Fe Al-Cu-Cr-Fe and Al-Mg-Si-Crand Al-Mg-Si-Cr compounds compounds formed formed during castingduring castingand were and rearranged were rearranged during the during hot forging the process.hot forg Bothing Al-Cu-Cr-Feprocess. Both and Al-C Al-Mg-Siu-Cr-Fe compounds and Al-Mg-Si were stable at 480 C. Thus, Al-Cu-Cr-Fe and Al-Mg-Si-Cr compounds remained after heat treatment at ◦ 480 ◦C for 120 min (Figure4b,c,e,f). Materials 2020, 13, x FOR PEER REVIEW 5 of 12

Materialscompounds2020, 13were, 1177 stable at 480 °C. Thus, Al-Cu-Cr-Fe and Al-Mg-Si-Cr compounds remained 5after of 12 heat treatment at 480 °C for 120 min (Figure 4b,c,e,f).

Figure 5. SEM observation results for various heat-treated specimens (forging A: (a–c), forging B: (d–f)). (Figurea) As-received 5. SEM observation A, (b) RA120, results (c) IA120, for various (d) as-received heat-treated B, ( especimens) RB120, and (forging (f) IB120. A: (a–c), forging B: (d– f)). (a) As-received A, (b) RA120, (c) IA120, (d) as-received B, (e) RB120, and (f) IB120. Table 4. EDS analysis results for specimens as-received A, RA120, and IA120, as shown in FigureTable 54.a,c,e, EDS respectively. analysis results for specimens as-received A, RA120, and IA120, as shown in Figure 5a,c,e, respectively. As-received A RA120 IA120 at.% ABCDEFAs-received A RA120 IA120 at.% Fe 7.8A 0.0B 8.3C 0.1D 0.2E F 11.5 Cu 3.1 0.1 3.1 0.2 0.2 3.1 ZnFe 1.2 7.8 1.1 0.0 1.4 8.3 1.70.1 0.50.2 11.5 1.8 MgCu 1.3 3.1 5.1 0.1 1.0 3.1 19.10.2 42.60.2 3.1 1.5 AlZn 80.6 1.2 50.7 1.1 82.3 1.4 46.61.7 32.50.5 1.8 81.4 Si 2.8 40.4 0.5 30.4 23.8 0.5 CrMg 3.21.3 2.6 5.1 3.41.0 1.919.1 42.6 0.2 1.5 0.2 Al 80.6 50.7 82.3 46.6 32.5 81.4 Table 5.Si EDS analysis2.8 results for 40.4specimens as-received0.5 B,30.4 RB120, and23.8 IB120, as0.5 shown in Figure5d–f,Cr respectively. 3.2 2.6 3.4 1.9 0.2 0.2

As-received B RB120 IB120 Tableat.% 5. EDS analysis results for specimens as-received B, RB120, and IB120, as shown in Figure 5d– f, respectively. GHIJKL Fe 8.0 0.2 8.5 0.0 12.2 0.2 As-received B RB120 IB120 at.%Cu 2.6 0.4 3.8 0.2 3.4 0.2 Zn 1.7G 1.0H 1.0I 0.9J 1.6K 0.5L MgFe 0.8 8.0 33.3 0.2 1.2 8.5 40.8 0.0 1.512.2 0.2 42.6 Al 82.8 41.6 82.6 36.0 80.6 32.5 CuSi 0.4 2.6 22.1 0.4 0.7 3.8 20.60.2 0.43.4 0.2 23.8 CrZn 3.7 1.7 1.4 1.0 2.2 1.0 1.50.9 0.31.6 0.5 0.2 Mg 0.8 33.3 1.2 40.8 1.5 42.6 FigureAl6 shows the 82.8 microstructure 41.6 features of 82.6 samples subjected36.0 to various80.6 heat32.5 treatments. Their microstructureSi features 0.4 are similar; 22.1 thus, hardness 0.7 tests were20.6 performed. 0.4 23.8 Cr 3.7 1.4 2.2 1.5 0.3 0.2

Figure 6 shows the microstructure features of samples subjected to various heat treatments. Their microstructure features are similar; thus, hardness tests were performed.

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Figure 6. Metallographic images of vari variousous heat-treated specimens. ( a) As-received A, ( b) as-received B, (c) RA120, ((d)) RB120,RB120, ((e)) IA120,IA120, andand ((ff)) IB120.IB120.

