Development of Precipitation Hardening Parameters for High Strength Alloy AA 7068

materials

Article Development of Precipitation Hardening Parameters for High Strength Alloy AA 7068

Julia Osten 1, Benjamin Milkereit 1,2 , Michael Reich 1,*, Bin Yang 1, Armin Springer 3, Karina Nowak 4 and Olaf Kessler 1,2

1 Materials Science, Faculty of Mechanical Engineering and Marine Technology, University of Rostock, Justus-von-Liebig-Weg 2, 18059 Rostock, Germany; werkstoﬀ[email protected] (J.O.); [email protected] (B.M.); [email protected] (B.Y.); [email protected] (O.K.) 2 Competence Centre ◦CALOR, Department Life, Light & Matter, Faculty of Interdisciplinary Research, University of Rostock, Albert-Einstein-Str. 25, 18059 Rostock, Germany 3 Electron Microscopic Centre, University Medical Centre Rostock, Strempelstraße 14, 18057 Rostock, Germany; [email protected] 4 Fraunhofer Research Institution for Large Structures in Production Engineering IGP, Albert Einstein-Str. 30, 18059 Rostock, Germany; [email protected] * Correspondence: [email protected]; Tel.: +49-381-498-9490

Received: 19 December 2019; Accepted: 5 February 2020; Published: 19 February 2020

Abstract: The mechanical properties after age hardening heat treatment and the kinetics of related phase transformations of high strength AlZnMgCu alloy AA 7068 were investigated. The experimental work includes differential scanning calorimetry (DSC), differential fast scanning calorimetry (DFSC), sophisticated differential dilatometry (DIL), scanning electron microscopy (SEM), as well as hardness and tensile tests. For the kinetic analysis of quench induced precipitation by dilatometry new metrological methods and evaluation procedures were established. Using DSC, dissolution behaviour during heating to solution annealing temperature was investigated. These experiments allowed for identification of the appropriate temperature and duration for the solution heat treatment. Continuous cooling experiments in DSC, DFSC, and DIL determined the kinetics of quench induced precipitation. DSC and DIL revealed several overlapping precipitation reactions. The critical cooling rate for a complete supersaturation of the solid solution has been identified to be 600 to 800 K/s. At slightly subcritical cooling rates quench induced precipitation results in a direct hardening effect resulting in a technological critical cooling rate of about 100 K/s, i.e., the hardness after ageing reaches a saturation level for cooling rates faster than 100 K/s. Maximum yield strength of above 600 MPa and tensile strength of up to 650 MPa were attained.

Keywords: AlZnMgCu; AA 7068; aluminium alloys; precipitation hardening; dilatometry; diﬀerential fast scanning calorimetry (DFSC); diﬀerential scanning calorimetry (DSC)

1. Introduction AlZnMgCu alloys attain highest strength levels among aluminium alloys and are particularly used in aviation applications [1]. The strength is typically adjusted through precipitation hardening, respectively, age hardening [2,3]. The age hardening process comprises three major steps: Solution annealing, quenching, and ageing [1]. The solution heat treatment intends to dissolve relevant alloying element atoms and is setting the initial structural condition for the following cooling or quenching. Due to quenching the alloying element atoms shall be retained in solution resulting in a supersaturation. From this unstable state, nanometre sized precipitates form during the ﬁnal ageing treatment, which hinder the dislocation glide and thereby increase the strength of the alloy. To obtain the intended

Materials 2020, 13, 918; doi:10.3390/ma13040918 www.mdpi.com/journal/materials Materials 2020, 13, 918 2 of 14 properties, for instance high strength or sufficient ductility, a number of heat treatment parameters for the age hardening process must be carefully selected. This includes reasonable heating and cooling rates as well as temperatures and durations of the solution annealing and the ageing step. In terms of technological application the cooling step is critical as undesired quench induced precipitation during cooling might severely reduce the strength as well as the ductility after ageing [4–6]. Until the end of the last century, the process parameters of age hardening were predominantly acquired by empirical ex situ tests [1,7]. Nowadays the kinetics of quench induced precipitation can be investigated by in situ differential scanning calorimetry (DSC, e.g., [8]), as well as using in situ dilatometry (DIL) [9]. Conventional DSCs are limited to cooling rates of about 5 K/s. However, high strength aluminium alloys have critical cooling rates for a complete supersaturation of the solid solution of up to some hundreds of K/s. Basically DIL should allow the in situ cooling analysis of precipitation with cooling rates of up to about 100 K/s [9]. Thus, in this work, the DIL method is further developed in order to enlarge the cooling rate range and to enhance the sensitivity. Another sophisticated technique for the kinetic analysis of quench induced precipitation is the chip-sensor based differential fast scanning calorimetry (DFSC [10]) using a differential reheating method (DRM) [11]. This work intends to enable a sound selection of appropriate heat treatment parameters for the high alloyed aluminium alloy AA 7068 based on the kinetic analysis of the related solid-solid phase transformations. The major objective was adjusting a high strength to allow the usage of AA 7068 as a material for solid self-piercing rivets [12,13] applied for medium to high strength AlMgSi and AlCuMg sheet material [14]. Therefore, the kinetic behaviour of dissolution and precipitation reactions during heating, cooling, and annealing was investigated by in situ DSC as well as in situ differential DIL in a broad range of scanning rates and durations. In addition, the DFSC-DRM was used to reveal the critical cooling rate. Quench induced precipitations of slower cooling rates were investigated by SEM. The continuous cooling precipitation diagram of AA 7068 has been recorded. Moreover, the hardness and tensile properties of selected states including ageing/stretching after quenching were tested resulting in a complete set of suitable age hardening heat treatment parameters, which obtain high strength for AA 7068.

2. Materials and Methods The investigated aluminium wrought alloy AA 7068 (AlZn7.5Mg2.5Cu2) was supplied as a rolled bar stock with a diameter of 8 mm. The mass fractions of alloying elements are given in Table1 (analysed by optical emission spectrometry). This composition was chosen as it allows to adjust very high strength. It should be noted, that the Cu content is at the lowermost boundary, whereas the Zn content is slightly above the boundary. The initial heat treatment condition was T651, which is solution annealed, quenched, stress relieved by stretching, and artiﬁcially aged.

Table 1. Mass fraction of alloying elements of the investigated material in %.

Mass Fraction in % Aluminium Alloy Si Fe Cu Mn Mg Cr Zn Ti Zr AA 7068 0.052 0.076 1.60 0.008 2.96 0.013 8.43 0.022 0.084 AMS 4331 0.12 0.15 1.6–2.4 0.10 2.2–3.0 0.05 7.3–8.3 0.10 0.05–0.15 ≤ ≤ ≤ ≤ ≤

DSC was performed to analyse the kinetic dissolution and precipitation behaviour during heating of the initial conditions T651 and “as-quenched”. In particular, a temperature was determined which is suitable for solution annealing. Additionally, the kinetic behaviour of quench-induced precipitation was analysed via DSC. Two instruments were used, namely a Setaram 121 DSC (Setaram, Caluire-et-Cuire, France) and a Pyris Diamond DSC (PerkinElmer, Waltham, MA, USA). The DSC experiments were performed as described in references [15,16]. For each condition at least four samples and two baselines were measured. Materials 2020, 13, 918 3 of 14

In addition to DSC, the quench-induced precipitation was analysed by DIL, which basically allows detection of precipitation reaction in Al alloys [17–19], although their volume eﬀect is very small. Materials 2020, 13, x FOR PEER REVIEW 3 of 14 Beyond that it enables an extension of the accessible cooling rates [9]. In this work the dilatometric methoddilatometric has further method been has developedfurther been to developed enlarge the to assessable enlarge the cooling assessable rate cooling range and rate to range increase and theto sensitivityincrease the for sensitivity the detection for the of precipitationdetection of precipitation during cooling. during cooling. DilatometricDilatometric measurementsmeasurements were performed using a a Bähr Bähr 805A/D 805A/D instrument instrument with with a a linear linear variablevariable didifferentialﬀerential transducertransducer sensorsensor (LVDT),(LVDT), utilising silica push rods rods to to transmit transmit the the change change in in lengthlength/elongation,/elongation, see seeFigure Figure1 .1.

