Hot Press As a Sustainable Direct Recycling Technique of Aluminium: Mechanical Properties and Surface Integrity

materials

Article Hot Press as a Sustainable Direct Recycling Technique of Aluminium: Mechanical Properties and Surface Integrity

Nur Kamilah Yusuf ID , Mohd Amri Lajis * and Azlan Ahmad

Sustainable Manufacturing and Recycling Technology, Advanced Manufacturing and Materials Center (SMART-AMMC), Universiti Tun Hussein Onn Malaysia (UTHM), Parit Raja 86400, BatuPahat, Johor, Malaysia; [email protected] (N.K.Y.); [email protected] (A.A.) * Correspondence: [email protected]; Tel.: +60-7-4538313; Fax: +60-7-4538399

Received: 5 June 2017; Accepted: 19 July 2017; Published: 3 August 2017

Abstract: Meltless recycling technique has been utilized to overcome the lack of primary resources, focusing on reducing the usage of energy and materials. Hot press was proposed as a novel direct recycling technique which results in astoundingly low energy usage in contrast with conventional recycling. The aim of this study is to prove the technical feasibility of this approach by characterizing the recycled samples. For this purpose, AA6061 aluminium chips were recycled by utilizing hot press ◦ process under various operating temperature (Ts = 430, 480, and 530 C) and holding times (ts = 60, 90, and 120 min). The maximum mechanical properties of recycled chip are Ultimate tensile strength (UTS) = 266.78 MPa, Elongation to failure (ETF) = 16.129%, while, for surface integrity of the chips, ◦ the calculated microhardness is 81.744 HV, exhibited at Ts = 530 C and ts = 120 min. It is comparable to theoretical AA6061 T4-temper where maximum UTS and microhardness is increased up to 9.27% and 20.48%, respectively. As the desired mechanical properties of forgings can only be obtained by means of a ﬁnal heat treatment, T5-temper, aging after forging process was employed. Heat treated recycled billet AA6061 (T5-temper) are considered comparable with as-received AA6061 T6, where the value of microhardness (98.649 HV) at 175 ◦C and 120 min of aging condition was revealed to be greater than 3.18%. Although it is quite early to put a base mainly on the observations in experimental settings, the potential for signiﬁcant improvement offered by the direct recycling methods for production aluminium scrap can be clearly demonstrated. This overtures perspectives for industrial development of solid state recycling processes as environmentally benign alternatives of current melting based practices.

Keywords: sustainable manufacturing; direct metal recycling; hot press (HP); aluminium AA6061; mechanical properties; surface integrity

1. Introduction The carbon dioxide emissions and vast release of solid waste by industrial processes has led to global warming, which brings about adverse effect to human activity. Various industrial processes accounted for approximately 14% of the total carbon dioxide emissions and 20% of the total greenhouse gas emissions in 2010 [1]. According to [2], the assembling division, which lies at the centre of the modern economy, must be made to protect elevated living standards accomplished by industrialized social orders and, in turn, to empower social order to achieve and sustain a similar level of afﬂuence. Hence, the need of decrement in power utilization in mechanical procedures has turned into a central point in the modern world. Aluminium alloys have been in exigency by the industrial practitioner due to their distinctive properties, which includes good strength and low density compared with steel. Most of them were

Materials 2017, 10, 902; doi:10.3390/ma10080902 www.mdpi.com/journal/materials Materials 2017, 10, 902 2 of 18 utilized for food packaging, transportation, food additives, beverage cans, cooking utensils, building medicines, and surgery materials, because of their unique physical and chemical properties [3]. Whenever the demand and application increased, the waste produced from the machining will also significantly increase [4–6]. This leads to the production of secondary aluminium to substitute the current use of primary aluminium. Correspondingly, different methods have emerged in producing secondary aluminium. The most viable aluminium recycling practices in most industries are based on the conventional recycling using a melting technique. It is forecasted that, in 2030, 6.1 megatons of scrap will not be recycled due to high concentration of aluminium alloying elements caused by their inefficient and/or challenging removal during re-melting [7]. The conventional recycling of aluminium is recently less favourable, as the method requires high energy and large number of operations which led to cost increment. Instead of using melting techniques that use a very high temperature to reach the melting point, recycling of wrought aluminium alloys by solid-state is preferable. High energy consumption for conventional aluminium recycling and subsequent refinement has been considered in previous studies. Initially, the solid-state recycling techniques employ the powder metallurgy processes by Gronostajski et al. [8], followed by research from Fogagnolo et al. [9]. Moreover, some of recycling techniques have been studied and show excellent mechanical responses by employing extrusion and powder metallurgy process [10–15]. This process chain requires a small amount of energy compared to conventional process chains, using only 5–6 GJ·ton−1, which is 5–6% of that needed for the conventional process chain. During the complete conversion of aluminium chips into a compact metal by extrusion, a portion of the chip from which impurities cannot be removed is wasted, amounting to approximately 2%; the extrusion waste can be as high as 3%. Thus, 95% of the aluminium chips are recovered [12]. Spark plasma sintering (SPS) is a new approach of solid-state recycling. The main characteristics of SPS are that the pulsed DC current directly passes through the graphite die and the chip is compacted. The dynamic scrap compaction, combined with electric current-based joule heating, achieved partial fracture of the stable surface oxides, desorption of the entrapped gases and activated the metallic surfaces, resulting in efficient solid-state chip welding eliminating residual porosity [16,17]. On the other hand, a wide research enthusiasm for maintainability is available in the modern technical studies: Life Cycle Assessment (LCA) technique and Design for Environment (DFE) methodology are these days broadly examined in many research laboratories throughout the world [18]. Moreover, Duflou et al. [19] compared the environmental performance of three innovative solid state recycling strategies with the environmental impact of the conventional re-melting approach. The paper demonstrates the noteworthy ecological benefits from utilizing the inventive reusing techniques. In conjunction with this, hot press showed promising alternative for recycling aluminium as the waste from the machining process [20–22]. To validate this, a comparative analysis of an alternative material recycling route hot press process, starting from the same waste materials as the conventional re-melting technique, was studied. The LCA model was created using the Simapro 8.0.5 software for life cycle assessment. The databases contained in the Simapro software provide the LCI data of the raw and process materials used in the background system. The data for conventional technique are collected by the Ecoinvent database combined with published data from the literature. The results of analysis justified that hot press process is evidently gives the significant environmental benefits as compared to the conventional re-melting technique where the Global Warming Potential (GWP) is reduced up to 69.2% [23]. Furthermore, hot press process allowed a good potential of strength and plasticity of aluminium. Recycled aluminium has shown virtuous mechanical and physical properties, when subjected to stern plastic deformation course. Many researchers have agreed that the most significant parameter that must be considered when dealing with aluminium alloys is temperature [14,15,24–26]. Theoretically, linear correlation of raise between temperature and mechanical properties of aluminium alloy are ought to occur. Hence, this paper has conducted solid-state direct recycling technique by utilizing hot press process of an aluminium chip with aim to investigate the effect of operating Materials 2017, 10, 902 3 of 18 temperature and holding time on the mechanical properties, as well as physical properties of direct recycling of AA6061 aluminium alloy that is potentially to be used as secondary resources.

2. Experimental Material and Procedure

2.1. Experimental Material and Processing The chemical composition of commercial aluminium bulk series AA6061-T6 is depicted in Table1. It was measured from energy-dispersive X-ray spectroscopy (EDS) using Hitachi SU8000 (Hitachi, Tokyo, Japan), and is the average of three EDS spots. Copper intermetallic compounds in low concentration detected in all EDS spots. Parts of copper, silicon, magnesium, manganese, and iron intermetallic have been detected. Aluminium exhibited as a main element (94.9–95.4 wt %). In comparison to the compounds of detected elements with ASM handbook material data (2006), this veriﬁed that the material used in this research is AA6061 aluminium alloy.

Table 1. Average chemical element of as-received AA6061-T6 matrix

Elements Weight Percent (wt %) Aluminium 95.03 Magnesium 1.17 Silicon 0.34 Iron 0.46 Copper 0.15 Manganese 0.14 Others Balance

The AA6061 bulk material was milled to produce medium size aluminium chips with average sizes 5.20 mm × 1.097 mm × 0.091 mm in Figure1. The milling took place in the Sodick-MC430L high speed machining, with the cutting speed, v, of 110 m/min; feed rate, f, of 0.05 mm/tooth; and the depth of cut of 1.0 mm. The aluminium chips produced by machining should not contain any impurities and dirt because these would alter their chemical composition and subsequently impair the diffusion bonding of the chips during solid-state recycling. A cleaning process in acetone (C3H6O) begins as soon the chip leaves the milling machine, following ASTM G131-96. The drying process is 30 min in thermal drying oven at 60 ◦C. The cleaned aluminium chip was poured into the mould and the plunge is fixed accordingly. Hot press process (Figure1) was executed with the steady pressure at 47 MPa (around 35 tons) and four times pre-compacting cycle. Indeed, Figure2 shows the process diagram of direct recycling hot press. The temperatures selected are 430, 480, and 530 ◦C after considering the optimum operating temperature for hot forging and heat treatment process. In addition, the three selected holding times are 60, 90 and 120 min. Immediately after the forging process (solution heat treatment), the specimen will be quenched in water at quench rate 100 ◦C/s to perform rapid cooling into room temperature followed by artificial aging at temperature 175 ◦C with 120-min duration. Samples with the corresponding designated different parameter setting are represented in Table2. The recycled specimen after hot forging process is denoted as T1-temper at which the aluminium were cooled from an elevated-temperature shaping process and naturally aged to a substantially stable condition [27]. These recycled specimens are considered comparable with theoretical AA6061-T4 temper in terms of aluminium being the solution heat treated and naturally aged to a substantially stable condition [27] to observe the potential of recycled material to be used as secondary resources. The complete cycle of heat treated recycled specimen is denoted as T5-temper, in which the aluminium was cooled from an elevated temperature shaping process and subsequently artificially aged [27]. This heat treated recycled billet are considered comparable with as-received AA6061-T6 and denoted as AR-T6. Analyses include tensile test, ultimate tensile strength, and elongation to failure for mechanical properties, while microhardness and density are considered for surface integrity analyses. Materials 2017, 10, 902 3 of 18

