Solid state recycling of Mg–Gd–Y–Zn–Zr alloy chips by spark plasma sintering

Bing Li,Bugng Teng,b,∗,Ziqing Zhu

aSchool of Materials Science and Engineering,Harbin Institute of Technology,Harbin 150001,China

b National Key Laboratory for Precision Hot Processing of Metals,Harbin Institute of Technology,Harbin 150001,China

Received 14 July 2019;received in revised form 5 September 2019;accepted 12 September 2019 Available online 24 June 2020

Abstract The consolidation Mg–Gd–Y–Zn–Zr billets containing long period-stacking ordered(LPSO)phase were recycled from the metal chips through the spark plasma sintering(SPS)process,which achieves the effective metallurgical bonding between metal chips.The effects of the sintering parameters on the microstructure characteristic and mechanical properties of the recycled billets were studied.The metal chips were effectively bonded in the recycled billets sintered at 500°C,however,the metal chips partly melted into semi-solid state as sintering temperature increased to 550°C.The oxidation films of rare earth(RE)element formed at the bond interface between metal chips during SPS recycling process.The consolidation recycled billets through SPS demonstrated the rival compression failure strain and superior compression stress compared with the referenced cast alloy.The lamellar 14H-LPSO phases hardly precipitate in the vicinity of the bond interface between metal chips after heat treatment with air cooling.However,the furnace cooling facilitates the precipitation of 14H-LPSO phases within the α-Mg grains,even theα-Mg matrix adjacent to the bond interface.The oxide films at the bond interface between the metal chips were zigzagged and fragmented during the isothermal compression.The cracks or holes were hardly observed adjacent to the bond interface during isothermal compression,which reveals superior bond properties and deformation consistency performance between metal chips.

Keywords:Solid state recycling;Spark plasma sintering;Heat treatment;Bond interface.

1.Introduction

The recycling of nonferrous metal or rare metals was attracting more attention due to the trend of utilizing nonrenewable resources effectively.The recycling method can be classified as liquid state recycling and solid state recycling[1].The liquid state recycling refers to that scrap metals were re-melted to obtain the recycled billet.However,the liquid state recycling through the re-melting was energy-intensive due to the massive consumption of fuel and emissions of hazardous gas[2].Furthermore,a large amounts of scrap metals were oxidized and burnt out,and the covering and refining agent were also required during the melting process,which can result in the lower utilization rate and higher cost.The solid state recycling refers to that scrap metals were recycled without melting process.Compared with the liquid state recycling,the solid state recycling can increase utilization rate and efficiency,decrease the loss of scrap metals.Currently the solid state recycling was achieved through cold or hot pre-pressing followed by hot extrusion[3].The bond between scrap metals was mechanical occlusion rather than metallurgical combination during cold or hot pre-pressing[4].Furthermore,only the higher compressive forces accompanied with long hold time is far from enough to achieve the superior bonding between metal chips during cold or hot pressing[5],and the relatively low densification of the recycled billets indicates the inferior mechanical properties of the recycled billets through the conventional solid state recycling method[6].