Figure 1. Schematic setup of the Bähr dilatometer in alpha-mode. Figure 1. Schematic setup of the Bähr dilatometer in alpha-mode. The specimens (hollow cylinders in 4 mm diameter, 10 mm length, and thickness 1 mm or 0.2 mm) The specimens (hollow cylinders in 4 mm diameter, 10 mm length, and thickness 1 mm or 0.2 mm) were inductively heated by an induction coil. An additional inner hollow coil is perforated and thus were inductively heated by an induction coil. An additional inner hollow coil is perforated and thus used for inert gas quenching. For cooling up to 10 K/s nitrogen and for more rapid cooling helium used for inert gas quenching. For cooling up to 10 K/s nitrogen and for more rapid cooling helium were used as inert and cooling gases. Hollow rods allow an internal cooling by slight gas volumes were used as inert and cooling gases. Hollow rods allow an internal cooling by slight gas volumes flow through hollow samples. The temperature of the sample was controlled by S-type thermocouples flow through hollow samples. The temperature of the sample was controlled by S-type spot-welded to the sample surface. Heating to solution annealing at 480 C for 10 min was carried out thermocouples spot-welded to the sample surface. Heating to solution annealing◦ at 480 °C for 10 min at 2 K/s. Cooling was realised at five different cooling rates, namely 0.1, 1, 10, 30, and 100 K/s. For each was carried out at 2 K/s. Cooling was realised at five different cooling rates, namely 0.1, 1, 10, 30, and experiment, a fresh sample was used. The same heat treatments were done on 99.9995% pure Al 100 K/s. For each experiment, a fresh sample was used. The same heat treatments were done on (Al5N5) alternating with AA 7068 samples. It is expected that the Al5N5 samples only show the thermal 99.9995% pure Al (Al5N5) alternating with AA 7068 samples. It is expected that the Al5N5 samples expansion of the aluminium matrix together with the push rods and can therefore be used for baseline only show the thermal expansion of the aluminium matrix together with the push rods and can measurement. The baseline is new for dilatometry of Al alloys and allows to correct the signal for the therefore be used for baseline measurement. The baseline is new for dilatometry of Al alloys and basicallows expansion to correct of the the signal aluminium for the matrix basic expansion lattice. Beyond of the that, aluminium it contains matrix all additional lattice. Beyond influences that, it of thecontains device all on theadditional dilatometry influences result, of e.g., the additional device on length the dilatometry changes of the result, silica e.g., push additional rods. The length strain ε ε ischanges evaluated of the dividing silica push the length rods. changeThe strain∆L ε by is theevaluated initial sampledividing length the lengthL0, =change∆L/L0 Δ.L For by a the reduction initial and a smoothing of raw data a stepwise linear description with 500 points between 50 and 478 C has sample length L0, ε = ΔL/L0. For a reduction and a smoothing of raw data a stepwise linear description◦ been used. The pure Al baseline strain εBL was subtracted from the sample strain εS (see Figure2a) with 500 points between 50 and 478 °C has been used. The pure Al baseline strain εBL was subtracted and the strain difference has been derived with respect to temperature (d∆ε/dT). The derivative is from the sample strain ε (see Figure 2a) and the strain difference has been derived with respect to further smoothed by theSSavitzky–Golay [20] filter method of the software OriginPro 2018 which performstemperature a local (dΔε polynomial/dT). The derivative regression is to further determine smoothed the smoothed by the Savitzky–Golay value for each temperature[20] filter method point. Theof the resulting software curve OriginPro can finally 2018 be usedwhich for performs evaluation, a local e.g., ofpolynomial precipitation regression start and to end determine temperatures the smoothed value for each temperature point. The resulting curve can finally be used for evaluation, (Figure2b). As a consequence of the baseline subtraction, the zero-level of the derivative curve should e.g., of precipitation start and end temperatures (Figure 2b). As a consequence of the baseline now be the reference value (being zero), allowing easy determination of the characteristic temperatures. subtraction, the zero-level of the derivative curve should now be the reference value (being zero), For each condition at least three samples and two baselines were measured by DIL. The dilatometric allowing easy determination of the characteristic temperatures. For each condition at least three experiments in general showed a very good reproducibility. Hence, one representative curve of three samples and two baselines were measured by DIL. The dilatometric experiments in general showed cooling tests was used for further discussion. a very good reproducibility. Hence, one representative curve of three cooling tests was used for As the in situ detection of precipitation during cooling by dilatometry is limited to cooling rates further discussion. of 100 K/s, even faster cooling experiments were performed by Differential Fast Scanning Calorimetry (DFSC) applying the Differential Reheating Method (DRM) introduced in [11,21]. In addition, hardness

Materials 2020, 13, 918 4 of 14 testing (HV1) in the as-quenched state as well as in the artiﬁcially aged state were performed. Tensile testing was performed using round tensile specimen with a screw head, a diameter of 5 mm and a gauge length of 25 mm, in accordance with DIN 50125:2016 form B following the test method B out of DINMaterials EN 2020 ISO, 13 6892-1:2017., x FOR PEER REVIEW (Testing machine Z50, ZwickRoell GmbH & Co.KG, Ulm, Germany).4 of 14 /K

0.00 -2 0.0020 0.0 Cooling with 0,1 K/s cooling 0.1 K/s -0.02 0.0018 red: Savitzky-Golay smoothing -0.2 -0.04 0.0016 -0.4 -0.06 0.0014 -0.6 -0.08 0.0012 0.0010 -0.8 -0.10 Al5N5 -0.12 0.0008 strain in% -1.0 7068 -0.14 0.0006 -1.2 0.0004 -0.16 strain difference in % in difference strain 0.0002 -1.4 -0.18 0.0000 -1.6 -0.20 0 100 200 300 400 500 -0.0002 0 100 200 300 400 500

(a) temperature in °C differencein 10 strain of derivative temperature in °C (b)

FigureFigure 2. 2.Evaluation Evaluation procedureprocedure dilatometricdilatometric analysis: analysis: Alloy Alloy 7068, 7068, solu solutiontion annealing annealing 480 480 °C ◦10min,C 10 min, andand cooling cooling raterate 0.10.1 KK/s./s. ( a)) raw raw strain strain data data of of sample sample and and baseline baseline measurement measurement as well as well as strain as strain didifferenceﬀerence of of these these twotwo andand ((b)) derivation of of strain strain difference. diﬀerence.