3.2. Effect Effect of High-Penetrating Infrared on Solution Heat Treatment Figures7 7and and8 show 8 show the hardnessthe hardness results results of forging of forging a and forging A and B forging after various B after heat various treatments. heat Thetreatments. hardness The values hardness of forging values aof specimens forging A were specim higherens were than higher the values than ofthe forging values Bof specimens. forging B Thisspecimens. result This was result due to was the due diff erenceto the difference in metal flowin metal during flow the during forging the processforging process (different (different shapes areshapes subjected are subjected to different to different stresses). stresses). It should It be should noted thatbe noted for both that RH- for andboth IR-heated RH- and specimens,IR-heated hardnessspecimens, after hardness 30 min after of treatment 30 min of was treatment the same was as the that same after as 120 that min. after 120 min.

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Figure 7. Hardness of various heat-treated forging A specimens. FigureFigure 7. Hardness 7. Hardness of variousof various heat-treated heat-treated forging forging A specimens. A specimens.

Figure 8. Hardness of various heat-treated forging B specimens. Figure 8. Hardness of various heat-treated forging B specimens. Figure 8. Hardness of various heat-treated forging B specimens. According to SEM (Figure5) and EDS (Tables4 and5) results, no coarse proeutectic MgZn 2 phases According to SEM (Figure 5) and EDS (Tables 4 and 5) results, no coarse proeutectic MgZn2 formed in the material, indicating that proeutectic MgZn formed a smaller precipitated phase during According to SEM (Figure 5) and EDS (Tables 24 and 5) results, no coarse proeutectic MgZn2 phases formed in the material, indicating that proeutectic MgZn2 formed a smaller precipitated phase the extrusion and forging process. Thus, only 30 min was needed for the solutionization. duringphases the formed extrusion in the and material, forging indicatingprocess. Thus, that proeutecticonly 30 min MgZn was needed2 formed for a thesmaller solutionization. precipitated phase Figure9 shows the hardness curves of IR- and RH-heated forging a specimens. a lot of research duringFigure the 9 showsextrusion the andhardness forging curves process. of IR- Thus, and RH-heatedonly 30 min forging was needed A specimens. for the solutionization. A lot of research has been reported on the aging hardening of the 7075 aluminum alloy. The highest values of hardness has beenFigure reported 9 shows on the the aging hardness hardening curves of of the IR- 7 075and aluminum RH-heated alloy. forging The A highest specimens. values A oflot hardness of research developed by age hardening could be attributed to precipitation of coherent and finely dispersed developedhas been byreported age hardening on the aging could hardening be attributed of the to7075 precipitation aluminum alloy.of coherent The highest and finely values dispersed of hardness precipitatesdeveloped (i.e., by MgZn age hardening2) phases. Thesecould precipitatesbe attributed cause to precipitation lattice distortions of coherent that make and the finely alloy dispersed harder precipitates (i.e., MgZn2) phases. These precipitates cause lattice distortions that make the alloy and act as obstacles to dislocation movement and thereby strengthen the alloy [10,13]. There was no harderprecipitates and act as(i.e., obstacles MgZn2 )to phases. dislocation These moveme precipitatesnt and causethereby latti strengthence distortions the alloy that [10,13].make theThere alloy difference in the peak age hardness value of IR- and RH-heated specimens, indicating that using the washarder no difference and act asin obstaclesthe peak toage dislocation hardness movemevalue of ntIR- and and thereby RH-heated strengthen specimens, the alloy indicating [10,13]. that There IR-heated method could provide the solution effect as RH-heated provide. usingwas the no IR-heated difference method in the peakcould age provide hardness the solutionvalue of effect IR- and as RH-heatedRH-heated provide.specimens, indicating that using the IR-heated method could provide the solution effect as RH-heated provide.