2. Experimental Material and Procedure

2.1. Experimental Material and Processing The chemical composition of commercial aluminium bulk series AA6061-T6 is depicted in Table 1. It was measured from energy-dispersive X-ray spectroscopy (EDS) using Hitachi SU8000 (Hitachi, Tokyo, Japan), and is the average of three EDS spots. Copper intermetallic compounds in low concentration detected in all EDS spots. Parts of copper, silicon, magnesium, manganese, and iron intermetallic have been detected. Aluminium exhibited as a main element (94.9–95.4 wt %). In comparison to the compounds of detected elements with ASM handbook material data (2006), this verified that the material used in this research is AA6061 aluminium alloy.

Table 1. Average chemical element of as-received AA6061-T6 matrix

Elements Weight Percent (wt %) Aluminium 95.03 Magnesium 1.17 Silicon 0.34 Iron 0.46 Copper 0.15 Manganese 0.14 Others Balance

Materials 2017, 10, 902 4 of 18

Hot press process (Figure 1) was executed with the steady pressure at 47 MPa (around 35 tons) and four times pre-compacting cycle. Indeed, Figure 2 shows the process diagram of direct recycling hot press. The temperatures selected are 430, 480, and 530 °C after considering the optimum operating temperature for hot forging and heat treatment process. In addition, the three selected holding times are 60, 90 and 120 min. Immediately after the forging process (solution heat treatment), the specimen will be quenched in water at quench rate 100 °C/s to perform rapid cooling into room temperature followed by artificial agingFigure atFigure temperature 1. 1.Direct Direct 175recycling recycling °C with hot hot 120-min press press process. duration. process.

Solution Heat Treatment Solution Heat Treatment + Aging (T1) (T5)

Hot Press Forging Quenching Aging

Temperature

Pressure TS= (430, 480, 530) ºC TS 47MPa

-1 (dT/dt)q= 100ºC· s Pressure Temperature Ta= 175 ºC Ta

ts= (60, 90, 120) min ta= 120 min

Time FigureFigure 2. Direct2. Direct recycling recycling hot press press diagram. diagram.

Samples withTable the correspo 2. Samplending designation designated different for different parameter parameter setting setting. are represented in Table 2. The recycled specimen after hot forging process is denoted as T1-temper at which the aluminium were cooled from an elevated-temperature shaping process and naturally aged to a substantially Solution Heat Treatment (Hot Press) Solution Heat Treatment + Aging stable condition [27]. These recycled specimens are considered comparable with theoretical AA6061- Sample Operating T4 temper in terms of aluminium being theHolding solution Time, heat t streatedAging and Temp,naturally Ta agedAging to a substantially Time, ta Designation Temperature, T s (min) (◦C) (min) stable condition [27] to observe(◦C) the potential of recycled material to be used as secondary resources. The completeS1-T1 cycle of heat 430 treated recycled specimen 60 is denoted - as T5-temper, in - which the aluminiumS2-T1 was cooled from 430 an elevated temperature 90 shaping process - and subsequently - artificially aged [27].S3-T1 This heat treated 430 recycled billet are considered 120 comparable - with as-received -AA6061-T6 and denotedS4-T1 as AR-T6. Analyses 480 include tensile 60 test, ultimate tensile - strength, and elongation - to S5-T1 480 90 - - failureS6-T1 for mechanical properties, 480 while microh 120ardness and density - are considered -for surface integrityS7-T1 analyses. 530 60 - - S8-T1 530 90 - - S9-T1Table 530 2. Sample designation 120 for different parameter - setting. - S10-T5 530 120 175 120 Theory-T4Solution Heat Treatment (Hot ASM Press) Theory AA6061-T4Solution Heat Treatment + Aging SampleAR-T6 Operating As-Received AA6061-T6 Holding Time, ts Aging Temp, Ta Aging Time, ta Designation Temperature, Ts (min) (°C) (min) (°C) S1-T1 430 60 - - S2-T1 430 90 - - S3-T1 430 120 - - S4-T1 480 60 - - S5-T1 480 90 - -

Materials 2017, 10, 902 5 of 18

S6-T1 480 120 - - S7-T1 530 60 - - S8-T1 530 90 - - S9-T1 530 120 - - S10-T5 530 120 175 120 MaterialsTheory-T42017, 10 , 902 ASM Theory AA6061-T4 5 of 18 AR-T6 As-Received AA6061-T6

2.2. Material Characterization Measurement

2.2.1. Mechanical Characterization The forming procedure for arrangement for tensile testing in this paper abide by the following standard of Standard Test MethodsMethods for Tension TestingTesting ofof MetallicMetallic MaterialMaterial (ASTM(ASTM E8M,E8M, 2012),2012), and the dimensions are shown in Figure3 3.. TensileTensile tests tests were were performed performed using using universal universal testing testing machinemachine (UN-7001-LS,(UN-7001-LS, GOTECHGOTECH TestingTesting MachinesMachines Inc.,Inc., Taichung,Taichung, Taiwan)Taiwan) usingusing aa 2525 kNkN loadload cellcell withwith gauge length 2525 mmmm waswas usedused toto recordrecord thethe strain.strain. TensileTensile teststests werewere performedperformed atat speedsspeeds 55 mm/min.mm/min.

Subsize Specimen (6 mm Wide) Dimension (mm) G—Gage length 25.0 W—Width 6.0 T—Thickness <6.0 R—Radius of fillet, min 6.0 L—Overall length, min 100.0 A—Length of reduced section, min 32.0 B—Length of grip section, min 30.0 C—Width of grip section, approximate 10.0

Before testing

After testing

Figure 3. Tensile specimen dimension after recycled process. Figure 3. Tensile specimen dimension after recycled process.

2.2.2. Microstructural Characterization 2.2.2. Microstructural Characterization The microstructure analysis followed the Standard Guide for Preparation of Metallographic The microstructure analysis followed the Standard Guide for Preparation of Metallographic Specimens (American Society for Testing and Materials E3, 2001) and Standard Test Method for Specimens (American Society for Testing and Materials E3, 2001) and Standard Test Method for Macroetching Metals and Alloys (American Society for Testing and Materials E340, 2006). Light Macroetching Metals and Alloys (American Society for Testing and Materials E340, 2006). Light optical optical microscope (Nikon Eclipse LV150 NL, Nikon Metrology NV, Tokyo, Japan) was used to microscope (Nikon Eclipse LV150 NL, Nikon Metrology NV, Tokyo, Japan) was used to quantify the quantify the microstructural of the recycled samples. The samples were prepared for metallographic microstructural of the recycled samples. The samples were prepared for metallographic observations observations by grinding the sample consecutively with 60, 240, 400, 600 and 1200 SiC papers, and by grinding the sample consecutively with 60, 240, 400, 600 and 1200 SiC papers, and then polishing then polishing with 6 and 1 µm DIAMAT Polycrystalline Diamond cloths. Then, 0.02 µm SIAMAT with 6 and 1 µm DIAMAT Polycrystalline Diamond cloths. Then, 0.02 µm SIAMAT Noncrystalline Noncrystalline Colloidal Silica was used for finishing. The samples were etched with Barker’s reagent Colloidal Silica was used for ﬁnishing. The samples were etched with Barker’s reagent to show the to show the grain boundary developed by dynamic recovery or recrystallization. grain boundary developed by dynamic recovery or recrystallization.

2.2.3. Hardness

The Vickers hardness test was selected to measure the microhardness of recycled AA6061 aluminium alloy in this study. It was conducted by pressing an indenter into the surface of the tested material using a controlled force, followed by removal of the indenter, and ﬁnally the measurement of the indentation size. The analysis was done based on the Standard Test Method for Knoop and Vickers Materials 2017, 10, 902 6 of 18

Hardness of Materials (American Society for Testing and Materials E384, 2011). Hardness test samples were cut into 10.0 mm × 10.0 mm × 15.0 mm using an abrasive cutter under the ﬂow of cooling liquid to prevent frictional heating. Vickers microhardness measurements was made with a 2.943 N load in the span of 10 seconds holding time were used to monitor the hardness and data reported represent an average of at least 10 measurements.

2.2.4. Density The experimental density of the composites was obtained using Archimedean method of weighing small pieces cut from the composite, ﬁrst in air and then in water. The density of the sample is measured using density balance (HR-250AZ) from, and the actual mass and apparent weight of specimen are calculated while immersed in water.