TheAs the microstructural in situ detection changes of precipitation caused byduring quench-induced cooling by dilatometry precipitation is limited were to investigatedcooling rates by scanningof 100 K/s, electron even faster microscopy cooling (SEM).experiments Samples were for pe SEMrformed and by energy Differential dispersive Fast X-rayScanning spectroscopy Calorimetry (EDS) analysis(DFSC) wereapplying cold embeddedthe Differential in epoxy Reheating resin andMeth preparedod (DRM) by standardintroduced grinding in [11,21]. and In polishing addition, with water-free,hardness ethanol-basedtesting (HV1) lubricantsin the as-quenched [22]. For final state polishing, as well aas 0.05 in µthem oxideartificially polishing aged suspension state were was used.performed. The embedded Tensile testing and polished was performed samples were using mounted round tensile on a SEM specimen carrier with adhesivea screw head, conductive a carbondiameter tape of (Co. 5 mm PLANO, and a gauge Wetzlar, length Germany). of 25 mm, Additionally, in accordance the with SEM DIN samples 50125:2016 were form coated B following with carbon (Leicathe test EM method SCD 500, B out Co. of Leica,DIN EN Wetzlar, ISO 6892-1:2017. Germany). (Testing SEM samples machine were Z50, analysedZwickRoell by GmbH field emission & Co.KG, SEM (MERLINUlm, Germany).® VP Compact, Co. Zeiss, Oberkochen, Germany) equipped with an EDS detector (XFlash The microstructural changes caused by quench-induced precipitation were investigated by 6130, Co. Bruker, Berlin, Germany). For SEM imaging, an Everhart–Thornley-type high-efficiency scanning electron microscopy (SEM). Samples for SEM and energy dispersive X-ray spectroscopy secondary electron detector (Co. Zeiss, Oberkochen, Germany) was used, applying an accelerating (EDS) analysis were cold embedded in epoxy resin and prepared by standard grinding and polishing voltage of 5 kV. The particle size was determined with the Imagic ims Client software. Representative with water-free, ethanol-based lubricants [22]. For final polishing, a 0.05 µm oxide polishing areas of the samples were analysed and mapped for elemental distribution on the basis of the EDS suspension was used. The embedded and polished samples were mounted on a SEM carrier with spectraadhesive obtained conductive at acceleration carbon tape voltages(Co. PLANO, of up Wetzlar, to 20 kV Germany). and evaluated Additionally, by the QUANTAXthe SEM samples ESPRIT Microanalysiswere coated with software carbon (version (Leica 2.0,EM Co. SCD Bruker, 500, Co. Berlin, Leica, Germany). Wetzlar, Germany). SEM samples were analysedThe di byfferent field emission analysis techniquesSEM (MERLIN used® VP in Compact, correlation Co. with Zeiss, the Oberkochen, particular applied Germany) heat equipped treatments arewithMaterials illustrated an 2020EDS, 13 indetector, x FigureFOR PEER (XFlash3. REVIEW 6130, Co. Bruker, Berlin, Germany). For SEM imaging, an Everhart–5 of 14 Thornley-type high-efficiency secondary electron detector (Co. Zeiss, Oberkochen, Germany) was used, applying an accelerating voltage of 5 kV. The particle size was determined with the Imagic ims Client software. Representative areas of the samples were analysed and mapped for elemental distribution on the basis of the EDS spectra obtained at acceleration voltages of up to 20 kV and evaluated by the QUANTAX ESPRIT Microanalysis software (version 2.0, Co. Bruker, Berlin, Germany). The different analysis techniques used in correlation with the particular applied heat treatments are illustrated in Figure 3.

FigureFigure 3. 3.Schematic Schematic temperature-timetemperature-time profile. proﬁle.

3. Results and Discussion

3.1. DSC Heating Curves: Determination of an Appropriate Solution Annealing Temperature The sequence of endothermic dissolution and exothermic precipitation reactions during heating of alloy 7068 has been analysed for various heating rates. Previous DSC melting tests with 0.3 K/s revealed the solidus temperature to be 491 °C [23]. Thus, the temperature range analysed extends from 20 °C (sometimes from −70 °C) up to 485 °C, keeping the sample in the solid state. For improved readability the DSC curves shown in Figure 4 are shifted, showing the curve with the lowest heating rate on top. Endothermic changes are plotted upwards. The dissolution behaviour of two different initial conditions, namely T651 (Figure 4a) and as-quenched (Figure 4b), was investigated.

(a) initial condition T651 (b) solution-annealed and water-quenched -1 -1 K K -1 -1 0.01 K/s 0.01 K/s endo 0.1 Jg 0.1 Jg

0.1 K/s 0.1 K/s excess specific excessheat capacity specific excessheat capacity specific 1 K/s 1 K/s endo heating curve

-100 0 100 200 300 400 500 -100 0 100 200 300 400 500 temperature in °C temperature in °C Figure 4. Differential scanning calorimetry (DSC) heating curves of alloy 7068: (a) initial condition T651 and (b) solution-annealed and water-quenched.

Materials 2020, 13, x FOR PEER REVIEW 5 of 14

Materials 2020, 13, 918 5 of 14 Figure 3. Schematic temperature-time profile.

3.3. Results and Discussion

3.1.3.1. DSC Heating Curves: Determination of an Appropriate Solution Annealing Temperature TheThe sequencesequence ofof endothermic dissolution andand exothermicexothermic precipitation reactions duringduring heatingheating ofof alloyalloy 70687068 hashas beenbeen analysedanalysed forfor variousvarious heatingheating rates.rates. Previous DSC melting tests with 0.3 KK/s/s revealedrevealed thethe solidussolidus temperaturetemperature toto bebe 491491 ◦°CC[ [2233].]. Thus,Thus, thethe temperaturetemperature rangerange analysedanalysed extendsextends fromfrom 2020 ◦°CC (sometimes fromfrom −70 ◦°C)C) up to 485 °C,◦C, keeping the sample in the solid state. For improved − readabilityreadability thethe DSCDSC curvescurves shownshown inin FigureFigure4 4 are are shifted, shifted, showing showing the the curve curve with with the the lowest lowest heating heating raterate onon top.top. EndothermicEndothermic changes are plottedplotted upwards.upwards. The dissolution behaviour of two didifferentﬀerent initialinitial conditions,conditions, namelynamely T651T651 (Figure(Figure4 4a)a) and and as-quenched as-quenched (Figure (Figure4b), 4b), was was investigated. investigated.

(a) initial condition T651 (b) solution-annealed and water-quenched -1 -1 K K -1 -1 0.01 K/s 0.01 K/s endo 0.1 Jg 0.1 Jg

0.1 K/s 0.1 K/s excess specific excessheat capacity specific excessheat capacity specific 1 K/s 1 K/s endo heating curve

-100 0 100 200 300 400 500 -100 0 100 200 300 400 500 temperature in °C temperature in °C FigureFigure 4.4. DiDifferentialﬀerential scanningscanning calorimetrycalorimetry (DSC)(DSC) heatingheating curvescurves ofof alloyalloy 7068:7068: ((aa)) initialinitial conditioncondition T651T651 andand ((bb)) solution-annealedsolution-annealed andand water-quenched.water-quenched.

The heating DSC curves reveal a complex dissolution and precipitation behaviour with alternating endo and exothermic regions. The single reactions severely overlap, making a deconvolution of single reactions difficult. However, between the two different initial conditions substantial differences are seen particularly within the temperature region below 300 ◦C. The T651 state mainly shows dissolution reactions of the initial aged state (precursors of η-Zn2Mg, S-Al2CuMg, etc.). The quenched state shows alternating precipitation and dissolution reactions according to the precipitation sequence of AlZnMgCu alloys (also precursors of η-Zn2Mg, S-Al2CuMg, etc.). Above about 300 ◦C dissolution processes of more stable precipitates (η-Zn2Mg, S-Al2CuMg, etc.) dominate in both cases. A suitable solution annealing temperature can therefore be determined from the slow warming DSC curves of 0.01 K/s, since the DSC signal of the main solution process drops abruptly towards zero at high temperatures of 472 ◦C. This means, the dissolution process is finished, and the heating rate specific solvus temperature is about 472 ◦C. It can therefore be assumed that an equilibrium solvus temperature Materials 2020, 13, x FOR PEER REVIEW 6 of 14

The heating DSC curves reveal a complex dissolution and precipitation behaviour with alternating endo and exothermic regions. The single reactions severely overlap, making a deconvolution of single reactions difficult. However, between the two different initial conditions substantial differences are seen particularly within the temperature region below 300 °C. The T651 state mainly shows dissolution reactions of the initial aged state (precursors of η-Zn2Mg, S-Al2CuMg, etc.). The quenched state shows alternating precipitation and dissolution reactions according to the precipitation sequence of AlZnMgCu alloys (also precursors of η-Zn2Mg, S-Al2CuMg, etc.). Above about 300 °C dissolution processes of more stable precipitates (η-Zn2Mg, S-Al2CuMg, etc.) dominate in both cases. A suitable solution annealing temperature can therefore be determined from the slow warming DSC curves of 0.01 K/s, since the DSC signal of the main solution process drops abruptly towards zero at high temperatures of 472 °C. This means, the dissolution process is finished, and the heatingMaterials 2020rate, 13 specific, 918 solvus temperature is about 472 °C. It can therefore be assumed that6 ofan 14 equilibrium solvus temperature is slightly below 472 °C. In the next step the influence of soaking within the temperature range between solvus and solidus temperature was assessed by subsequent is slightly below 472 ◦C. In the next step the inﬂuence of soaking within the temperature range between DSCsolvus cooling and solidus curves. temperature was assessed by subsequent DSC cooling curves.