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FigureFigure 9. 9.E Effectffect of of aging aging temperature temperature and and aging aging method method on on hardness hardness profiles profiles of of specimens. specimens. Figure 9. Effect of aging temperature and aging method on hardness profiles of specimens. However,However, the the IR-heated IR-heated specimens specimens showed showed a a higher higher hardness hardness value value at at the the early early stage stage of of aging aging hardeninghardeningHowever, compared compared the IR-heated to to that that obtained specimens obtained with with showed RH. RH. Similar Similara high resultser results hardness can becan found valuebe found in at our the in previous ourearly previous stage research of research aging [8,9]. hardeningThis[8,9]. phenomenon This compared phenomenon might to that bemight obtained associated be associated with with RH. a with high Similar a concentration high results concentration can of be solutes found of solutes in theourmatrix. previousin the matrix. research [8,9]. FigureThisFigure phenomenon 10 10 showsshows the might results be associated of of the the tensile tensile with test. test.a high It could It concentration could be beobserved observed of solutes that that IR in heat IR the heat treatmentmatrix. treatment gave gavetheFigure material the material 10 tensile shows tensile strengththe results strength and of ductility the and tensile ductility like test. that like It of could thatthe RH-heated ofbe theobserved RH-heated material. that IR material. It heat should treatment Itbe should noted gave that be thenotedthe material IR-heated that thetensile specimens IR-heated strength artificially specimens and ductility aged artificially like for 12that h of(IA60-120-12H/IB60-120-12H) aged the forRH-heated 12 h (IA60-120-12H material. It should and/IB60-120-12H) had be strength noted that and and thehadelongation IR-heated strength similar specimens and elongationto that artificially of RH-heated similar aged to forspecimens that 12 h of (IA60-120-12H/IB60-120-12H) RH-heated artificially aged specimens for 24 h artificially (RA60-120-24H/RB60-120- and had aged strength for 24and h elongation(RA60-120-24H24H). Thus, similar IR/ RB60-120-24H).heat to that treatment of RH-heated could Thus, shorten IR specimens heat treatmentthe artificialartificially could aging aged shorten time for 24 to the h12 (RA60-120-24H/RB60-120- artificial h. aging time to 12 h. 24H). Thus, IR heat treatment could shorten the artificial aging time to 12 h.

Figure 10. Tensile strength curves of various specimens. FigureFigure 10. 10. TensileTensile strength strength curves curves of of various various specimens. specimens.

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Materials 2020, 13, 1177 9 of 12 IR heat treatment enhanced Young’s modulus of the specimens (Table 6). As IR-heated and RH- heated specimens were of similar chemical composition, the difference of Young’s modulus was influencedIR heat treatment by the other enhanced difference Young’s of the modulus material, of for the example, specimens the (Table concentration6). As IR-heated of precipitates, and the RH-heatedtype of specimens precipitates, were and of the similar related chemical effect occurred composition, by precipitates. the difference Xie ofet Young’sal. [14] found modulus that wasYoung’s influencedmodulus by theof a other material difference is related of the to material, the concentration for example, of thesolutes concentration and precipitates. of precipitates, Thus, theaccording type to of precipitates,the results and of theTable related 6, we effect find occurred that using by precipitates. IR-heated Xiecan et result al. [14 ]in found the thatdifference Young’s of modulus precipitate. of a materialIncreasing is related Young’s to themodulus concentration can enhance of solutes the and pote precipitates.ntial of thinning Thus, accordingthickness to of the workpiece results of and Tablevibration6, we find resistance that using [15]. IR-heated Thus, IR can heat result treatment in the improves di fference rolling of precipitate. and vibration Increasing resistance. Young’s modulus can enhance the potential of thinning thickness of workpiece and vibration resistance [15]. Thus, IR heat treatmentTable improves 6. Specific rolling tensile and strength, vibration elongation, resistance. and hardness from Figure 10. Young’s Material PropertiesTable 6. Specific tensileYS strength, elongation,UTS andUE hardness fromTE Figure 10. Hardness Modulus (MPa) (MPa) (%) (%) (HRB) Material PropertiesSample YS UTS UE TE Young’s(σ/ε, MPa)Hardness (MPa) (MPa) (%) (%) Modulus (HRB) Sample IA60-120-12H 574 630 7 10 (σ/ε, MPa)121 89 IA60-120-12HIA60-120-24H 574507 630 570 7 7 10 10 121108 89 88 IA60-120-24H 507 570 7 10 108 88 RA60-120-24HRA60-120-24H 512512 554 554 4 4 55 100100 89 89 IB60-120-12HIB60-120-12H 528528 613 613 9 9 10 10 122122 90 90 IB60-120-24H 533 634 7 10 161 88 RB60-120-24HIB60-120-24H 554533 638 634 9 7 12 10 107161 89 88 RB60-120-24H 554 638 9 12 107 89 3.3. High-Temperature Resistance 3.3. High-Temperature Resistance In this study, 200 ◦C was used as the high-temperature resistance test temperature. In this study, 200 °C was used as the high-temperature resistance test temperature. Karaaslan et Karaaslan et al. [16] found that when the temperature reaches 200 ◦C, GP zones dissolve, and the materialal. [16] softens. found that However, when the the temperature material is reaches strengthened 200 °C, by GP the zones precipitation dissolve, and of thethe materialη’ phase softens. in the followingHowever, stage.the material Strength is isstrength enhancedened by byη the’ precipitates, precipitation and of the the material η’ phase softens in the whenfollowing the ηstage.’ precipitatesStrength become is enhanced the η phase.by η’ precipitates, The material and also the softens material when softens over-aging when the occurs η’ precipitates [5]. Danh et become al. [5] the η phase. The material also softens when over-aging occurs [5]. Danh et al. [5] found that at 200 °C, found that at 200 ◦C, GP zones dissolve within 10 min and over-aging occurs after 30 min. Figure 11 GP zones dissolve within 10 min and over-aging occurs after 30 min. Figure 11 shows the time and shows the time and hardness profiles of 200 ◦C heating of T6 (IA60-120-12H and RA60-120-12H) specimens.hardness The profiles hardness of 200 increases °C heating within of 30T6 min(IA60-120-12H due to the re-precipitation and RA60-120-12H) and phase specimens. transformation The hardness of GPincreases zones. within Over-aging 30 min also due occurs to the re-precipitation within 30 min. and It shouldphase transformation be noted that of the GP IR-heat-treated zones. Over-aging specimenalso occurs has a slowerwithin softening30 min. It rateshould at high be noted temperatures. that the IR-heat-treated specimen has a slower softening rate at high temperatures.