The spark plasma sintering(SPS),as a rapid consolidation method,was widely used for the consolidation of advanced materials powders[7,8].The high energy pulsed Direct Current(DC)applied to the powder particles enables the individual particles to uniformly generate Joule heat.The spark plasma generated by the discharge of the pulsed DC current between the powders activates the surface of the particles,and the SPS process can utilize the Joule heat generated by the powder after being energize,thus resulting in higher heating efficiency[9–12].Park et al.[13]fabricated the Al–Mg composite materials through the SPS process with pure Al and Mg powders,which can act as lightweight and high-efficiency materials in the automobile and aerospace.Minarik et al.[14]prepared the AE42 magnesium through SPS process and concluded that SPS temperature can significantly affect the mechanical properties of the fabricated materials through modifying the morphology and size of A11RE3particles.In addition,it is concluded that spheroidization kinetics of the A11RE3secondary phase particles during SPS was obviously faster than that during the conventional annealing.Guan et al.[15]improved the ductility and strength of nanostructured Mg alloy through SPS process which was incorporated with in-situ powder casting.Furthermore,the effects of the post-SPS treatment on the mechanical properties of the recycled billets were also studied.Mathieu et al.[16]optimizes the mechanical properties of the SPS Mg alloy by post-sintering in-situ precipitation treatment.Zhang et al.[17]fabricated the Mg matrix composites reinforced with Al and nano SiC particles through SPS and following hot extrusion.From all mentioned above,the SPS process were mainly implemented in the consolidation process of power particles and fabrication of Mg matrix composite.However,the research regarding SPS consolidation recycling of scrap metals was rarely reported,only Paraskevas et al.[18,19]recycled the aluminum alloy scrap,pure Mg and AZ31 machining chips through the SPS process.The Mg-RE alloy has attracted more researchers’attention due to its higher specific strength and better heat resistance[20,21].The RE elements were inclined to segregate during the melting process of the Mg-RE alloy,and thus Mg-RE alloys are difficult to be recycled through the liquid state recycling process[22].In addition,it is necessary to recycle the Mg-RE alloy through SPS due to the higher value of the Mg-RE alloy.However,the consolidation recycling of the Mg-RE alloy via SPS has not been documented so far.Furthermore,the Mg-RE alloy containing LPSO phases has received more attention due to its strengthening effect of the LPSO phases[23,24].The addition of the massive RE and Zn element not only facilitates the precipitation of the intragranular lamellar LPSO phase,but also contribute to the formation of the interdendritic LPSO phase[25–27].Furthermore,the SPS recycling of the Mg-RE alloy containing LPSO phases demonstrated great economic benefits due to the massive addition of the RE element.Thus,the SPS recycling of Mg–Gd–Y–n–Zr containing LPSO phases should be studied to obtain fully densified recycled billets with superior mechanical properties.

Table 1The actual composition of Mg-7Gd-4Y-2Zn-0.4Zr alloy(wt%).

The paper systematically fabricated the consolidation Mg–Gd–Y–Zn–Zr alloy billets directly from the metal chips through SPS process.The effects of the sintering parameters on the microstructure and mechanical properties of consolidation recycled Mg–Gd–Y–Zn–Zr billet,including sintering temperature and time,were studied.The bond mechanism between metal chips during the SPS process and the effect of heat treatment on the recycled Mg–Gd–Y–Zn–Zr billets were also illuminated.In addition,the evolution of the bond interface between metal chips during the isothermal compression was also investigated.The paper can provide a basis for understanding and guidance of the spark plasma sintering application on the solid state recycling of Mg-RE alloy.

2.Experiment procedure

2.1.Materials

The selected material in the paper was cast Mg–7Gd–4Y–2Zn–0.4Zr alloy.The actual chemical composition of the selected alloy was demonstrated in Table 1.The cast ingot was also used as the reference material.The metal chips with 5mm in length,2mm in width and<0.1mm in thickness,were machined from the cast alloy through turning process.The metal chips were rinsed with distilled water and then chemical cleaned with the alcohol and acetone under ultrasonic wave to remove the surface impurities and grease contaminations.The cleaned metal chips were poured into graphite die with 30mm in diameters,and were placed in the vacuum chamber.Then the metal chips were subjected to the cold pressing process in the graphite die at a pressure of 40MPa and hold for 5min before the SPS process.

2.2.Spark plasma sintering

The schematic diagram of the spark plasma process was illustrated in Fig.1(a).The spark plasma sintering was implemented on the Labox 325 SPS apparatus affiliated with 30-kN press,pulsed or constant DC current supply system,temperature feedback system,vacuum chamber and thermal process controller,as shown in Fig.1(b).The interfaces between samples and die/punch were covered with the graphite paper to facilitate the Joule heating and demolding form the graphite mold.The sintering process can be listed as three steps:(1)the pressure gradually increases to 30kN(40MPa)(2)considering the heat inertia,the pressed chips-based billets were heated to 320°C with heating rate of 80°C/min through the application of pulsed DC current,and then were heated to the 400°C,450°C,500°C and 550°C with heating rate of 25°C/min,and the pressure and temperature were hold for 10min.The curves of setting and actual temperature and pressure variations were shown in Fig.2.The billets sintered at different temperature were denoted as ST400,ST450,ST500 and ST550.In addition,the pre-pressed chips billets were sintered at the selected optimum temperature for 5min,10min and 15min,which were denoted as SH5,SH10 and SH15.(3)finally cooling down with the reduced pressure in the SPS chamber with vacuum.