3. Results and Discussion

3.1. Mechanical Properties Table3 shows the mechanical properties, including ultimate tensile strength (UTS), elongation to failure (ETF) and microhardness as a function of artiﬁcial aging time for recycled specimen. Figure4 shows the result for UTS and ETF for different operating temperature and holding time. The results in Figure4 show that the tensile strength is very sensitive to temperature, with an increasing trend when the temperature increases from 430 to 530 ◦C where UTS increased with the increment of temperature. The inclination trend is depicted as holding time at its maximum (120 min) where at S3-T1 (28.44 MPa) surged up to 266.78 MPa at S9-T1. At holding time of 60 min, the value of UTS increased with increasing operating temperature ranging from S1-T1 (14.97 MPa) to S9-T1 (230.55 MPa). The ETF has a linear trend with increment of temperature, which is similar to the UTS trend. As can be seen, at the maximum holding time (120 min), ETF for S3-T1 (2.027%) elevated up to S9-T1 (16.129%) at 530 ◦C. At holding time 60 min, the value of ETF increased with the increasing operating temperature, exhibiting 6.769% from S1-T1 (0.091%) to S7-T1 (6.860%).

Table 3. Results for tensile properties for all samples.

Ultimate Tensile Elongation to Sample Microhardness Density Strength, UTS Failure, ETF Designation (MPa) (%) HV g/cc S1-T1 14.97 0.09 69.76 1.92 S2-T1 16.91 1.04 70.37 2.08 S3-T1 28.44 2.02 71.45 2.16 S4-T1 48.11 0.12 72.62 2.29 S5-T1 72.84 2.22 74.69 2.38 S6-T1 90.42 3.22 75.63 2.51 S7-T1 230.55 6.86 77.64 2.56 S8-T1 240.27 9.50 79.81 2.63 S9-T1 266.78 16.12 81.74 2.65 S10-T5 340.47 21.70 98.64 2.69 Theory-T4 240.00 22.00 65.00 2.70 AR-T6 343.43 24.62 95.51 2.67 Materials 2017, 10, 902 7 of 18 Materials 2017, 10, 902 7 of 18

FigureFigure 4. Tensile 4. Tensile results results of of recycled recycled specimen specimen atat different operating operating time time and and temperature. temperature.

As depicted in Figure 4, the results on the effect of operating temperature clearly give the significantAs depicted impact in Figureto the forging4, the resultsprocess onby proving the effect the offact operating that at constant temperature holding times, clearly value give of the significantUTS and impact surged to theup forgingto 215.58–238.34 process by MPa proving and theETF fact 6.769–14.102% that at constant by increasing holding times, operating value of UTStemperature and surged upfrom to 430 215.58–238.34 °C to 530 °C. MPa The and primary ETF 6.769–14.102%cause of the increase by increasing in strength operating is the operating temperature fromtemperature 430 ◦C to 530 exceeded◦C. The theprimary solvus temperature: cause of the this increase causes the in precipitates strength is to the re-dissolve operating and temperature results exceededin solute the solution solvus hardening. temperature: The operating this causes forging the temperatures precipitates in to this re-dissolve study were and selected results between in solute solutionthe solidus hardening. and the The recrystallization operating forging temperature. temperatures The process in this requires study were high selected temperature, between however the solidus it and theshould recrystallization not be higher than temperature. liquid temperature The process as well requires as be given high sufficient temperature, time for however phases containing it should not be highercopper than and/or liquid magnesium temperature to dissolve as well completely as be given so that sufficient a nearly time homogenous for phases solid containing solution copper is achieved. The temperature and time must be well controlled, because either insufficiency or and/or magnesium to dissolve completely so that a nearly homogenous solid solution is achieved. excessiveness may cause coarsening and decreasing precipitate [28]. Solubility increases with The temperature and time must be well controlled, because either insufficiency or excessiveness may increase in temperature, so conducting in lower temperatures will give worse properties [29]. causeFurthermore, coarsening andat high decreasing temperature, precipitate solid solution [28]. Solubility strengthening increases is expected with increase to have ina major temperature, effect. so conductingFriis et inal. lower[30] and temperatures Langkruis et will al. give[31] worsestated that properties the tensile [29]. tests Furthermore, represent the at highoccurrence temperature, of solidsignificant solution strengthening hardening, caused is expected by severe to plastic have astrain major induced effect. in Friis the etsamples, al. [30] in and the Langkruis deformed state. et al. [31] statedRometsch that the and tensile Schaffer tests represent[32] had analysed the occurrence the yield of strength significant with hardening, an assumption caused of byindependent severe plastic straincontributions induced in theof solid samples, solution in the hardening, deformed precipitation state. Rometsch hardening, and Schafferand intrinsic [32] strength. had analysed Meng the [33] yield strengthalso withagreed an that assumption the increment of independent of UTS were affected contributions by the up of solidrise of solution heating temperature hardening, precipitationby stated hardening,that during and intrinsicsolutionizing, strength. solute Meng atoms [33 were] also dissolved agreed that in the the Aluminium increment matrix of UTS and were retained affected in bya the up risemetastable of heating state. temperature These fine cluste by statedrs (precipitates) that during act solutionizing, as a barrier for solute dislocation atoms movement, were dissolved causing in the an enhancement of initial strength. Moreover, Oppenheim et al. [34] agreed that, below 470 °C, lowest Aluminium matrix and retained in a metastable state. These fine clusters (precipitates) act as a barrier strength and hardness were produced by indicating that a significant amount of the solute did not for dislocation movement, causing an enhancement of initial strength. Moreover, Oppenheim et al. [34] go in solid solution ◦at this low temperature, whereas higher strength and hardness were exhibited at agreedhigh that, temperature below 470 upC, to lowest500 °C. strength and hardness were produced by indicating that a significant amount ofThe the same solute trend did notis observed go in solid for solutionthe holding at this time low effects temperature, in comparison whereas to UTS higher at strengthwhich it and ◦ hardnessdemonstrated were exhibited the effect at of high holding temperature time, as shown up to 500in FigureC. 4. At the operating temperature 430 °C, theThe value same of UTS trend marginally is observed raisedfor (13.47 the MPa) holding with the time increase effects of holding in comparison time from S1-T1 to UTS (14.97 at MPa) which it demonstratedto S3-T1 (28.44 the effectMPa). ofAt holding the maximum time, asoperating shown intemperature Figure4. Atof 530 the °C, operating the value temperature of UTS slightly 430 ◦C, the valueincreased of UTS from marginally 230.55 MPa raised at S7-T1 (13.47 to 266.78 MPa) MPa with at the S9-T1. increase Meanwhile, of holding a similar time fromascending S1-T1 trend (14.97 for MPa) to S3-T1ETF (28.44demonstrated MPa). the At theeffect maximum of holding operating time. At the temperature operating temperature of 530 ◦C, 430°C, the value the value of UTS of ETF slightly increased from 230.55 MPa at S7-T1 to 266.78 MPa at S9-T1. Meanwhile, a similar ascending trend for ETF demonstrated the effect of holding time. At the operating temperature 430◦C, the value of Materials 2017, 10, 902 8 of 18

ETFMaterials slightly 2017 raised, 10, 902 (1.936%) with the risen of holding time from S1-T1 (0.091%) to S3-T18 (2.027%). of 18 ◦ At theslightly maximum raised operating(1.936%) with temperature the risen of of holding 530 C, time the valuefrom S1-T1 of ETF (0.091%) increased to S3-T1 from (2.027%). 6.860% inAt S7-T1the to 16.129%maximum in S9-T1. operating On the temperature other hand, of when530 °C, comparing the value of the ETF minimum–maximum increased from 6.860% parameter in S7-T1 to (S1-T1 with16.129% S9-T1), thein S9-T1. value On of the UTS other is observed hand, when to increase comparing dramatically the minimum–maxi from 14.97mum MPa parameter to 266.78 (S1-T1 MPa. ◦ ◦ withFigure S9-T1),5 shows the value microstructure of UTS is observed at S1-T1 to (430 increaseC/60 dramatically min) and S9-T1 from 14.97 (530 MPaC/120 to 266.78 min). AtMPa. S9-T1 the average grainFigure diameter 5 shows microstructure is 30.78 µm, whereas, at S1-T1 (430 at S1-T1, °C/60 themin) grain and S9-T1 diameter (530 °C/120 revealed min). to At be S9-T1 bigger the (61.70 µm).average At a minimum grain diameter parameter, is 30.78 the µm, porosity whereas, between at S1-T1, the the chips grai aren diameter greatly revealed observable, to be as bigger shown in Figure(61.705a. Thisµm). shows At a minimum that, at low parameter, temperature, the porosi chip istysimply between interlocking the chips are instead greatly of consolidating.observable, as On the contrary,shown in theFigure recycled 5a. This aluminium shows that, profiles at low at temper differentature, operating chip is simply temperature interlocking show instead that the of high consolidating. On the contrary, the recycled aluminium profiles at different operating temperature temperature leads to recrystallized grain boundary where porosity between the chips is less apparent show that the high temperature leads to recrystallized grain boundary where porosity between the at S9-T1 in Figure5b. The size of the grain boundary became closer between the chips, which shows chips is less apparent at S9-T1 in Figure 5b. The size of the grain boundary became closer between that,the at maximumchips, which parameter, shows that, theat maximum solid-state parameter, welding the of thesolid-state chip is welding enhanced. of the The chip orientation is enhanced. of the ◦ grainThe boundary orientation is closerof the grain between boundary the chips, is closer which between proved the that,chips, at which maximum proved temperature that, at maximum of 530 C, the solid-statetemperature welding of 530 °C, of thethe chipsolid-state is enhanced. welding Russelof the chip and is Kok enhanced. [35] have Russel compared and Kok the [35] stress-strain have behaviourcompared of singlethe stress-strain crystal and behaviour polycrystalline of single aluminium crystal and tensile polycrystalline specimens foraluminium different tensile grain size. In general,specimens small for grain different size producedgrain size. close In general, space barrier small tograin dislocation size produced and their close study space conclude barrier thatto the closerdislocation the spaced and barrier their tostudy dislocations, conclude thethat greater the closer strength the spaced will be barrier produced. to dislocations, However, itthe is knowngreater that dynamicstrength recrystallization will be produced. occurs However, during it hot is known extrusion that for dynamic Al-Mg recrystalliz alloys. Thus,ation it occurs is acceptable during thathot the higherextrusion strength for produced Al-Mg alloys. at 530 Thus,◦C it recycled is acceptable specimen that the is higher attributed strength to grain produced refinement at 530°C strengthening recycled specimen is attributed to grain reﬁnement strengthening and particle-dispersion strengthening [36]. and particle-dispersion strengthening [36]. Thus, the fine-grained microstructures of recycled specimens Thus, the ﬁne-grained microstructures of recycled specimens from dynamic recrystallization during from dynamic recrystallization during hot working will eventually lead to higher UTS. hot working will eventually lead to higher UTS.