FigureFigure 11. E ﬀ11.ect Effect of heating of heating method method on hardness on hardness proﬁles profiles of specimens of specimen ageds ataged 200 at◦ C.200 °C.

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Materials 2020, 13, x FOR PEER REVIEW 10 of 12 Figure 12 and Table7 show the results of the tensile test of high-temperature resistance test specimens.Figure The12 and tensile Table test was7 show carried the outresults after heatof the treatment tensile test at 200 of◦ Chigh-temperature for 4 h. We found resistance that the tensile test strengthspecimens. decreased, The tensile and test ductility was carried increased, out after indicating heat treatment that over-aging at 200 °C occurred for 4 h. for We both found specimens. that the Additionally,tensile strength IA60-120-12H_200-4H decreased, and ductility showed increased, higher strengthindicating (yield that strength, over-aging YS: 400occurred MPa, ultimatefor both tensilespecimens. strength, Additionally, UTS: 474 IA60-120-12H_200-4H MPa) than that of RA60-120-1F2H_200-4H showed higher strength (YS: (yield 367 strength, MPa, UTS: YS: 458 400 MPa), MPa, indicatingultimate tensile better strength, high-temperature UTS: 474 MPa) resistance. than that Even of RA60-120-1F2H_200-4H after heat treatment at(YS: 200 367◦C MPa, for UTS: 240 min, 458 theMPa), IR-heated indicating specimen better high-tempera showed a betterture Young’s resistance. modulus Even after than thatheat oftreatment the RH-heated at 200 °C specimen. for 240 min, the IR-heated specimen showed a better Young’s modulus than that of the RH-heated specimen.

Figure 12. Strain-stress curves of specimens after the high-temperature test (200 ◦ C, 4 h). Figure 12. Strain-stress curves of specimens after the high-temperature test (200 °C, 4 h). Table 7. Speciﬁc tensile strength, elongation, and hardness of high-temperature (200 ◦C) over-aged specimens. Table 7. Specific tensile strength, elongation, and hardness of high-temperature (200 °C) over-aged specimens.Material Properties YS UTS UE TE Young’s Hardness (MPa) (MPa) (%) (%) Modulus (HRB) Sample (σYoung’s/ε, MPa) IA60-120-12H_200-4HMaterial Properties 400 474 7 12 124 76 RA60-120-12H_200-4H 367YS 458UTS 7UE 13TE Modulus 89 Hardness 83 (MPa) (MPa) (%) (%) (σ/ε, (HRB) Sample MPa) In the XRD patterns (shown in Figure 13), according to our previous research [11], IA60-120-12H_200-4H 400 474 7 12 124 76 MgZn2 and Al-Cu(Cr)Fe compounds could be observed. Comparing RA60-120-12H_200-4H and RA60-120-12H_200-4H 367 458 7 13 89 83 IA60-120-12H_200-4H specimens, RA60-120-12H_200-4H has a more obvious MgZn2 peak compared to that for IA60-120-12H_200-4H. By comparing to the results in Figure 11, there are more MgZn2 In the XRD patterns (shown in Figure 13), according to our previous research [11], MgZn2 and formed in the RA60-120-12H_200-4H; while MgZn2 is representative of over-aging. Al-Cu(Cr)Fe compounds could be observed. Comparing RA60-120-12H_200-4H and IA60-120- 12H_200-4H specimens, RA60-120-12H_200-4H has a more obvious MgZn2 peak compared to that for IA60-120-12H_200-4H. By comparing to the results in Figure 11, there are more MgZn2 formed in the RA60-120-12H_200-4H; while MgZn2 is representative of over-aging.