3.2.3.2. DSC DSC Cooling Cooling Curves: Curves: Assessing Assessing the the Influence Inﬂuence of of Solution Solution Annealing Annealing Temperature Temperature and Duration ToTo allow a sound selection of the solution annealing temperature and duration, DSC cooling curvescurves after after solution solution treatment treatment at at 470, 470, 475, 475, and and 480 480 °C◦ wereC were compared, compared, see see Figure Figure 5a.5 Ina. general, In general, the DSCthe DSC cooling cooling curves curves are arevery very similar. similar. They They show show at atleast least two two overlapping overlapping precipitation precipitation reactions (details(details are givengiven inin ChapterChapter 3.3.). 3.3.). However, However, in in detail detail it it can can be be seen seen that that after after solution solution treatment treatment at 470 at 470◦C for°C 20for min,20 min, an instantaneousan instantaneous onset onset of precipitationof precipitation with with onset onset of coolingof coolin occurs.g occurs. This This indicates indicates an anincomplete incomplete dissolution dissolution of the of mainthe main alloying alloying elements, elements, acting acting as nuclei as nuclei for precipitation for precipitation [24]. To [24]. ensure To ensurea complete a complete solution, solution, the temperature the temperature for the solution for the solution treatment treatment was set atwas 475 setC( at 475480 °CC). (−480 In the °C). next In ◦ − ◦ thestep next the step soaking the soaking duration duration was varied was between varied betw 10 andeen 6010 min,and 60 see min, Figure see5 Figureb. The 5b. DSC The cooling DSC cooling curves curvesfrom any from soaking any soaking duration duration were highly were similar. highly 30similar. min at 30 475 min◦C at (10 475 min °C at (10 480 min◦C) at were 480 deﬁned °C) were as definedsolution as treatment solution duration.treatment duration.

(a) 0.5 (b) 0.5 470°C 20´ 475°C 10´ 475°C 30´ 475°C 30´ 0.4 480°C 20´ 0.4 475°C 60´ -1 -1 K 0.3 K 0.3 -1 -1 inJg in Jg p

p 0.2 0.2

0.1 0.1 excess c excess excess c excess exo exo 0.0 0.0 cooling curve -0.1 -0.1 100 200 300 400 500 100 200 300 400 500 temperature in °C temperature in °C

Figure 5. DSC cooling curves of alloy 7068 (cooling rate 0.01 K/s): (a) different solution annealing Figure 5. DSC cooling curves of alloy 7068 (cooling rate 0.01 K/s): (a) different solution annealing temperatures and (b) different solution annealing times. temperatures and (b) different solution annealing times. 3.3. Kinetic Behaviour of Quench Induced Precipitation Analysed by DSC, DIL, DFSC, and Hardness Testing 3.3. Kinetic Behaviour of Quench Induced Precipitation Analysed by DSC, DIL, DFSC, and Hardness Testing The kinetic behaviour of quench-induced precipitation during cooling with various rates from the solutionThe kinetic annealing behaviour temperature of quench-induced has been analysed precipitat byion DSC during (Figure cooling6a) and with dilatometry various rates (Figure from6 b).the solutionThe cooling annealing curves temperature in Figure6 arehas shiftedbeen analysed and the by slowest DSC (Figure rates are6a) shownand dilatometry on top. Be (Figure aware 6b). of The the coolingdifferent curves rates usedin Figure in DSC 6 are and shifted dilatometry. and the slowest rates are shown on top. Be aware of the different rates Theused DSC in DSC cooling and curvesdilatometry. show exothermic precipitation when the signal exceeds the zero level, since only exothermic precipitation reactions can occur during cooling. DSC cooling (Figure6a) reveals a relatively sharp onset after slight undercooling (below the solvus) followed by a very broad reaction shoulder, similar to other highly concentrated AlZnMgCu alloys [8,25]. This undercooling is in the range of few K at the slow rate of 0.01 K/s and increases with increasing cooling rate. At the rate of 3 K/s the onset of precipitation is seen at about 440 ◦C and thus the undercooling is about 30 K. The second broad exothermic precipitation during slow cooling ranges from about 465 to 180 ◦C. This indicates a severe overlap of multiple exothermic precipitation reactions. With an increasing cooling rate, the peak area decreases, i.e., in total the precipitation reactions are supressed. The critical cooling rate cannot be quantified by conventional DSC measurement, since at the fastest rate of 3 K/s a precipitation enthalpy of about 20 J/g is still measured. Materials 2020, 13, 918 7 of 14 Materials 2020, 13, x FOR PEER REVIEW 7 of 14

(a) cooling after SHT 475 ° C 30 min (b) cooling after SHT 480 ° C 10 min

0.003 K/s

-5

0.01 K/s 1x10

-1 0.1 K/s

-1

0.03 K/s

0.5 Jg

-1 1 K/s 0.1 K/s

decrease in length DL- 10 K/s 0.3 K/s

increase in length DL+ 1 K/s difference deviation in K 100 K/s

excess specific heat capacity exo 3 K/s

0 100 200 300 400 500 0 100 200 300 400 500 temperature in ° C temperature in ° C Figure 6.6.Cooling Cooling curves curves of alloy of alloy 7068 7068after solution after solution annealing: annealing: (a) DSC and (a) (DSCb) di ffanderential (b) dilatometry.differential dilatometry. Differential dilatometry has been found to be useful for studying higher cooling rates of up to 100 KThe/s and DSC thus cooling can extendcurves theshow bandwidth exothermic of precipitation analysed cooling when rates. the signal Figure exceeds6b compares the zero cooling level, sincedilatometry only exothermic curves in the precipitation range of 0.1 to reactions 100 K/s. Acan relative occur shortening during cooling. of the sample DSC cooling is plotted (Figure upwards. 6a) revealsIt has been a relatively described sharp earlier onset [9] thatafter certain slight precipitationundercooling reactions (below the cause solvus) an increase followed in theby a total very volume broad reactionand thus shoulder, an elongation similar of theto other sample, highly while concentrated precipitation AlZnMgCu of a different alloys phase [8,25]. might This cause undercooling a shrinkage is inof the volumerange of andfew thereforeK at the slow a shortening rate of 0.01 of the K/s sample. and increases That means, with increasing opposing volumecooling rate. changes At the caused rate ofby 3 precipitation K/s the onset of of di precipitationfferent phases is mightseen at superimpose about 440 °C each and otherthus the and undercooling a signal value is ofabout zero 30 does K. The not secondnecessarily broad mean exothermic that no reaction precipitation is occurring. during Therefore slow cooling cooling range dilatometrys from about bears 465 the to same 180 issue°C. This like indicatesheating DSC a severe in terms overlap of its of signal multiple interpretation exothermic [16 precipitation]. reactions. With an increasing cooling rate, Forthe peak cooling area of decreases, aluminium i.e. alloy, in total 7068, the dilatometry precipitation reveals reactions very are similar supressed. results The compared critical cooling to DSC. rateFor instance, cannot be comparing quantified the by cooling conventional curves of DSC 0.1 K measurement,/s, reveals a sharp since onset at ofthe precipitation fastest rate resulting of 3 K/s in a precipitationa peak of about enthalpy 435 ◦C inof both about cases. 20 J/g Also, is still for measured. the inflection towards the shoulder 410 ◦C are revealed as characteristicDifferential temperature dilatometry in both has cases.been found For 1 Kto/s be the useful general for curve studying shapes higher are similar, cooling too. rates In addition,of up to 100dilatometry K/s and helpsthus can to discuss extend temperature the bandwidth regions of analysed of overlapping cooling reactions. rates. Figure The latter6b compares is observed cooling at a dilatometrycooling rate of curves 10 K/ s in where the range the dilatometer of 0.1 to 100 reveals K/s. a A first relative precipitation shortening region of between the sample about is 440 plotted and upwards.320 ◦C causing It has a been shortening described of the earlier sample. [9] that This certain is followed precipitation by another reactions reaction cause region an betweenincrease in about the total320 to volume 150 ◦C, and causing thus another an elongation elongation. of the sample, while precipitation of a different phase might causeAs a shrinkage a very positive of the aspect volume cooling and therefore dilatometry a shortening helps to substantiallyof the sample. expand That means, the cooling opposing rate volumespectrum changes up to 100caused K/s. by For precipitatio high alloyedn of AlZnMgCu different phases alloy AAmight 7068 superimpose even this fast each cooling other rateand ofa 100signal K/ s value reveals of a zeroprecipitation does not signal necessarily between mean about that 380 and no reaction 220 ◦C. This is occurring. finding shows Therefore that the cooling upper dilatometrycritical cooling bears rate the is abovesame issue 100 K like/s. Fasterheating cooling DSC in of terms hollow of samplesits signal may interpretation be possible [16] in quenching. dilatometry.For cooling However, of aluminium the decreasing alloy 7068, sti dilatometryffness with reveals deceasing very wall similar thickness results complicates compared to length DSC. Formeasurements. instance, comparing Thus, fast the scanning cooling calorimetrycurves of 0.1 (DFSC) K/s, reveals and hardness a sharp onset testing of afterprecipitation cooling asresulting well as inafter a peak cooling of andabout artificial 435 °C ageing in both (130 cases.◦C forAlso, 16 h)for are the used inflection to identify towards the upper the shoulder critical cooling 410 °C rate. are revealed as characteristic temperature in both cases. For 1 K/s the general curve shapes are similar, too. In addition, dilatometry helps to discuss temperature regions of overlapping reactions. The latter