Obvious porosity and boundary lines

i ii (a)

Smaller chip boundaries

i ii (b)

FigureFigure 5. Microstructure 5. Microstructure of of minimum–maximum minimum–maximum parameter: (i ()i bright) bright field; ﬁeld; and and (ii) (underii) under polarized polarized light. (a) S1-T1 (430 °C/60 min); (b) S9-T1 (530 °C/120 min). light. (a) S1-T1 (430 ◦C/60 min); (b) S9-T1 (530 ◦C/120 min).

Materials 2017, 10, 902 9 of 18

Materials 2017, 10, 902 9 of 18 During solutionizing at temperature of 530 ◦C, the cold-deformed specimens led to recrystallization.During solutionizing It was known at thattemperature the higher of the 53 strains,0 °C, thethe lower cold-deformed the recrystallized specimens grain led size to [37 ]. Therecrystallization. results obtained It duringwas known tensile that tests the showshigher thatthe st therains, structure the lower affects the recrystallized mechanical properties. grain size [37]. Coarse structureThe results led toobtained the worst during tensile tensile strength tests in shows comparison that the with structure the finer affects microstructures. mechanical Thisproperties. is proven byCoarse UTS exhibiting structure led only to 14.97 the worst MPa tensile at minimum strength parameter in comparison (S1-T1). with However, the finer microstructures. the grain size is This shown tois be proven the biggest by UTS at exhibiting 61.70 µm. only As 14.97 mentioned MPa at minimum before, the parameter operating (S1-T1). forging However, temperatures the grain for size hot workingis shown were to be selected the biggest to comprise at 61.70 theµm. temperature As mentioned between before, solidusthe operating and recrystallization. forging temperatures In the for case ofhot AA6061 working aluminium were selected alloy physicalto comprise properties the temperat designation,ure between recrystallization solidus and recrystallization. temperature started In the from 450case◦C andof AA6061 solidus aluminium region started alloy at 580physical◦C[27 properties]. This is becausedesignation, during recrystallization recrystallization temperature temperature, thestarted microstructure from 450 condition°C and solidus started region to enter started the at second 580 °C phase [27]. ofThis precipitation. is because during Precipitation recrystallization is achieved bytemperature, heating the alloythe microstructure below the solidus condition line at astarte suitabled to temperatureenter the second and time. phase In thisof precipitation. process, solute atomsPrecipitation diffuse tois formachieved small by precipitates heating the [alloy38]. Thesebelow finethe solidus precipitates line at act a suitable as a barrier temperature for dislocation and time. In this process, solute atoms diffuse to form small precipitates [38]. These fine precipitates act movement, causing an enhancement of initial strength [33]. as a barrier for dislocation movement, causing an enhancement of initial strength [33]. As illustrated in Figure4, when comparing the minimum–maximum parameter (S1-T1 and S9-T1), As illustrated in Figure 4, when comparing the minimum–maximum parameter (S1-T1 and S9-T1), the value of ETF increased dramatically from 0.091% to 16.129%. Figure6 shows the FESEM image of the value of ETF increased dramatically from 0.091% to 16.129%. Figure 6 shows the FESEM image the fractured surfaces of the minimum (SI-T1) and maximum (S9-T1) parameter setting. Magnification of the fractured surfaces of the minimum (SI-T1) and maximum (S9-T1) parameter setting. × × ofMagnification 100 is an overview of 100× is of an the overview fracture of surface, the fractu 250re surface,is the periphery 250× is the coarseperiphery topology coarse oftopology the fracture of surfacethe fracture and 1000 surface× is and equiaxed 1000× dimples.is equiaxed dimples.

Fine grooves, ridges, and fine cups

(a) (d)

Microvoids and dimples

(b) (e)

Figure 6. Cont.

Materials 2017, 10, 902 10 of 18 Materials 2017, 10, 902 10 of 18

Figure 6. FESEM micrographs showing the fracture surface of minimum–maximum parameter S1-T1 Figure 6. FESEM micrographs showing the fracture surface of minimum–maximum parameter S1-T1 (430 °C/60 min) and S9-T1 (530 °C/120 min): (a) S1-T1 (430 °C/60 min); (b) S1-T1 (430 °C/60 min); (430 ◦C/60 min) and S9-T1 (530 ◦C/120 min): (a) S1-T1 (430 ◦C/60 min); (b) S1-T1 (430 ◦C/60 min); (c) S1-T1 (430 °C/60 min); (d) S9-T1 (530 °C/120 min); (e) S9-T1 (530 °C/120 min); and (f) S9-T1 (c) S1-T1 (430 ◦C/60 min); (d) S9-T1 (530 ◦C/120 min); (e) S9-T1 (530 ◦C/120 min); and (f) S9-T1 (530 °C/120 min). (530 ◦C/120 min). At low operating temperatures and holding times (S1-T1), the fractured surfaces obviously revealAt low as flat operating and shallow, temperatures and there and are holding no dimples times at (S1-T1), all. This the type fractured of tear surfacesfracture suggested obviously that reveal asthe flat deformation and shallow, occurs and in there bands are through no dimples the grains, at all. which This is expected type oftear when fracture the dislocation suggested governed that the deformationthe deformation occurs by in cutting bands through through the the strengthening grains, which particles. is expected It is also when observed the dislocation that a little governed gross theof deformation plastic deformation by cutting and throughthe fracture the takes strengthening place along particles. the crystallographic It is also observed plane (cleavage that a little plane) gross ofin plastic which deformation the normal stress and the is maximized. fracture takes It appears place along that theno gross crystallographic plastic deformation plane (cleavage and fracture plane) intakes which place the normal along stressthe crystallographic is maximized. plane It appears (cleavage that noplane) gross on plastic which deformation the normal and stress fracture is takesmaximized. place along At low the magnification, crystallographic these plane flat (cleavageareas exhibit plane) fine ongrooves, which ridges the normal and fine stress cups, is as maximized. shown Atin low Figure magnification, 6a. This supports these flat the areas results exhibit of ETF fine at grooves,which atridges S1-T1, andthe fineresult cups, is as as low shown as 0.09%. in Figure This6 a. Thistype supports of fracture the is results normally of ETF classified at which as brittle at S1-T1, deformation the result [13]. is as As low observed as 0.09%. at maximum This type parameter of fracture is normally(S9-T1), classifiedit consists as of brittle many deformation microvoids [ 13and]. As dimples observed indicating at maximum the ductile parameter fracture (S9-T1), mode. it consists The presence of large and deep dimples in the fractured surface in Figure 6e is evidence for large fluent of many microvoids and dimples indicating the ductile fracture mode. The presence of large and strain of the specimen (ETF 16.13%). However, the dimples did not form uniformly in this condition deep dimples in the fractured surface in Figure6e is evidence for large fluent strain of the specimen and the dimples of different size are apparent in this figure. (ETF 16.13%). However, the dimples did not form uniformly in this condition and the dimples of different3.2. Surface size Integrity are apparent in this figure. 3.2. SurfaceFigure Integrity 7 shows the overall results for the effects of operating temperature and holding time on the surface integrity. Surface integrity is the sum of all the elements that describe all the conditions Figure7 shows the overall results for the effects of operating temperature and holding time on the existing on or at the surface of the material resulting from controlled manufacturing process. surfaceThe surface integrity. integrity Surface of integritymaterial greatly is the sum influences of all the mechanical elements properties that describe of alloys. all the From conditions the results, existing onit or shows at the that surface the value of the of materialmicrohardness resulting increase fromd controlledwith the increment manufacturing of temperature. process. At The holding surface integritytime 60 of min, material the value greatly of microhardness influences mechanical increased properties with increasi of alloys.ng operating From thetemperature, results, it showsyielding that thefrom value S1-T1 of microhardness (69.761 HV) to increasedS7-T1 (77.642 with HV). the The increment same trend of temperature. is observed Atas the holding holding time time 60 at min, themaximum value of of microhardness 120 min: S3-T1 increased (71.455 HV) with surged increasing up to 81.744 operating HV temperature,at S9-T1. The effect yielding of parameter from S1-T1 (69.761obviously HV) gives to S7-T1 the (77.642notable HV).impact The to samethe forgin trendg isprocess observed when as the the increment holding time in microhardness at maximum of 120raised min: up S3-T1 to 11.983 (71.455 HV. HV) From surged the upprevious to 81.744 study, HV the at S9-T1. relationship The effect between of parameter hardness obviouslyand strength gives thewas notable studied impact by several to the researchers forging process [13], which when was the incrementconfirmed inby microhardness the tensile test results raised where up to 11.983similar HV. Fromtrends the were previous observed study, in hardness the relationship and UTS. between Tan and hardnessOgel [37] stated and strength that, as temperature was studied increases, by several researchersmore solid [ 13dissolves], which in was the matrix confirmed for super by the saturation, tensile test due results to increased where diffusion similar trends kinetics were and observedhigher insolubility hardness limits and UTS. at higher Tan andtemperatures. Ogel [37] stated that, as temperature increases, more solid dissolves in the matrixMeanwhile, for super as illustrated saturation, in Figure due to 7, increased a similar diffusionupward trend kinetics for mi andcrohardness higher solubility demonstrated limits at higherthe effect temperatures. of holding time. At the operating temperature 430 °C, the value of microhardness marginallyMeanwhile, raised as with illustrated the increased in Figure of7 holding, a similar time upward of 60 min trend at forS1-T1 microhardness (69.761 HV) to demonstrated 120 min at S3-T1 (71.455 HV). At the maximum operating temperature 530 °C, the value of microhardness the effect of holding time. At the operating temperature 430 ◦C, the value of microhardness marginally