MaterialsMaterials2020 2020, ,13 13,, 1177 x FOR PEER REVIEW 1111 of of 12 12

Figure 13. XRD patterns of specimens after the high-temperature test (200 ◦C, 4 h). Figure 13. XRD patterns of specimens after the high-temperature test (200 °C, 4 h). This study investigated over-aging with RH and IR heat treatments at 200 ◦C. The IR-heated specimenThis showedstudy investigated better high-temperature over-aging with resistance. RH and IR According heat treatments to previous at 200 investigations°C. The IR-heated [7], IR-heatedspecimen specimensshowed better had relativelyhigh-temperature uniform andresistance dense. precipitationsAccording to duringprevious the investigations precipitate process. [7], IR- heatedThus, specimens it is reasonable had relatively to infer uniform that the abilityand dense to withstand precipitations high temperatureduring the precipitate is due to the process. uniform and denseThus, precipitationsit is reasonable which to infer formed that the due ability to the to withstand IR heated process.high temperature That is, these is due fine to the and uniform larger amountand dense of precipitations precipitations (compared which formed to the due RH to heated the IR sample) heated consumed process. That the Mgis, these and Zn fine atoms and inlarger the matrixamount and of delayedprecipitations the transformation (compared to to theη-MgZn RH he2.ated sample) consumed the Mg and Zn atoms in the matrixMoreover, and delayed from the the results transformation of mechanical to η-MgZn tests2. as presented in Figures9 and 10; higher hardnessMoreover, values from at the the initial results aging of mechanical stage, the enhancement tests as presented of tensile in Figures strength, 9 and and 10; Young’s higher modulus, hardness ourvalues experiment at the initial results aging chime stage, in with the theenhancemen results usingt of tensile IR as a strength, heated method and Young’s that can modulus, bring about our aexperiment fine and larger results amount chime of in precipitations. with the results using IR as a heated method that can bring about a fine and larger amount of precipitations. 4. Conclusions 4. Conclusions In this study, a systemic study on infrared heat treatment on hot forging 7075 aluminum alloy, the followingIn this study, findings a systemic were obtained: study on infrared heat treatment on hot forging 7075 aluminum alloy, the following findings were obtained: 1. IR heat treatment can be used as a heat treatment method on hot forging 7075 aluminum alloy. 1 Moreover,IR heat treatment compared can tobe theused RH-heated as a heat treatm method,ent method the IR-heated on hot methodforging 7075 results aluminum in better alloy. age hardeningMoreover, atcompared the initial to aging the stage.RH-heated method, the IR-heated method results in better age 2. IRhardening heat treatment at the initial shortens aging the stage. heat treatment time and produces strength and elongation in 12 h, 2 likeIR heat that treatment obtained withshortens RH inthe 24 heat h. treatment time and produces strength and elongation in 12 3. IR-heatedh, like that specimens obtained hadwith a RH higher in 24 Young’s h. modulus than that of RH-heated specimens. IR-heated 3 canIR-heated change specimens the precipitates’ had a conditionhigher Young’s of the specimens. modulus than that of RH-heated specimens. IR- 4. IR-heatedheated can specimens change the had precipitat better highes' condition temperature of the softening specimens. resistance than RA-heated specimens. 4 IR-heated specimens had better high temperature softening resistance than RA-heated specimens. Author Contributions: Conceptualization, F.-Y.H. and Y.-L.C.; methodology, Y.-L.C.; validation, Y.-L.C.; formal analysis, Y.-L.C.; investigation, Y.-L.C.; resources, T.-S.L.; data curation, F.-Y.H.; writing—original draft preparation, Author Contributions: Conceptualization, F.-Y.H. and Y.-L.C.; methodology, Y.-L.C.; validation, Y.-L.C.; formal Y.-L.C.; writing—review and editing, Y.-L.C. and F.-Y.H.; visualization, Y.-L.C.; supervision, F.-Y.H. and T.-S.L.; analysis, Y.-L.C; investigation, Y.-L.C; resources, T.-S.L.; data curation, F.-Y.H.; writing—original draft preparation, Y.-L.C; writing—review and editing, Y.-L.C and F.-Y.H.; visualization, Y.-L.C; supervision, F.-Y.H.

Materials 2020, 13, 1177 12 of 12 project administration, F.-Y.H.; funding acquisition, F.-Y.H. All authors have read and agreed to the published version of the manuscript. Research idea about IR rapid heating process on this material given by T.-S.L. Funding: This research received no external funding. Acknowledgments: The authors are grateful to the Instrument Center of National Cheng Kung University and the Ministry of Science and Technology, Taiwan (107-2221-E-006-012-MY2), for their financial support. Conflicts of Interest: The authors declare no conflict of interest.

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