Fig.1.The SPS process(a)schematic diagram(b)experiment apparatus(c)the consolidation recycled billets sintered at 400°C,450°C and 500°C(d)the liquid metal was squeezed out the mold after sintering process at 550°C(e)the polished recycled billets sintered at 550°C.

Fig.2.The schematic of(a)the setting(b)the realistic temperature and pressure variation during the SPS process.

2.3.The heat treatment and isothermal compression of the recycled billets

Fig.3.The SEM image and TEM observation of the raw Mg–Gd–Y–Zn–Zr alloy(a)SEM images(b)TEM images.

The consolidation billets were recycled from the cast metal chips,and solute atoms were inclined to segregate during the casting process.The heat treatment followed by SPS process should be carried out to improve the mechanical properties of the recycled billets.The recycled billets sintered at the optimum sintering parameters were selected and subjected to the heat treatment at different parameters,including the temperature and time.The cylinder samples with 12mm in height and 8mm in diameters were machined from the recycled billets with optimum sintering parameters and subjected to the isothermal compression test.The isothermal compression tests were carried out on the DSI Gleeble 1500D thermodynamic simulation test machine.The cylinder samples were electrochemical polished before the compression test to eliminate the oxidation film.The experimental temperatures were 350°C,400 °C and 450°C and the strain rates were 0.001 s−1,0.01 s−1,0.1 s−1and 1 s−1.The cylinder samples were compressed to the 40% of the initial height in vacuum,and thus a total true strain of 0.9 can be obtained.The specimens were heated to the specified temperature with heating rate of 5K/s and hold for 2min.Furthermore,the specimens after compression tests were immediately quenched with water to preserve the microstructure.

2.4.The microstructure characterization and mechanical properties evaluation

The samples for microstructure observation were machined from the center of the consolidation recycled billets.The samples were subjected to the optical microstructure(OM)observation(Leica DMI3000Moptical microscope),scan electron microscope(SEM)(Quanta 200FEG)and transmission electron microscope(TEM)(Talos F200)and the energy dispersive spectrometer(EDS)measurement was also conducted.The OM samples should be etched in the solution(5g picric acid,5ml alcohol,100ml acetic acid and 10ml distilled water).The samples for SEM observation were electro-polished in the prepared solution(the volume ratio of perchloric acid and the alcohol was 1:9).The thin foils for TEM observation were prepared through the ion polishing machinery.

The mechanical properties of the recycled billets were evaluated through the compression test at room temperature.The cylinder samples of 6mm in diameters and 9mm in height were wire-electrical discharge machined from the consolidation recycled billets and then subjected to the compression test at room temperature.The cast alloys were also subjected to the compression test,which was used as the reference.The three repeated compression tests for each recycled billet were implemented to guarantee the repeatability.The compression tests were carried out on the Shimadzu Autograph AG-X plus test machine with a constant speed of 1mm/min.

Fig.4.The SEM observations of recycled billets sintered at different temperature(a)and(b)400°C(c)and(d)450°C(e)and(f)500°C(g)and(h)550°C.

3.Results

3.1.Microstructure characterization of the recycled billets via SPS

The SEM and TEM observations of the raw Mg–Gd–Y–Zn–Zr alloys were illustrated in Fig.3.The morphology of raw Mg–Gd–Y–Zn–Zr chips was demonstrated in Fig.3(a).The statistically average grain size of the raw Mg–Gd–Y–Zn–Zr chips was approximately 30∼40μm.The eutectic Mg3(Gd,Y)phases[25]and interdendritic LPSO phases were observed in the raw Mg–Gd–Y–Zn–Zr alloy,and the intragranular 14H-LPSO phases were scarcely observed.The selected area electron diffraction(SAED)patterns in Fig.3(b)indicates that the interdendritic LPSO phase can be verified to be 18R-LPSO phases.The metal chips were firstly subjected to the cold pre-pressing process in the graphite die at a pressure of 40MPa,and then SPS process was implemented on the cold pre-pressed billets.