Materials 2020, 13, x FOR PEER REVIEW 8 of 14

is observed at a cooling rate of 10 K/s where the dilatometer reveals a first precipitation region between about 440 and 320 °C causing a shortening of the sample. This is followed by another reaction region between about 320 to 150 °C, causing another elongation. As a very positive aspect cooling dilatometry helps to substantially expand the cooling rate spectrum up to 100 K/s. For high alloyed AlZnMgCu alloy AA 7068 even this fast cooling rate of 100 K/s reveals a precipitation signal between about 380 and 220 °C. This finding shows that the upper critical cooling rate is above 100 K/s. Faster cooling of hollow samples may be possible in quenching dilatometry. However, the decreasing stiffness with deceasing wall thickness complicates length measurements. Thus, fast scanning calorimetry (DFSC) and hardness testing after cooling as well as after cooling and Materials 2020, 13, 918 8 of 14 artificial ageing (130 °C for 16 h) are used to identify the upper critical cooling rate. Figure 7 shows the results obtained by the DFSC-DRM method. Three individual samples were usedFigure to determine7 shows the the precipitation results obtained enthalpy by the during DFSC-DRM cooling method. at various Three rates. individual One issue samples of the were used usedmethod to determine is, that the thesamples precipitation are very enthalpysmall with during only about cooling 100 at by various 100 by rates. 40 µm One3. Currently issue of no the device used methodis available is, that which the samples allows are to very determine small with thes onlye samples about 100’ mass. by 100 Thus, by 40 theµm 3 normalization. Currently no device of the iscalorimetric available which signal, allows which to determine is normally these carried samples’ out mass. via Thus, the sample the normalization mass, has of to the be calorimetric based on signal,assumptions. which is In normally this work, carried we were out viaable the to sample measure mass, the hascooling to be rate based of 3 on K/s assumptions. in both, DSC In and this DFSC. work, weWe were therefore able to used measure the value the cooling of the ratespecific of 3 precipitation K/s in both, DSC enthalpy and DFSC. obtained We therefore by DSC (integral used the valueof the ofnormali the specificsed DSC precipitation curve, average enthalpy value obtained = 21 byJ/g) DSC to (integral calibrate of the the specific normalised precipitation DSC curve, enthalpies average valuemeasured= 21 by J/g) DFSC. to calibrate This is the the specific raw DFSC precipitation signal of each enthalpies sample measured in µJ as shown by DFSC. in Figure This 7 isa thedivided raw DFSCby the signal specific of eachprecipitation sample in enthalpyµJ as shown of 21 in FigureJ/g obtained7a divided by DSC by the to specificestimate precipitation the individual enthalpy DFSC ofsample 21 J/g obtainedmasses. With by DSC these to estimate estimated the masses individual and DFSC the cooling sample masses.rates applied, With these the DFSC estimated signals masses are andnormalised the cooling and ratesthe values applied, plotted the DFSC in Figure signals 7b are normalisedobtained. Figure and the7 shows values substantial plotted in deviations Figure7b arebetween obtained. the Figure three 7 samples. shows substantial Those deviations deviations are between related the to three the tiny samples. sample Those size deviations and probably are relatedoriginate to thefrom tiny slight sample variation size and in probablythe chemical originate composition from slight (by variationlocal segregations) in the chemical or from composition variations (byin t localhe density segregations) of nucleation or from sites variations (dispersoid in the distributiondensity of nucleation and particularly sites (dispersoid the amount distribution of grain andboundary particularly surface the within amount the of samples). grain boundary Nevertheless, surface all within samples the show samples). a similar Nevertheless, trend and allthe samples specific showprecipitation a similar enthalpy trend and is the dropping specific precipitation towards zero enthalpy with increasi is droppingng cooling towards rate. zero The with upper increasing critical coolingcooling rate.rate Thewas upperfound criticalto be about cooling 600 rate K/s was to 800 found K/s, to which be about allows 600 Ka /completes to 800 K supersaturation/s, which allows of a completethe alloy. supersaturation This is one of the of highest the alloy. critical This iscooling one of rates the highest ever found critical for cooling an aluminium rates ever alloy, found compare for an aluminium[11,21,25,26]. alloy, compare [11,21,25,26].

(a) 8 25 sample 1 (b) sample 1 7 sample 2 J/g sample 2 20 6 sample 3 sample 3 5 15 4 critical cooling rate critical cooling rate » 600–800 K/s 3 » 600–800 K/s 10 2 1 5 0 0

absolute enthalpy change in µ J -1 specific enthalpy change in 1000 100 10 1 1000 100 10 1 cooling rate in K/s cooling rate in K/s FigureFigure 7. 7. DiDifferentialﬀerential fast fast scanning scanning calorimetry calorimetry(DFSC) (DFSC)results results of of three three samples samples 7068: 7068: (a (a)) absolute absolute enthalpyenthalpy changechange andand ((b)) speciﬁcspecific enthalpy change. change.

ToTo compare compare the the precipitation precipitation enthalpies enthalpies from from DSC DSC and DFSC and DFSC with hardness with hardness values of values the distinct of the coolingdistinct rates cooling Figure rates8 is Figure plotted. 8 is Average, plotted. minimum, Average, minimu and maximumm, and valuesmaximum are shown values for are the shown enthalpies. for the Theenthalpies. averaged The hardness averaged values hardness are plotted values with are theirplotted standard with their deviations standar fromd deviations six single from indentations. six single Theindentations. cooling rate The axis cooling is scaled rate logarithmic axis is scaled with logarithmic high cooling with rates high on thecooling leftin rates order on to the allow left alignmentin order to withallow the alignment time axis withof the the following time axis continuous of the following cooling precipitation continuous coolingdiagram precipitation in Figure9. diagram in FigureThe 9. specific precipitation enthalpies determined for slow cooling rates are quite high (up to 40 J/g), which is reasonable for this high concentrated alloy. With increasing cooling rate, the total precipitation enthalpy is decreasing as precipitation is suppressed. This behaviour is common for age hardening aluminium alloys [25]. As indicated above the upper critical cooling rate, which is the slowest cooling rate at which any quench-induced precipitation is supressed completely, is about 600 to 800 K/s. This is the highest value ever reported for an aluminium wrought alloy, which is reasonable, as AA 7068 is a quite high concentrated alloy. However, the hardness after ageing indicates a technical critical cooling rate to be just about 100 K/s. This means, that after artificial ageing (130 ◦C for 16 h) there is no further increase in hardness with an increasing cooling rate above 100 K/s. This can be attributed to a certain direct hardening effect of quench induced precipitation for cooling rates above 100 K/s. This indeed is confirmed by the as-quenched hardness, which first rises with increasing cooling rate, then shows Materials 2020, 13, 918 9 of 14