Materials 2017, 10, 902 11 of 18 raised with the increased of holding time of 60 min at S1-T1 (69.761 HV) to 120 min at S3-T1 Materials 2017, 10, 902 11 of 18 (71.455 HV). At the maximum operating temperature 530 ◦C, the value of microhardness slightly ◦ increasedslightly increased from at S7-T1from at (77.642 S7-T1 HV)(77.642 to 81.744HV) to HV 81.744 at S9-T1. HV at Below S9-T1. 530 BelowC, there530 °C, was there a tendency was a towardstendency increasing towards increasing hardness hardness with increasing with incr theeasing holding the time,holding however time, however the trend the was trend opposite was foropposite temperatures for temperatures above and above this and was this agreed was agreed by several by several researchers researchers [28,29 ,[28,29,39].39]. Furthermore, Furthermore, when comparingwhen comparing the minimum–maximum the minimum–maximum parameter parameter (S1-T1 and(S1-T1 S9-T1) and theS9-T1) value the of value microhardness of microhardness increased fromincreased 69.761 from HV 69.761 to 81.744 HV HV. to 81.744 The elastic HV. propertiesThe elastic ofproperties specimens of werespecimens slightly were higher slightly compared higher to thecompared expected to the and expected measured and values measured of the values theoretical of the theoretical value, as a value, result as of a the result precipitation of the precipitation and oxide dispersionand oxide hardening.dispersion Thehardening. material The is stiffer material due is to stiffer the high due oxide to the content high whichoxide iscontent dispersed which within is thedispersed material within [16]. the material [16].

Figure 7. Results of of microhardness microhardness and and density density at at different different operating operating times times and and temperatures. temperatures.

As observed for density analysis in Figure 7, at holding time of 120 min, the value of density As observed for density analysis in Figure7, at holding time of 120 min, the value of density increased with the increasing of operating temperature, showing the increment from S3-T1 (2.16 g/cc) increased with the increasing of operating temperature, showing the increment from S3-T1 (2.16 g/cc) elevated to 2.65 g/cc at S9-T1. At the holding time of 60 min, density raised up from S1-T1 (1.93 g/cc) elevated to 2.65 g/cc at S9-T1. At the holding time of 60 min, density raised up from S1-T1 (1.93 g/cc) to S7-T1 (2.56 g/cc). Meanwhile, a similar ascending trend for density is demonstrated as the effect of to S7-T1 (2.56 g/cc). Meanwhile, a similar ascending trend for density is demonstrated as the effect holding time. At operating temperature of 430 °C, the value of density increased with increasing of holding time. At operating temperature of 430 ◦C, the value of density increased with increasing holding time at S1-T1 (1.92 g/cc) to S3-T1 (2.16 g/cc). At the maximum operating temperature 530 °C, holding time at S1-T1 (1.92 g/cc) to S3-T1 (2.16 g/cc). At the maximum operating temperature 530 ◦C, the value of density slightly increased about from 2.56 g/cc at S7-T1 to 2.65 g/cc at S9-T1. It is the value of density slightly increased about from 2.56 g/cc at S7-T1 to 2.65 g/cc at S9-T1. It is obviously obviously shown that the density increase whenever the holding time and the temperature increase. shown that the density increase whenever the holding time and the temperature increase. Insufﬁcient Insufficient temperature and holding time will eventually result in the alloy having large voids, temperature and holding time will eventually result in the alloy having large voids, which leads to which leads to low density [40]. This retention process is assisted by raising the holding time, low density [40]. This retention process is assisted by raising the holding time, allowing the alloy to allowing the alloy to have better bonding between the chips. Great combination of high temperature have better bonding between the chips. Great combination of high temperature and high holding time and high holding time permits the composite to exert high density. permitsWhen the comparing composite tothe exert minimum–maximum high density. parameter (S1-T1 and S9-T1), as shown in Figure 7, the valueWhen of comparing density increased the minimum–maximum from 1.92 g/cc to 2.65 parameter g/cc. This (S1-T1 can be and referred S9-T1), to as the shown microstructure in Figure7 , theanalysis; value the of density obvious increased porosity fromin Figure 1.92 g/cc5a gives to 2.65 the g/cc. lower This density can bevalue referred at minimum to the microstructure parameter, analysis;while closer the grain obvious boundary porosity structure in Figure in 5Figurea gives 5b the at maximum lower density parameter value atproduced minimum high parameter, density. Unfortunately, the value of maximum density only exhibited 2.65 g/cc at maximum parameter (S9-T1) which it was below the theoretical value; 2.7 g/cc (AA6061 T4—from ASM, 2009). This phenomena agreed well with the study from Ceschini et al. [41], which showed a slight reduction of the porosity,

Materials 2017, 10, 902 12 of 18 while closer grain boundary structure in Figure5b at maximum parameter produced high density. Unfortunately, the value of maximum density only exhibited 2.65 g/cc at maximum parameter (S9-T1) which it was below the theoretical value; 2.7 g/cc (AA6061 T4—from ASM, 2009). This phenomena agreed well with the study from Ceschini et al. [41], which showed a slight reduction of the porosity, afterMaterials forging, 2017 without, 10, 902 evidence of material damage induced by the plastic deformation, both in12 termsof 18 of reinforcement/matrix de-cohesion or particle cracking. Moreover, the high oxide content of the chips canafter explain forging, this. without Fine-form evidence scrap of has material a very damage high surface induced area-to-volume by the plastic deformation, ratio, and, both thus, in theterms oxide contentof reinforcement/matrix is much higher than de-cohesion for solid samples or particle [16 ].cracking. Moreover, the high oxide content of the chips can explain this. Fine-form scrap has a very high surface area-to-volume ratio, and, thus, the In addition, the maximum UTS peaked at 266.78 MPa (S9-T1), which shows an increment of 9.27% oxide content is much higher than for solid samples [16]. from the theoretical value (240 MPa (Theory T4)), as shown in Figure8. Unfortunately, the value of In addition, the maximum UTS peaked at 266.78 MPa (S9-T1), which shows an increment of maximum ETF only exhibited 16.129% at maximum parameter (S9-T1) which was 22% below the 9.27% from the theoretical value (240 MPa (Theory T4)), as shown in Figure 8. Unfortunately, the theoreticalvalue of value maximum (Theory ETF T4). only The exhi valuebited 16.129% of maximum at maximum microhardness parameter reaches (S9-T1) 81.744which HVwas (S9-T1),22% below which is 20.48%the theoretical above of value the theoretical (Theory T4). (65 The HV) value (Theory of ma T4).ximum The microhardness increment of UTS reaches and 81.744 microhardness HV (S9-T1), value clarifywhich the is potential 20.48% above ability of the of hottheoretical press process(65 HV) to(Theory give aT4). notable The increment potential of toUTS be and utilized microhardness as one of the recyclingvalue clarify method the of potential aluminium. ability of hot press process to give a notable potential to be utilized as one of the recycling method of aluminium. 3.3. Post-Process (Artificial Aging-T5 Temper) 3.3. Post-Process (Artificial Aging-T5 Temper) The desired mechanical properties of forgings can be obtained only by means of final heat treatment.The By desired performing mechanical heat treatmentproperties toof theforgings recycled can samplebe obtained immediately only by means after hot of pressfinal heat process S10-T5,treatment. it is interesting By performing to note heat thattreatment the strength to the recycled behaviour sample changes immediately dramatically after hot comparedpress process to the heat-treatedS10-T5, it sample, is interesting reaching to note the strengththat the streng of 340.47th behaviour MPa and changes the ETF dramatically increased to 21.70%,compared as to shown the in Figureheat-treated9. This value sample, is comparable reaching the to strength as-received of 340.47 AR-T6. MPa The and maximum the ETF increased hardness to exhibited21.70%, as 98.649shown HV in Figure 9. This value is comparable to as-received AR-T6. The maximum hardness exhibited 98.649 HV for heat-treated sample S10-T5 at 120 min aging time and 175 ◦C aging temperature. An increase in for heat-treated sample S10-T5 at 120 min aging time and 175 °C aging temperature. An increase in hardness and strength could be explained by a diffusion assisted mechanism, and by the hindrance hardness and strength could be explained by a diffusion assisted mechanism, and by the hindrance of dislocationof dislocation by by impurity impurity atoms, atoms, i.e., i.e., foreignforeign particle of of the the second second phase, phase, since since the the material material after after ◦ quenchingquenching from from 530 530C °C (solution (solution heatheat treatment)treatment) will will have have excessive excessive vacancy vacancy concentration concentration [42]. [42 ]. RafiqRafiq et al. et [ 43al. ][43] proved proved that that the the increase increase of of aging aging time time and and temperature temperature will will leadlead toto thethe increasing of of GP ZoneGP density. Zone density. Hence, theHence, degree the ofdegree irregularity of irregularity in the lattices in the willlattices cause will an cause increase an increase in the mechanical in the propertiesmechanical of the properties aluminium of the alloy. aluminium Compared alloy. Compared to the maximum to the maximum value of value microhardness of microhardness at S10-T5, the valueat S10-T5, is 3.18% the value greater is 3.18% to as-received greater to as-received AR-T6 (95.512 AR-T6 HV). (95.512 HV).