The recycled billets sintered at 550°C were stripped from the graphite mold,as demonstrated in Fig.1(d).The metal chips were partly melted into semi-solid state at 550°C and the liquid metals can damage the mold and SPS machine during the sintering process,and thus the selected temperature should be below 550°C.The SEM observations of the recycled billets sintered at different temperature were illustrated in Fig.4.Deduced from the Fig.4(b)and(d),a few holes can be observed in the recycled billets sintered at 400°C and 450°C.However,the holes or defects were rarely observed at the recycled billets sintered at 500°C and 550°C,which indicates that the metal chips were effectively bonded through atoms diffusion.The bond interface between metal chips was obviously observed regardless of the sintering temperature.In addition,the eutectic Mg3(Gd,Y)phases were remained in the recycled billets sintered at the 400°C and 450°C,however,the eutectic Mg3(Gd,Y)phases were scarcely observed in the recycled billets sintered at 500°C or 550°C which was above the eutectic temperature.Thus,the eutectic Mg3(Gd,Y)phases in the metal chips dissolved intoα-Mg matrix during the consolidation SPS process as the sintering temperature increased to 500 or 550°C.The block interdendritic 18-LPSO phases cannot be detected and only the bond interface between the metal chips can be observed in the recycled billets sintered at 550°C.The temperature distributions in the billets are heterogeneous and the actual temperature in partial zone of the billets exceeds the setting temperature due to the heating inertia,and thus the metal chips were partially heated to liquid state as the sintering temperature increased to the setting 550°C which was close to the solidus temperature.The liquid metal was squeezed out the graphite mold during the SPS process,as demonstrated in Fig.1(d),and thus the block interdendritic 18R-LPSO phases cannot be observed in the consolidation recycled billets sintered at 550°C.The intragranular lamellar 14H-LPSO phases were observed as sintering temperature exceeds 500°C,as illustrated in Fig.4(g)and(h).

Fig.5.The EDS distributions of recycled billets sintered at(a)450°C(b)500°C high magnification(c)500°C low magnification(d)550°C.

Fig.6.The TEM images of the recycled billets sintered at 500°C(a)intragranular lamellar LPSO phase(b)corresponding SAED pattern(c)interdendritic LPSO phase(d)corresponding SAED pattern(e)the TEM observation of the bond interface between metal chips(f)corresponding SAED pattern of the bond interface(g-l)the EDS results of the bond interface.

The EDS analyses of the recycled billets sintered at 450°C,500°C and 550°C were illustrated in Fig.5.The Y element evidently accumulates at the bond interface between the metal chips,as illustrated in Fig.5(b)and(c),which indicates the oxidation of Y element at the bond interface between metal chips during the SPS process.Furthermore,the EDS distribution of the recycled billets sintered at 550°C was illustrated in Fig.5(d).Similarly,the Y element also accumulates around the bond interface between the metal chips in the recycled billets sintered at 550°C.In addition to the Y element,the Zr element also segregated at the bond interface,which indicated that oxide of Zr element massively formed at the bond interface in the recycled billets sintered at 550°C.The oxidation of the RE element might result from the surface oxidation of the metal chips during the turning machining and then spread into the oxide films at bond interface during SPS process.