a maximum/plateau for cooling rates between about 30 to 300 K/s. For even faster cooling rates the as-quenched hardness finally drops again when approaching the physical upper critical cooling rate. A direct hardening contribution of quench-induced precipitation has already been shown for alloy AA7150 [27]. In detail for AA7150 very similar hardness values in similar cooling rate regions were achieved. For AA7150 the direct hardening effect of quench induced precipitation was confirmed by Materialstensile 2020 tests, 13, x in FOR the PEER as-quenched REVIEW condition and is primarily referred to the quench-induced precipitation9 of 14 Materialsof thin 2020 Zn, 13 and, x FOR Cu PEER rich Y-phaseREVIEW platelets [27,28]. 9 of 14 250 SHT DSC 475° C 30 min 50 250 SHTSHT DFSCDSC 475° 475° C C 30 10 min min 50 SHT DFSC 475° C 10HV min after 130°C 16h 200 HV after 130°C 16h 40 200 40 water

quenching J/gin water Dh from DSC 150 30 h quenching J/gin

Dh from DSC 150 HV as quenched 30 h

D HV as quenched 100 20 hardness HV1 hardness 100 20

hardness HV1 hardness (ice) water 50 10 enthalpy change quenching(ice) water

enthalpy change 50 quenching 10 Dh from DFSC 0 Dh from DFSC 0 0104 103 102 101 100 10-1 10-2 10-3 10-40 4 3 2 1 0 -1 -2 -3 -4 10 10 10 cooling10 10rate in10 K/s 10 10 10 cooling rate in K/s Figure 8. Enthalpy change of alloy 7068 from quench induced precipitation as well as hardness in as- quenchedFigureFigure 8. Entand 8. halpyEnthalpy aged change condition. change of alloy of alloy 7068 7068 from from quench quench induced induced precipitation precipitation asas wellwell asas hardness hardness in in as- quenchedas-quenched and aged and agedcondition. condition. SHT DFSC & DIL: 480° C 10 min SHT DSC: 475° C 30 min 500 SHT DFSC & DIL: 480° C 10 min SHT DSC: 475° C 30 min 500 a 400 a 400 a a + X 300 a a + X 300 200

temperature in °temperature C 200 CCR temperature in °temperature C 100 CCR 100 0 4 12 21 37 38 39 2010 1984 19412 13221 9937 7338 5339 0 201 198 194 132 99 73 53 00.1 1 10 100 1000 10000 100000 0.1 1 10 100 1000 10000 100000time in s HV1 after ageing 130 ° C 16 h time in s HV1 after ageing 130 ° C 16 h Dhtotal in J/g

Dhtotal in J/g Figure 9. Continuous Cooling Precipitation Diagram of AA 7068 from DSC, dilatometry (DIL), and Figure 9. Continuous Cooling Precipitation Diagram of AA 7068 from DSC, dilatometry (DIL), and DFSC. Within the reaction α α + X, the X comprises any quench induced phase. DFSC.Figure Within 9. Continuous the reaction Cooling α → Precipitationα + X, the X comprises Diagram ofany AA quench 7068 frominduced DSC, phase. dilatometry (DIL), and DFSC.All Within cooling the results reaction (DSC, α  DIL, α + DFSC,X, the X and comprises hardness) any are quench summarised induced in phase. the continuous cooling precipitationThe specific (CCP)precipitation diagram enthalpies of aluminium determined alloy 7068 for in slow Figure cooling9, being rates a powerful are quite tool high for practical(up to 40 J/g), whichheatThe is reasonable treatmentspecific precipitation as for well this as high for enthalpies heat concentrated treatment determined simulation alloy. With for [29 slowincreasing]. cooling cooling rates arerate, quite the totalhigh precipita(up to 40 tionJ/g), enthalpywhich is reasonableis decreasing for thisas precipitation high concentrated is suppressed. alloy. With This increasing behaviour cooling is common rate, the for total age precipita hardeningtion aluminiumenthalpy is alloysdecreasing [25]. As as indicatedprecipitation above is suppressed.the upper critical This coolingbehaviour rate, is whichcommon is the for slowest age hardening cooling ratealuminium at which alloys any quench [25]. As-induced indicated precipitation above the upper is supressed critical coolingcompletely, rate, iswhich about is 600the toslowest 800 K/s. cooling This israte the at highest which anyvalue quench ever reported-induced for precipitation an aluminium is supressed wrought completely, alloy, which is is about reasonable, 600 to 800 as AAK/s. 7068 This isis athe quite highest high value concentrated ever reported alloy. forHowever, an aluminium the hardness wrought after alloy, ageing which indicates is reasonable, a technical as AA critical 7068 cois olinga quite rate high to beconcentrated just about 100 alloy. K/s. However, This means, the thathardness after artificialafter ageing ageing indicates (130 °C a fortechnical 16 h) there critical is noco olingfurther rate increase to be just in hardnessabout 100 with K/s. anThis increasing means, that cooling after rate artificial above ageing 100 K/s. (130 This °C can for be16 attributedh) there is tono a further certain increase direct hardening in hardness effect with of an quench increasing induced cooling precipitation rate above for 100 cooling K/s. This rates can above be attributed 100 K/s. Thisto a certainindeed directis confirmed hardening by the effect as-quenched of quench hardness, induced precipitationwhich first rises for withcooling increasing rates above cooling 100 rate, K/s. thenThis showsindeed a is maximum/plateau confirmed by the foras-quenched cooling rates hardness, between which about first 30 torises 300 with K/s. increasingFor even faster cooling cooling rate, rthenates showsthe as -aquenched maximum/plateau hardness forfinally cooling drops rates again between when about approaching 30 to 300 theK/s. physical For even upper faster coolingcritical coolingrates the rate. as- quenched A direct hardeninghardness finally contribution drops again of quench when- inducedapproaching precipitation the physical has upper already critical been cooling rate. A direct hardening contribution of quench-induced precipitation has already been

Materials 2020, 13, x FOR PEER REVIEW 10 of 14

shown for alloy AA7150 [27]. In detail for AA7150 very similar hardness values in similar cooling rate regions were achieved. For AA7150 the direct hardening effect of quench induced precipitation was confirmed by tensile tests in the as-quenched condition and is primarily referred to the quench- induced precipitation of thin Zn and Cu rich Y-phase platelets [27,28]. All cooling results (DSC, DIL, DFSC, and hardness) are summarised in the continuous cooling precipitation (CCP) diagram of aluminium alloy 7068 in Figure 9, being a powerful tool for practical

Materialsheat treatment2020, 13, 918as well as for heat treatment simulation [29]. 10 of 14