Figure 8. Results of recycled aluminium and theoretical value and as-received AA6061: (a) ultimate Figure 8. Results of recycled aluminium and theoretical value and as-received AA6061: (a) ultimate tensile strength; (b) elongation to failure; and (c) microhardness. tensile strength; (b) elongation to failure; and (c) microhardness.

Materials 2017, 10, 902 13 of 18 Materials 2017, 10, 902 13 of 18

FigureFigure 9. Results9. Results of of heat heat treated treated recycled recycled aluminiumaluminium and and as-received as-received AA6061: AA6061: (a) (ultimatea) ultimate tensile tensile strength;strength; (b) ( elongationb) elongation to to failure; failure; and and ( c(c)) microhardness.microhardness.

Consequently, Figure 10 shows the FESEM images of the fractured surfaces for S10-T5 at 120 min ofConsequently, aging and as-received Figure 10 showsAR-T6. theFrom FESEM S10-T5, images fractured of the surfaces fractured reveal surfaces as flat, for S10-T5shallow, at 120and min of agingfeatureless and as-received dimples. This AR-T6. type of From tear S10-T5,fracture fracturedsuggests that surfaces the deformation reveal as flat, occurs shallow, in bands and through featureless dimples.the grains, This which type ofis tearexpected fracture as suggestsdislocation that governs the deformation the deformation, occurs cutting in bands through through the the grains,strengthening which is expected particles [44]. as dislocation At high magnification, governs the these deformation, flat areas cuttingexhibit fine through grooves, the ridges strengthening and particlesfine cups, [44]. as At shown high magnification, in Figure 10b. theseThis type flat areasof fracture exhibit is normally fine grooves, classi ridgesfied as and brittl finee deformation cups, as shown in Figure[13]. This 10b. parallels This type the of results fracture for isETF normally S10-T5, classifiedwhich was as 22.39% brittle lower deformation than the [as-received13]. This parallels AR-T6. the resultsIn as-received for ETF S10-T5, AR-T6 whichimages, was numerous 22.39% microvoids lower than and the dimples as-received can be AR-T6. observed, In as-received indicating the AR-T6 images,ductile numerous fracture mode. microvoids The presence and dimples of large can and be deep observed, dimples indicating in the fractured the ductile surface fracture in Figure mode. 10f The proved the large fluent strain of the specimen, which is 24.62%. However, the dimples are not formed presence of large and deep dimples in the fractured surface in Figure 10f proved the large fluent strain uniformly in this condition and the dimples of different sizes are apparent in this figure. of the specimen, which is 24.62%. However, the dimples are not formed uniformly in this condition and the dimples of different sizes are apparent in this figure. The EDX analysis of the white regions on the fractured surfaces in Table1 revealed the presence of elements Fe, Mn and Cu in the basis phase. As anticipated, phases containing significant quantities of heavy elements appeared to be brighter than phases containing light elements (lower atomic number elements) [45]. To detect the constituting phases made up of these elements, X-ray diffraction (XRD) analysis was performed for both solutionised S9-T1 and heat treated recycled billet S10-T5 (Figure 11). X-ray diffraction (XRD) technique can be used to characterize the microstructural evolutions in terms of studying the maximum peak intensity/changes. Based on composition, the predominant equilibrium second phase in AA6061 are shown to be Al, AlFeSi and AlFeMn, as depicted in the Appendix. Comparing the intensity of the detected phases for S9-T1 and S10-T5 heat treated sample, intensity of the (111) and (200) pole increased, as shown in Figure 11. Shokuhfar et al. [45], Kuijpers et al. [46], and B. Lin et al. [47] define(a) (200) pole as β-AlFeSi, which is also known as( ad) hard phase. This analysis demonstrated the relative number of phases in the alloy confirms that the β-AlFeSi phase increased by heat treatment at S10-T5. The solutionised sample S9-T1 shows that the precipitates are not completely dissolved. It is known that grain and sub-grain boundary migration has an important role in the development of the (200) texture [48]. The soft phase has less resistance to the plastic flow than the hard phase. Moreover, the load redistribution is taken up by the hard phase which also weakens the ductility and workability of 6xxx alloys. In addition to AlFeSi phase, in 6xxx aluminium alloys, Mn is interchangeable with Fe atoms to form the α-Al(FeMn)Si phase. As the contrast between these two phases is very low, distinguishing the phase type is very difficult by automated systems [49]. Shokuhfar et al. [45] and Ezatpour et al. [48] agreed with these results, demonstrating the relative amount of phases in the alloy confirms that the hard β-AlFeSi phase increased by heat treatment. This observation confirms both increasing strength and microhardness Materials 2017, 10, 902 13 of 18

Figure 9. Results of heat treated recycled aluminium and as-received AA6061: (a) ultimate tensile strength; (b) elongation to failure; and (c) microhardness.

Consequently, Figure 10 shows the FESEM images of the fractured surfaces for S10-T5 at 120 min of aging and as-received AR-T6. From S10-T5, fractured surfaces reveal as flat, shallow, and featureless dimples. This type of tear fracture suggests that the deformation occurs in bands through the grains, which is expected as dislocation governs the deformation, cutting through the strengthening particles [44]. At high magnification, these flat areas exhibit fine grooves, ridges and fine cups, as shown in Figure 10b. This type of fracture is normally classified as brittle deformation Materials[13]. 2017This, 10parallels, 902 the results for ETF S10-T5, which was 22.39% lower than the as-received AR-T6.14 of 18 In as-received AR-T6 images, numerous microvoids and dimples can be observed, indicating the ductile fracture mode. The presence of large and deep dimples in the fractured surface in Figure 10f after heat treatment. Therefore, the increased strength and microhardness, comparing S9-T1 and heat proved the large fluent strain of the specimen, which is 24.62%. However, the dimples are not formed treated S10-T5 condition, are due to the increase in the hard phase. uniformly in this condition and the dimples of different sizes are apparent in this figure.

Materials 2017, 10, 902 14 of 18 (a) (d)

Flat, shallow, and featureless dimples

(b) (e)

Large and deep dimples

Figure 10. FESEM micrographs showing the fracture surface of the profiles at the heat-treated sample Figure 10. FESEM micrographs showing the fracture surface of the proﬁles at the heat-treated S10-T5 and as-received AR-T6: (a) S10-T5 (heat treated)−100×; (b) S10-T5 (heat treated)-250×; sample S10-T5 and as-received AR-T6: (a) S10-T5 (heat treated)−100×;(b) S10-T5 (heat treated)-250×; (c) S10-T5 (heat treated)-1000×; (d) AR-T6 (As-received)-100×; (e) AR-T6 (As-received)-250×; and (c) S10-T5 (heat treated)-1000×;(d) AR-T6 (As-received)-100×;(e) AR-T6 (As-received)-250×; and (f) AR-T6 (As-received)-1000×. (f) AR-T6 (As-received)-1000×. The EDX analysis of the white regions on the fractured surfaces in Table 1 revealed the presence of elements Fe, Mn and Cu in the basis phase. As anticipated, phases containing significant quantities of heavy elements appeared to be brighter than phases containing light elements (lower atomic number elements) [45]. To detect the constituting phases made up of these elements, X-ray diffraction (XRD) analysis was performed for both solutionised S9-T1 and heat treated recycled billet S10-T5 (Figure 11). X-ray diffraction (XRD) technique can be used to characterize the microstructural evolutions in terms of studying the maximum peak intensity/changes. Based on composition, the predominant equilibrium second phase in AA6061 are shown to be Al, AlFeSi and AlFeMn, as depicted in the Appendix. Comparing the intensity of the detected phases for S9-T1 and S10-T5 heat treated sample, intensity of the (111) and (200) pole increased, as shown in Figure 11. Shokuhfar et al. [45], Kuijpers et al. [46], and B. Lin et al. [47] define (200) pole as β-AlFeSi, which is also known as a hard phase. This analysis demonstrated the relative number of phases in the alloy confirms that the β-AlFeSi phase increased by heat treatment at S10-T5. The solutionised sample S9-T1 shows that the precipitates are not completely dissolved. It is known that grain and sub-grain boundary migration has an important role in the development of the (200) texture [48]. The soft phase has less resistance to the plastic flow than the hard phase. Moreover, the load redistribution is taken up by the hard phase which also weakens the ductility and workability of 6xxx alloys. In addition to AlFeSi phase, in 6xxx aluminium alloys, Mn is interchangeable with Fe atoms to form the α-Al(FeMn)Si phase. As the contrast between these two phases is very low, distinguishing the phase type is very difficult by automated systems [49]. Shokuhfar et al. [45] and Ezatpour et al. [48] agreed with these results, demonstrating the relative amount of phases in the alloy confirms that the hard β-AlFeSi phase increased by heat treatment. This observation confirms both increasing strength and microhardness