The TEM observations of the recycled billets sintered at 500°C were illustrated in Fig.6.The intragranular lamellar 14H-LPSO phase precipitates across theα-Mg grains,as illustrated in Fig.6(a)and(b).Furthermore,the interdendritic 18R-LPSO phases were also remained and the eutectic Mg3(Gd,Y)phases were scarcely observed at the grain boundaries during the recycled sintering process,as indicated in Fig.6(d).The TEM observation in Fig.6(e)indicates the formation of oxide films of RE element at the bond interface between the metal chips,and the corresponding SAED pattern in Fig.6(f)indicates the nano-sized amorphous films at the bond interface.In addition,the EDS analyses of the bond interface between metal chips were also demonstrated in Fig.6(g-l).The Gd,Y and Zr elements accumulate at the bond interface,especially the Y and Zr elements,which also verified that the formation of the nanosized amorphous oxidation of RE elements(mainly Gd2O3,Y2O3or ZrO2)[28–30]at the bond interface.The surface oxidation of the metal chips during the turning machining process transformed into the oxide films at bond interface during SPS.The precipitation of the lamellar 14H-LPSO phases within theα-Mg grains and the aggregation of the RE element at the bond interface between the metal chips consumes the RE element,which requires more solute RE element into theα-Mg matrix.The dissolution of the eutectic Mg3(Gd,Y)phase provides the necessary RE element for the precipitation of the intragranular lamellar 14H-LPSO phase and the RE accumulation at the bond interface between metal chips.

The SEM observations of the recycled billets sintered at 500°C for different duration in Fig.7 indicated the sintering duration slightly affects the microstructure of the recycled billets.The thickness of the bond interface increases as the sintering duration increased from 10min to 15min,as presented in Fig.7(e)and(f),which indicates more RE element accumulation at the bond interface between metal chips and is unfavorable to the precipitation of the lamellar 14H-LPSO phase across theα-Mg grains.

Fig.7.The SEM images of the recycled billets at 500°C with different sintering duration(a)and(d)5min(b)and(e)10min(c)and(f)15min.

3.2.Effects of sintering parameters on the mechanical properties of the recycled billets

The compression stress-strain curves of the recycled billets sintered at different parameters were demonstrated in Fig.8,and the compression stress-strain curve of the cast alloy was used as reference.The compression yield strength(CYS)and ultimate compression strength(UCS)of the recycled billets improved with the increase of the sintering temperature until 500°C.The optimum mechanical properties are obtained as the sintering temperature increased to 500°C.The recycled billets ST500 exhibits higher CYS(217MPa vs.174MPa or 191MPa)and UCS(467MPa vs.351MPa or 412MPa)compared with the recycled billets ST400 or ST450.The recycled billets regardless of sintering temperature demonstrated the rival or even superior CYS compared with the cast billets,indicating the strong bonding properties and the strengthening effects of the oxide films at bond interface.The recycled billets ST400 demonstrated inferior UCS and compression failure strain(CFS)compared with the cast billets,which indicated the numerous defects in the recycled billets and meant that the lower sintering temperature is unfavorable to the fully atoms diffusion and metallurgical bonding.The effects of the sintering duration on the mechanical properties of the recycled billets were illustrated in Fig.8(b).The compression property varies slightly as the sintering duration increased from 5min to 10min,however,the CYS significantly declined as the sintering duration increased to 15min.It is concluded that the prolonging sintering duration is unfavorable to improving the mechanical properties of the recycled billets.The thickness of the bond interface was elevated as the sintering duration increased to 15min,which consumed more solute RE atoms in the matrix and weakened the solution strengthening effects,and thus the recycled billets with prolonging sintering duration showed the decreased CYS.Based on all mentioned above,considering the effective bond between the metal chips,the optimum sintering parameters were determined to be 500°C for 10min.

Fig.8.The compression strain-stress response curves of the recycled billets sintered(a)at different temperature for 10min(b)at 500°C for different time.

3.3.Effects of post-SPS heat treatment on the microstructure and mechanical properties of the recycled billets