3.4. Microstructure of Different Cooled Samples through Scanning Electron Microscopy (SEM) 3.4. Microstructure of Diﬀerent Cooled Samples through Scanning Electron Microscopy (SEM) The microstructures of alloy 7068 after various cooling rates have been analysed by SEM- secondaryThe microstructures electron images of and alloy selected 7068 after images various are cooling shownrates in Figure have 10 been for analysed cooling rates by SEM-secondary ranging from electron0.01 to 10 images K/s. Next and selectedto the aluminium images are matrix shown which in Figure appears 10 for dark, cooling there rates are ranging two main from types 0.01 toof 10precipitates K/s. Next appearing to the aluminium brighter: matrix Precipitates which on appears grain dark,boundaries there areand two precipitates main types inside of precipitates the grains. appearingParticularly brighter: for the Precipitatesslower rates, on the grain grain boundaries boundary and particles precipitates become inside fairly the large. grains. Within Particularly those forparticles the slower often rates, even the brighter grain boundary particles particlesare visible, become which fairly are large. probably Within Fe those rich particlesprimary often particles. even brighterObviously, particles the quench are visible, induced which particles are probably become sm Fealler rich primarywith increasing particles. cooling Obviously, rate. This the quenchapplies inducedespecially particles to the grain become boundary smaller precipitates. with increasing However, cooling precipitates rate. This inside applies the grains especially from to0.01 the to grain1 K/s boundaryfirst appear precipitates. to increase However, in size and precipitates particle densit insidey, the but grains with from further 0.01 increasing to 1 K/s ﬁrst rate appear these to particles increase inbecome size and substantially particle density, smaller. butWithin with this further work increasingno detailed ratephase these analysis particles was carried become out. substantially According smaller.to previous Within work this workprecipitation no detailed of phase the analysisfollowing was phases carried out.can Accordingbe expected: to previous S-Al2CuMg, work precipitationη-Mg(Zn,Al,Cu) of2 the, ZnCu following rich Y-phase phases canplatelets be expected: [8,27,28,30], S-Al and2CuMg, potentiallyη-Mg(Zn,Al,Cu) also precursor/cluster2, ZnCu rich Y-phaseformation platelets [31,32]. [ 8The,27, latter28,30], two and particle potentially types also are precursorprobably /toocluster small formation to be detectable [31,32]. by The SEM. latter Thus, two particleit seems types likely are that probably the majority too small of the to bepartic detectableles observed by SEM. in our Thus, SEM it seems work likelyare either that theS-Al majority2CuMg or of theη-Mg(Zn,Al,Cu) particles observed2 phase in particles. our SEM EDS work indicates are either an S-Al enrichment2CuMg or ofη -Mg(Zn,Al,Cu)Zn, Mg, and Cu2 phasein any particles. quench EDSinduced indicates particle an observed enrichment in SEM of Zn, (Figure Mg, and11) and Cu th inus any does quench not allow induced to distinguish particle observed between inthe SEM two (Figurephases. 11Within) and thus the doesCCP not diagram allow to (Figure distinguish 9) al betweenl quench the induced two phases. precipitation Within the CCPreactions diagram are (Figuresummarised9) all quenchto as α  induced α + X. precipitation reactions are summarised to as α α + X. →

Figure 10. SEM-secondary electron images of alloy 7068: Cooling rates (a): 0.01 K/s; (b): 0.1 K/s; (c) 1 K/s; and (d) 10 K/s. Materials 2020, 13, x FOR PEER REVIEW 11 of 14

MaterialsFigure2020 ,10.13, SEM-secondary 918 electron images of alloy 7068: Cooling rates (a): 0.01 K/s; (b): 0.1 K/s; (c) 111 of 14 K/s; and (d) 10 K/s.

Figure 11. SEM-EDSSEM-EDS maps maps for for Zn, Zn, Mg, Mg, and and Cu Cu of of alloy alloy 7068 7068 after cooling at ( a)) 1 1 K/s K/s and ( b) 10 K/s. K/s.

3.5. Mechanical Mechanical Properties Properties after Ageing: Tensile Tensile Tests The tensile properties properties of of alumin aluminiumium alloy alloy 7068 7068 were were tested tested for for diff dierentﬀerent ageing ageing durations durations at 130 at 130°C. Plain◦C. Plain quenched quenched samples samples were werecompar compareded with withsamples samples which which have seen have an seen additional an additional stretching stretching of 3% afterof 3% quenching after quenching and prior and ageing, prior ageing, see Figure see Figure 12. For 12 each. For condition each condition six samples six samples were weretested. tested. The resultsThe results from from tensile tensile tests, tests, yield yield strength strength Rp0,2 Rp0,2, tensile, tensile strength strength R Rm,m and, and fracture fracture elongation elongation are summarised in in Table Table 2 for T6 T6 and and T651 T651 states. states. Next Next to to the the average average values values the the standard standard deviations deviations out ofout six of tests six tests are areshown. shown. Maximum Maximum yield yield strength strength an andd tensile tensile strength strength were were obtained obtained for for an an ageing treatment at 130 ◦C for 10 h following on the stretching operation (T651). These strengths are slightly

Materials 2020, 13, 918 12 of 14 lower than for aluminium alloy 7068 in the T6511 state according to AMS4311. However, it must be considered that that the Cu content of the investigated batch of alloy 7068 is at the lowermost boundary (Table1). Beyond that an additional stretching after aging (T6511) may result in a further increase in yield strength compared to T651. The fracture elongation is relatively high in all cases, while it is decreasing with increasing ageing duration.

Figure 12. Mechanical properties of alloy 7068: With or without stretching 3%, diﬀerent ageing times 1,

3, 10, and 30 h at 130 ◦C.

Table 2. Parameters from tensile testing: Heat treatment with or without stretching.

T6 475 ◦C 30 min, Quenching T651 475 ◦C 30 min, Quenching in in Water, 130 ◦C 30 h Water, Stretching 3%, 130 ◦C 10 h yield strength in N/mm2 605 5 626 2 ± ± tensile strength in N/mm2 649 2 651 3 ± ± elongation at fracture in % 9.8 0.8 10.0 1.2 ± ±

4. Conclusions Based on the experimental work comprising differential scanning calorimetry, differential dilatometry, differential fast scanning calorimetry, scanning electron microscopy, as well as hardness and tensile tests after age hardening heat treatment of a highly concentrated AlZnMgCu alloy AA 7068 the following findings can be stated:

An appropriate temperature for solution treatment is determined to be 475 C( 480 C), as the • ◦ − ◦ heating rate specific solvus at 0.01 K/s is 470 ◦C; Cooling DSC after soaking durations of 10 to 60 min at 475 C showed no differences, and a • ◦ soaking duration of 30 min was chosen; The application of differential dilatometry allows the in-situ detection of quench induced • precipitation in a wide cooling rate range of 0.1 K/s to 100 K/s. At fast cooling of 100 K/s dilatometry still reveals quenched induced precipitation; While DSC only shows one very broad precipitation peak for a wide range of cooling rates, • 1 dilatometry indicates the overlap of several quench induced precipitation reactions;

The physical upper critical cooling rate (CCR) at which quench induced precipitation is fully • suppressed was determined by DFSC analysis to be about 600 to 800 K/s; Materials 2020, 13, 918 13 of 14

A technological CCR is found to be 100 K/s (i.e., hardness after artiﬁcial ageing remains on • ≈ a saturation level for higher cooling rates including WQ). This indicates, that quench induced precipitation at cooling rates faster than 100 K/s contributes to a direct hardening eﬀect; For the investigated batch of AA 7068 the age hardening treatment comprising of solution heat • treatment 475 ◦C 30 min, water quenching, stretching 3%, and ageing (130 ◦C 10 h) results in a 2 2 yield strength Rp0.2 of 626 N/mm , a tensile strength Rm of 651 N/mm , and a fracture elongation of 10%.

Author Contributions: The experiments were designed by J.O., M.R., O.K. and B.M.; DSC and hardness tests were performed by J.O.; dilatometry by M.R.; DFSC by B.Y.; The tensile tests were done by K.N.; SEM analyses was done by A.S. and Marcus Frank. All authors contributed to the discussion of the results. The manuscript was written by B.M., J.O., and M.R. All authors have revised the text and given their approval for the final version of the manuscript. Funding: The project’ Joining of high-strength aluminium alloy structural components by means of aluminium self-piercing rivets’ is funded by the German Federal Ministry for Economic Affairs and Energy based on a resolution of the German Parliament (grant number 19432BR) via ‘Stifterverband Metalle e. V.’. Conflicts of Interest: The authors declare no conflict of interest.