Materials 2017, 10, 902 15 of 18

after heat treatment. Therefore, the increased strength and microhardness, comparing S9-T1 and heat Materials 2017 10 treated, S10-T5, 902 condition, are due to the increase in the hard phase. 15 of 18

FigureFigure 11. 11.XRD XRD spectra spectra of of recycled recycled specimenspecimen for for S9-T1 S9-T1 and and S10-T5. S10-T5.

4. Conclusions4. Conclusion Conclusively, direct recycled AA6061-T6 chip by utilizing hot press process shows great effects Conclusively, direct recycled AA6061-T6 chip by utilizing hot press process shows great effects on on the mechanical properties and surface integrity. The conclusions can be summarized as follow: the mechanical properties and surface integrity. The conclusions can be summarized as follow: 1. Results on the effect of operating temperature give most significant impact to the forging process: 1. Results on the effect of operating temperature give most signiﬁcant impact to the forging process: • UTS increased 89.51–93.35% from 14.97–266.78 MPa by increasing the operating temperature from 430 °C to 530 °C. • UTS increased 89.51–93.35% from 14.97–266.78 MPa by increasing the operating temperature • ETF increased 86.32–98.67% from 0.091–16.12% by increasing the operating temperature from 430 ◦C to 530 ◦C. from 430 °C to 530 °C. • •ETF Microhardness increased 86.32–98.67% reached 7.88–10. from28 0.091–16.12% HV by increasing by increasing the operating the operating temperature temperature from ◦ ◦ from430 430 °C toC 530 to 530 °C. C. • ◦ 2. MaximumMicrohardness mechanical reached properties 7.88–10.28 and surface HV by integrit increasingy of recycled the operating chip (T1-temper) temperature are considered from 430 C ◦ comparableto 530 C. with theoretical AA6061 T4-temper: 2. Maximum• Maximum mechanical UTS properties are notably and raised surface up integrity to 9.27% of recycled(266.78 MPa) chip (T1-temper)at parameter are condition considered comparable(530 with°C/120 theoretical min). AA6061 T4-temper: • Maximum hardness surged up to 20.48% (81.74 HV) at parameter condition (530 °C/120 min). 3.• HeatMaximum treated recycled UTS are billet notably (T5-temper) raised are up co tonsidered 9.27% (266.78comparable MPa) with at as-received parameter AA6061 condition ◦ T6-temper:(530 C/120 min). • Maximum hardness surged up to 20.48% (81.74 HV) at parameter condition • Value◦ of microhardness peak after 175 °C for 120 min aging increased 3.18% (98.64 HV). •(530 ValueC/120 of UTS min). (340.47 MPa) and ETF (21.70%) reached close to the as-received value. 3. HeatAlthough treated this recycled is only the billet foundation (T5-temper) of experime arental considered observations, comparable its potential as with an alternative as-received techniqueAA6061T6-temper: to the direct recycling methods for producing scrap can be clearly demonstrated. This analysis justified that hot press process evidently has significant benefits compared to conventional recycling.• Value Improved of microhardness energy efficiency peak after is current 175 ◦Cly for the 120 focus min of aging on-going increased research, 3.18% which (98.64 allows HV). • Value of UTS (340.47 MPa) and ETF (21.70%) reached close to the as-received value.

Although this is only the foundation of experimental observations, its potential as an alternative technique to the direct recycling methods for producing scrap can be clearly demonstrated. This analysis justified that hot press process evidently has significant benefits compared to conventional recycling. Improved energy efficiency is currently the focus of on-going research, which allows further Materials 2017, 10, 902 16 of 18 impact reductions. This study offers perspectives for industrial development of solid state recycling processes as environmentally benign alternatives for the current melting based practices.

Acknowledgments: The authors would like to express the deepest appreciation to the Ministry of Higher Education (MOHE), Malaysia, for funding this project through the Fundamental Research Grant Schemes (FRGS, vot numbers 1426, 1463, and 1496). Additional support in terms of facilities was also provided by Sustainable Manufacturing and Recycling Technology, Advanced Manufacturing and Materials Center (SMART-AMMC), Universiti Tun Hussein Onn Malaysia (UTHM). Author Contributions: N.K.Y. performed the experiments, analysed the data and prepared the manuscript. A.A. jointly conceived the study with N.K.Y. and gave a technical support. M.A.L. supervised its analysis and edited the manuscript. Conﬂicts of Interest: The authors declare no conﬂict of interest.

References

1. National Research Council. Advancing the Science of Climate Change; The National Academies Press: Washington, DC, USA, 2010. [CrossRef] 2. Jayal, D.; Badurdeen, F.; Dillon, O.W.; Jawahir, I.S. Sustainable manufacturing: Modeling and optimization challenges at the product, process and system levels. CIRP J. Manuf. Sci. Technol. 2010, 2, 144–152. [CrossRef] 3. Rashid, M.W.A.; Yacob, F.F.; Lajis, M.A.; Abid, M.A.A.M.; Mohamad, E.; Ito, T. A review: The potential of powder metallurgy in recycling aluminum chips (Al 6061 & Al 7075). In Proceedings of the 24th Design Engineering Systems Division JSME Conference Japan Society of Mechanical Engineers, Tokushima, Japan, 17–19 September 2014. 4. Fuziana, Y.F.; Warikh, A.R.M.; Lajis, M.A.; Azam, M.A.; Muhammad, N.A. Recycling aluminium (Al 6061) chip through powder metallurgy route. Mater. Res. Innov. 2014, 18, 354–358. [CrossRef] 5. Mashhadi, H.A.; Moloodi, A.; Golestanipour, M.; Karimi, E.Z.V. Recycling of aluminium alloy turning scrap via cold pressing and melting with salt flux. J. Mater. Process. Technol. 2009, 209, 3138–3142. [CrossRef] 6. Schlesinger, M.E. Aluminum Recycling; CRC Press: Boca Raton, FL, USA, 2013. 7. Hatayama, H.; Daigo, I.; Matsuno, Y.; Adachi, Y. Evolution of aluminum recycling initiated by the introduction of next-generation vehicles and scrap sorting technology. Resour. Conserv. Recycl. 2012, 66, 8–14. [CrossRef] 8. Gronostajski, J.Z.; Kaczmar, J.W.; Marciniak, H.; Matuszak, A. Direct recycling of aluminium chips into extruded products. J. Mater. Process. Technol. 1997, 64, 149–156. [CrossRef] 9. Fogagnolo, J.; Ruiz-Navas, E.; Simón, M.; Martinez, M. Recycling of aluminium alloy and aluminium matrix composite chips by pressing and hot extrusion. J. Mater. Process. Technol. 2003, 143, 792–795. [CrossRef] 10. Haase, M.; Tekkaya, A.E. Cold extrusion of hot extruded aluminum chips. J. Mater. Process. Technol. 2015, 217, 356–367. [CrossRef] 11. Gronostajski, J.; Marciniak, H.; Matuszak, A. New methods of aluminium and aluminium-alloy chips recycling. J. Mater. Process. Technol. 2000, 106, 34–39. [CrossRef] 12. Tekkaya, A.E.; Schikorra, M.; Becker, D.; Biermann, D.; Hammer, N.; Pantke, K. Hot profile extrusion of AA-6060 aluminum chips. J. Mater. Process. Technol. 2009, 209, 3343–3350. [CrossRef] 13. Güley, V.; Güzel, A.; Jäger, A.; Khalifa, N.B.; Tekkaya, A.E.; Misiolek, W.Z. Effect of die design on the welding quality during solid state recycling of AA6060 chips by hot extrusion. Mater. Sci. Eng. A 2013, 574, 163–175. [CrossRef] 14. Ab Rahim, S.N.; Lajis, M.A.; Ariffin, S. A review on recycling aluminum chips by hot extrusion process. Procedia CIRP 2015, 26, 761–766. [CrossRef] 15. Ab Rahim, S.N.; Lajis, M.A.; Ariffin, S. Effect of extrusion speed and temperature on hot extrusion process of 6061 aluminum alloy chip. ARPN J. Eng. Appl. Sci. 2016, 11, 2272–2277. 16. Paraskevas, D.; Vanmeensel, K.; Vleugels, J.; Dewulf, W.; Deng, Y.; Duflou, J.R. Spark plasma sintering as a solid-state recycling technique: The case of aluminum alloy scrap consolidation. Materials 2014, 7, 5664–5687. [CrossRef] 17. Paraskevas, D.; Dadbakhsh, S.; Vleugels, J.; Vanmeensel, K.; Dewulf, W.; Duflou, J.R. Solid state recycling of pure Mg and AZ31 Mg machining chips via spark plasma sintering. Mater. Des. 2016, 109, 520–529. [CrossRef] Materials 2017, 10, 902 17 of 18