The recycled billets were consolidation sintered from the cast metal chips,and the segregation of the solute RE atoms might remain in the consolidation recycled billets,which required the appropriate heat treatment followed by SPS process.The heat treatments with different parameters were implemented on the recycled billets sintered at 500°C for 10min,and the SEM images of the recycled billets after heat treatment with air cooling or furnace cooling were illustrated in Fig.9.The selected heat treatment temperature was 450°C,which facilitates the solution of the RE element into theα-Mg matrix and was below the eutectic temperature.The results indicated that the lamellar 14H-LPSO phases rarely precipitate at the vicinity of the bond interface between metal chips.The precipitation of the lamellar 14H-LPSO phase within theα-Mg grains requires the sufficient RE atoms solution into theα-Mg matrix.The RE element(mainly Y element)clustered at the bond interface between the metal chips due to the formation of oxide films of RE element during consolidation SPS process,which resulted in the absence of the RE element and thus the rare precipitation of lamellar LPSO phase at the vicinity of the bond interface between metal chips during post-SPS heat treatment,as demonstrated in Fig.9(a)(c)(e)(g)and(i).Furthermore,the furnace cooling facilitates the precipitation of lamellar 14H-LPSO phase within theα-Mg grains,even theα-Mg matrix adjacent to the bond interface between metal chips,as illustrated in Fig.9(b)(d)(f)(h)and(j).The precipitation of the intragranular lamellar 14H-LPSO phase consumes more solute RE atoms during the furnace cooling.The cooling methods are inclined to slightly affect the precipitation of lamellar 14H-LPSO phases with the increase of heat treatment duration,as the Fig.9(e–j)demonstrated.

Fig.9.The SEM images of the recycled billets with different heat treatment methods(a)450°C×4h air cooling(b)450°C×4h furnace cooling(c)450°C×8h air cooling(d)450°C×8h furnace cooling(e)450°C×12h air cooling(f)450°C×12h furnace cooling(g)450°C×16h air cooling(h)450°C×16h furnace cooling(i)450°C×20h air cooling(j)450°C×20h furnace cooling.

The mechanical properties of recycled billets after post-SPS heat treatment were illustrated in Fig.10.The air cooling can effectively strengthen the UCS and improve the CFS compared with the furnace cooling.The furnace cooling facilitates the precipitation of the lamellar 14H-LPSO phase adjacent to the bond interface between metal chips,which consumes more solute RE atoms in theα-Mg matrix.The intragranular lamellar 14H-LPSO phases can effectively inhibit the basal dislocation slip[25,31]and dislocations are prone to accumulate at lamellar 14H-LPSO phase adjacent to the bond interface,and thus the cracks are inclined to nucleate at the bond interface between metal chips,which might account for the lower UCS and CFS of the recycled billets after heat treatment with furnace cooling.The reduced thickness of the bond interface also decreased the compression strength of the recycled billets with furnace cooling.Thus,the recycled billet,which was subjected to heat treatment for 8h and following air cooling,demonstrated the superior UCS and CFS(472.3MPa and 22.54%),and the CFS was improved by 18%compared with the SPS recycled billet.

3.4.The microstructure evolution of the recycled billets during the isothermal compression process

The recycled billets sintered at 500°C for 10min were subjected to the hot compression process at different parameters.The flow stress curves of the consolidation recycled billets during the isothermal compression were demonstrated in Fig.11.The samples were severely crushed during the compression at 350°C with strain rate of 1 s−1.However,the integral stress-stain response curves at elevated temperature or lower strain rate were obtained,which indicates the better formability and deformation behavior of the recycled billets.The lower temperature and elevated strain rate are unfavorable to the hot deformation behavior and metal flow of the consolidation recycled billets.Compared with the Mg–Gd–Y–Zn–Zr alloy containing higher fraction of RE element in the previous literature[31],the obvious dynamic softening effect cannot be observed at lower temperature and higher strain rate,as illustrated in Fig.11(a)and(b).Compared with the cast and homogenized Mg–7Gd–5Y–0.6Zn–0.8Zr alloy in the literature[32],the recycled billets demonstrated the apparent dynamic softening phenomenon during the isothermal compression.The fragmented oxide films at bond interface can facilitate the DRX nucleation through the particle-stimulated nucleation during the isothermal compression,which results in the apparent dynamic softening effect compared with the cast and homogenized Mg–7Gd–5Y–0.6Zn–0.8Zr.Furthermore,the peak stress of the recycled billets was slightly below that of the cast Mg–7Gd–5Y–0.6Zn–0.8Zr alloy.

Fig.10.The compression strain-stress response curves of the recycled billets after heat treatment with different time and cooling method.

Fig.11.The flow stress-strain response curves of the recycled billets compressed at different parameters(a)350°C(b)400°C(c)450°C.