References

1. Polmear, I.J. Light Alloys. From Traditional Alloys to Nanocrystals, 4th ed.; Elsevier Butterworth-Heinemann: Amsterdam, The Netherlands, 2006; ISBN 0750663715. 2. Hornbogen, E. Hundred years of precipitation hardening. J. Light Met. 2001, 1, 127–132. [CrossRef] 3. Hornbogen, E. Precipitation hardening—The oldest nanotechnology. Metall-Berlin 2001, 55, 522–526. 4. Baba, Y. Influence of Additional Elements on the Quench-Sensitivity and Nucleation of Precipitates in Al-Zn-Mg Alloys. Nippon Kinzoku Gakkaishi 1967, 31, 910–915. [CrossRef] 5. Baba, Y. Quench-Sensitivity and Age-Hardening of Al-Zn-Mg Alloys Containing Trace Elements. Trans. Jpn. Inst. Met. 1968, S9, 356. 6. Conserva, M.; Fiorini, P. Interpretation of quench-sensitivity in Al-Zn-Mg-Cu alloys. Metall. Trans. A 1973, 4, 857–862. [CrossRef] 7. Shuey, R.T.; Tiryakio˘glu,M. Quenching of Aluminum Alloys. Quenching Theory and Technology, 2nd ed.; CRC Press: Boca Raton, FL, USA, 2011. 8. Starink, M.J.; Milkereit, B.; Zhang, Y.; Rometsch, P.A. Predicting the quench sensitivity of Al-Zn-Mg-Cu alloys: A model for linear cooling and strengthening. Mater. Des. 2015, 88, 958–971. [CrossRef] 9. Milkereit, B.; Reich, M.; Kessler, O. Detection of Quench Induced Precipitation in Al Alloys by Dilatometry. Mater. Sci. Forum. 2016, 877, 147–152. [CrossRef] 10. Zhuravlev, E.; Schick, C. Fast scanning power compensated differential scanning nano-calorimeter: 1. The device. Thermochim. Acta 2010, 50, 1–13. [CrossRef] 11. Yang, B.; Milkereit, B.; Zhang, Y.; Rometsch, P.A.; Kessler, O.; Schick, C. Continuous cooling precipitation diagram of aluminium alloy AA7150 based on a new fast scanning calorimetry and interrupted quenching method. Mater. Charact. 2016, 120, 30–37. [CrossRef] 12. Mucha, J. The effect of material properties and joining process parameters on behavior of self-pierce riveting joints made with the solid rivet. Mater. Des. 2013, 52, 932–946. [CrossRef] 13. Jäckel, M.; Grimm, T.; Niegsch, R.; Drossel, W.-G. Overview of Current Challenges in Self-Pierce Riveting of Lightweight Materials. Proceedings 2018, 2, 384. [CrossRef] 14. Gröber, D.; Georgi, W.; Sieber, M.; Scharf, I.; Hellmig, R.J.; Leidich, E.; Lampke, T.; Mayr, P. The effect of anodising on the fatigue performance of self-tapping aluminium screws. Int. J. Fatigue 2015, 75, 108–114. [CrossRef] 15. Milkereit, B.; Kessler, O.; Schick, C. Recording of continuous cooling precipitation diagrams of aluminium alloys. Thermochim. Acta 2009, 492, 73–78. [CrossRef] 16. Osten, J.; Milkereit, B.; Schick, C.; Kessler, O. Dissolution and precipitation behaviour during continuous heating of Al-Mg-Si alloys in a wide range of heating rates. Materials 2015, 8, 2830–2848. [CrossRef] Materials 2020, 13, 918 14 of 14

17. Hengge, E.; Enzinger, R.; Luckabauer, M.; Sprengel, W.; Würschum, R. Quantitative volumetric identification of precipitates in dilute alloys using high-precision isothermal dilatometry. Philos. Mag. Lett. 2018, 98, 301–309. [CrossRef] 18. Luckabauer, M.; Hengge, E.; Klinser, G.; Sprengel, W.; Würschum, R. In Situ Real-Time Monitoring of Aging Processes in an Aluminum Alloy by High-Precision Dilatometry. In Magnesium Technology 2017; The Minerals, Metals & Materials Series; Solanki, K., Orlov, D., Singh, A., Neelameggham, N., Eds.; Springer: Cham, Switzerland, 2017; pp. 669–674. [CrossRef] 19. Resch, L.; Klinser, G.; Hengge, E.; Enzinger, R.; Luckabauer, M.; Sprengel, W.; Würschum, R. Precipitation processes in Al–Mg–Si extending down to initial clustering revealed by the complementary techniques of positron lifetime spectroscopy and dilatometry. J. Mater. Sci. 2018, 53, 14657–14665. [CrossRef] 20. Savitzky, A.; Golay, M.J.E. Smoothing and Differentiation of Data by Simplified Least Squares Procedures. Anal. Chem. 1964, 36, 1627–1639. [CrossRef] 21. Zohrabyan, D.; Milkereit, B.; Kessler, O.; Schick, C. Precipitation enthalpy during cooling of aluminum alloys obtained from calorimetric reheating experiments. Thermochim. Acta 2012, 529, 51–58. [CrossRef] 22. Milkereit, B.; Jonas, L.; Schick, C.; Kessler, O. The continuous cooling precipitation diagram of an aluminum alloy en AW-6005A. HTM J. Heat Treat. Mat. 2010, 65, 159–171. [CrossRef] 23. Osten, J.; Reich, M.; Milkereit, B.; Kessler, O. Adapting age hardening parameters of high-alloyed 7xxx aluminium alloys through thermal analysis. In Proceedings of the 16th International Conference on Aluminium Alloys, Montreal, QC, Canada, 17–21 June 2018. 24. Fröck, H.; Milkereit, B.; Wiechmann, P.; Springer, A.; Sander, M.; Kessler, O.; Reich, M. Influence of Solution-Annealing Parameters on the Continuous Cooling Precipitation of Aluminum Alloy 6082. Metals 2018, 8, 265. [CrossRef] 25. Milkereit, B.; Starink, M.J.; Rometsch, P.A.; Schick, C.; Kessler, O. Review of the Quench Sensitivity of Aluminium Alloys: Analysis of the Kinetics and Nature of Quench-Induced Precipitation. Materials 2019, 12, 4083. [CrossRef] 26. Milkereit, B.; Kessler, O.; Schick, C. Quench Sensitivity and Continuous Cooling Precipitation Diagrams. In Encyclopedia of Aluminum and Its Alloys; CRC Press: Boca Raton, FL, USA, 2019; ISBN 9781351045636. 27. Zhang, Y.; Weyland, M.; Milkereit, B.; Reich, M.; Rometsch, P.A. Precipitation of a new platelet phase during the quenching of an Al-Zn-Mg-Cu alloy. Sci. Rep. 2016, 6, 23109. [CrossRef] 28. Liu, S.; Li, Q.; Lin, H.; Sun, L.; Long, T.; Ye, L.; Deng, Y. Effect of quench-induced precipitation on microstructure and mechanical properties of 7085 aluminum alloy. Mater. Des. 2017, 132, 119–128. [CrossRef] 29. Reich, M.; Kessler, O. Quenching Simulation of Aluminum Alloys Including Mechanical Properties of the Undercooled States. Mater. Perform. Charact. 2012, 1, 104632. [CrossRef] 30. Deschamps, A.; Texier, G.; Ringeval, S.; Delfaut-Durut, L. Influence of cooling rate on the precipitation microstructure in a medium strength Al-Zn-Mg alloy. Mater. Sci. Eng. A 2009, 501, 133–139. [CrossRef] 31. Schloth, P. Precipitation in the High Strength AA7449 Aluminium Alloy: Implications on Internal Stresses on Different Length Scales. Ph.D. Thesis, EPFL, Lausanne, Switzerland, 2015. 32. Schloth, P.; Wagner, J.N.; Fife, J.L.; Menzel, A.; Drezet, J.M.; van Swygenhoven, H. Early precipitation during cooling of an Al-Zn-Mg-Cu alloy revealed by in situ small angle X-ray scattering. Appl. Phys. Lett. 2014, 105, 101908. [CrossRef]

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