18. Ingarao, G.; Di Lorenzo, R.; Micari, F. Sustainability issues in sheet metal forming processes: An overview. J. Clean. Prod. 2011, 19, 337–347. [CrossRef] 19. Duﬂou, J.R.; Tekkaya, A.E.; Haase, M.; Welo, T.; Vanmeensel, K.; Kellens, K.; Dewulf, W.; Paraskevas, D. Environmental assessment of solid state recycling routes for aluminium alloys: Can solid state processes signiﬁcantly reduce the environmental impact of aluminium recycling? CIRP Ann. Manuf. Technol. 2015, 64, 37–40. [CrossRef] 20. Yusuf, N.K.; Lajis, M.A.; Daud, M.I.; Noh, M.Z. Effect of Operating Temperature on Direct Recycling Aluminium Chips (AA6061) in Hot Press Forging Process. Appl. Mech. Mater. 2013, 315, 728–732. [CrossRef] 21. Lajis, M.A.; Khamis, S.S.; Yusuf, N.K. Optimization of Hot Press Forging Parameters in Direct Recycling of Aluminium Chip (AA 6061). Key Eng. Mater. 2014, 622, 223–230. 22. Ahmad, A.; Lajis, M.A.; Yusuf, N.K.; Wagiman, A. Hot Press Forging as The Direct Recycling Technique of Aluminium—A Review. ARPN J. Eng. Appl. Sci. 2016, 11, 2258–2265. 23. Yusuf, N.K. Effects of Hot Press Forging Parameter And Life Cycle Assessment In Direct Recycling of AA6061 Aluminium. Ph.D. Thesis, Universiti Tun Hussein Onn Malaysia, Johor, Malaysia, May 2017. 24. Naka, T.; Nakayama, Y.; Uemori, T.; Hino, R.; Yoshida, F. Effects of temperature on yield locus for 5083 aluminum alloy sheet. J. Mater. Process. Technol. 2003, 140, 494–499. [CrossRef] 25. Tiryakiouglu, M.; Campbell, J.; Staley, J.T. On macrohardness testing of Al-7 wt % Si-Mg alloys: II. An evaluation of models for hardness-yield strength relationships. Mater. Sci. Eng. A 2003, 361, 240–248. [CrossRef] 26. Zhang, H.; Li, L.; Yuan, D.; Peng, D. Hot deformation behavior of the new Al-Mg-Si-Cu aluminum alloy during compression at elevated temperatures. Mater. Charact. 2007, 58, 168–173. [CrossRef] 27. Handbook Committee. ASM Handbook; ASM International: Materials Park, OH, USA, 2006; Volume 4. 28. Monteiro, V.M.; Diniz, S.B.; Meirelles, B.G.; Silva, L.C.; Paula, A.S. Microstructural and Mechanical study of aluminium alloys submitted to distinct soaking times during solution heat treatment. Tecnol. Metal. Mater. Miner. 2014, 11, 332–339. [CrossRef] 29. Pezda, J. The effect of the T6 heat treatment on hardness and microstructure of the En Ac-AlSi12CuNiMg alloy. Metalurgija 2014, 53, 63–66. 30. Friis, J.; Marthinsen, K.; Furu, T.; Nes, E.; Myhr, O.R.; Grong, Ø.; Holmedal, B.; Ryen, Ø. Work hardening behaviour of heat-treatable Al-Mg-Si-alloys. Mater. Sci. Forum 2006.[CrossRef]

31. Van de Langkruis, J.; Kuijpers, N.C.W.; Kool, W.H.; Vermolen, F.J.; van der Zwaag, S. Modelling Mg2Si Dissolution in an AA6063 Alloy During Pre-heating to the Extrusion Temperature. In Proceedings of the International Aluminum Extrusion Technology Seminar, Chicago, IL, USA, 16–19 May 2000. 32. Rometsch, P.A.; Schaffer, G.B. An age hardening model for Al-7Si-Mg casting alloys. Mater. Sci. Eng. A 2002, 325, 424–434. [CrossRef] 33. Meng, C. Effect of Preheating Condition on Strength of AA6060 Aluminium Alloy for Extrusion. Master’s Thesis, Auckland University of Technology, Auckland, New Zealand, 2010. 34. Oppenheim, T.; Tewﬁc, S.; Scheck, T.; Klee, V.; Lomeli, S.; Dahir, W.; Youngren, P.; Aizpuru, N.; Clark, R.; Lee, E.W.; et al. On the correlation of mechanical and physical properties of 6061-T6 and 7249-T76 aluminum alloys. Eng. Fail. Anal. 2007, 14, 218–225. [CrossRef] 35. Russell, A.M.; Lee, K.L. Structure-Property Relations in Nonferrous Metals; John Wiley & Sons: Hoboken, NJ, USA, 2005. 36. Chino, Y.; Mabuchi, M.; Otsuka, S.; Shimojima, K.; Hosokawa, H.; Yamada, Y.; Wen, C.; Iwasaki, H. Corrosion and mechanical properties of recycled 5083 aluminum alloy by solid state recycling. Mater. Trans. 2003, 44, 1284–1289. [CrossRef] 37. Tan, E.; Ogel, B. Inﬂuence of heat treatment on the mechanical properties of AA6066 alloy. Turkish J. Eng. Environ. Sci. 2007, 31, 53–60. 38. Rashid, M. Mathematical Modeling and Optimization of Precipitation Hardening of Extrudable Aluminum Alloys. Ph.D. Thesis, King Fahd University of Petroleum & Minerals, Dhahran, Saudi Arabia, June 1997. 39. Krishnan, K.N. The effect of post weld heat treatment on the properties of 6061 friction stir welded joints. J. Mater. Sci. 2002, 7, 473–480. [CrossRef] 40. Moustafa, S.; Daoush, W.; Ibrahim, A.; Neubaur, E. Hot Forging and Hot Pressing of AlSi Powder Compared to Conventional Powder Metallurgy Route. Mater. Sci. Appl. 2011, 2, 1127. [CrossRef] Materials 2017, 10, 902 18 of 18

41. Ceschini, L.; Minak, G.; Morri, A.; Tarterini, F. Forging of the AA6061/23 vol.%Al2O3p composite: Effects on microstructure and tensile properties. Mater. Sci. Eng. A 2009, 513, 176–184. [CrossRef] 42. Demir, H.; Gündüz, S. The effects of aging on machinability of 6061 aluminium alloy. Mater. Des. 2009, 30, 1480–1483. [CrossRef] 43. Siddiqui, R.A.; Abdullah, H.A.; Al-Belushi, K.R. Influence of aging parameters on the mechanical properties of 6063 aluminium alloy. J. Mater. Process. Technol. 2000, 102, 234–240. [CrossRef] 44. Risanti, D.D.; Yin, M.; del Castillo, P.E.J.R.D.; van der Zwaag, S. A systematic study of the effect of interrupted ageing conditions on the strength and toughness development of AA6061. Mater. Sci. Eng. A 2009, 523, 99–111. [CrossRef] 45. Shokuhfar, A.; Nejadseyfi, O. A comparison of the effects of severe plastic deformation and heat treatment on the tensile properties and impact toughness of aluminum alloy 6061. Mater. Sci. Eng. A 2014, 594, 140–148. [CrossRef] 46. Kuijpers, N.C.W.; Kool, W.H.; Koenis, P.T.G.; Nilsen, K.E.; Todd, I.; van der Zwaag, S. Assessment of different techniques for quantification of α-Al(FeMn)Si and β-AlFeSi intermetallics in AA 6xxx alloys. Mater. Charact. 2002, 49, 409–420. [CrossRef] 47. Lin, B.; Zhang, W.; Zhao, Y.; Li, Y. Solid-state transformation of Fe-rich intermetallic phases in Al-5.0Cu-0.6Mn squeeze cast alloy with variable Fe contents during solution heat treatment. Mater. Charact. 2015, 104, 124–131. [CrossRef] 48. Ezatpour, H.R.; Haddad Sabzevar, M.; Sajjadi, S.A.; Huang, Y. Investigation of work softening mechanisms and texture in a hot deformed 6061 aluminum alloy at high temperature. Mater. Sci. Eng. A 2014, 606, 240–247. [CrossRef] 49. Kuijpers, N.C.W.; Vermolen, F.J.; Vuik, C.; Koenis, P.T.G.; Nilsen, K.E.; van der Zwaag, S. The dependence of the α-AlFeSi to β-Al(FeMn)Si transformation kinetics in Al-Mg-Si alloys on the alloying elements. Mater. Sci. Eng. A 2005, 394, 9–19. [CrossRef]

© 2017 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).