In addition,the SEM observations of the recycled billets after the isothermal compression tests were demonstrated in Fig.12.The oxide films at bond interface were kinked and zigzagged at lower temperature and higher strain rate,as illustrated in Fig.12(a),(d),and(g).The interdendritic 18RLPSO phases were effectively broken into rod-shaped 18RLPSO phases at elevated temperature and lower strain rate,as demonstrated in Fig.12(f)(h)and(i).Furthermore,the oxide films at the bond interface were fragmented at elevated compression temperature and lower strain rate,which was consistent with the evolution of theα-Mg matrix and interdendritic 18R-LPSO phases.The finer DRX grains were also observed at the coarse deformed grain boundaries or the vicinity of the bond interface,as illustrated in Fig.12(f)and(i),which also verified that the fragmented oxide films facilitated the DRX nucleation and contributed to the dynamic softening effects during the isothermal compression process.The residual fragmented oxide films might affect the mechanical properties of the recycled billets through dispersion strengthening.The cracks or holes were hardly observed adjacent to the bond interface after isothermal compression,and thus the bond interface between metal chips showed the superior bond performance and deformation consistency performance withα-Mg matrix.

4.Discussion

The cast Mg–Gd–Y–Zn–Zr chips were successfully recycled through the SPS process.The numerous holes or defects can be observed in the consolidation recycled billets sintered at temperature below 500°C,however,the cracks or defects were hardly observed as the sintering temperature exceeds 500°C.The elevated sintering temperature facilitates the atoms diffusion and thus contributes to the effective bonding process between metal chips.However,the metal chips were partly melted into semi-solid state as the sintering temperature increased to 550°C which was close to the solidus temperature.The heat inertia during the SPS heating process can result that the realistic temperature exceeds the setting temperature,and the realistic temperature exceeds the 550°C or the solidus temperature,and thus the metal chips were partly melted into semi-solid state.Thus,the optimum sintering temperature was selected to be 500°C.The oxidation of the RE elements,such as Gd2O3,Y2O3and ZrO2,formed at the bond interface between metal chips during the turning machining and spark plasma sintering process,and thus the RE element accumulate at the bond interface between metal chips.The bond interface thickness was elevated with the increase of the sintering duration,which requires the more solute RE atoms accumulation at the bond interface and decreases the solution strengthening effects of theα-Mg matrix,and thus the compression strength slightly decreased as the sintering duration increased to 15min.Furthermore,it can be concluded that the sintering temperature can significantly affect the microstructure and mechanical properties of the recycled billets and sintering duration slightly affects the microstructure and mechanical properties of the recycled billets.

Fig.12.The SEM images of the recycled billets after isothermal compression with different parameters(a)350°C/0.1 s−1(b)350°C/0.01 s−1(c)350°C/0.001 s−1(d)400°C/0.1 s−1(e)400°C/0.01 s−1(f)400°C/0.001 s−1(g)450°C/0.1 s−1(h)450°C/0.01 s−1(i)450°C/0.001 s−1.

Generally speaking,the SPS simultaneously introduced the combined effect of the thermal,mechanical and electrical field during the SPS consolidation process[28].The spark plasma can purify the metal surface through eliminating surface impurities,and facilitates the atoms diffusion[32,33].The Joule heating from the SPS can facilitate the metal transfer,which assists effective consolidation and interface bonding between metal chips during the SPS[17,33].The SPS pulsed power offers dispersed discharge sites and facilitates the rapid cooling of the bonding between metal chips[34].The mentioned above factors contribute to the consolidation recycled billets with superior mechanical properties and effective interface bonding.

The cooling methods after heat treatment significantly influence the microstructure characteristic and compression properties of the recycled billets.The furnace cooling facilitates the precipitation of the intragranular lamellar 14H-LPSO phases within theα-Mg matrix,even the matrix adjacent to the bond interface,which requires more solute RE element in theα-Mg matrix.The recycled billets with furnace cooling demonstrated lower UCS and CFS compared with the air cooling.The dislocations accumulate at the lamellar 14HLPSO phases adjacent to the bond interface during compression at room temperature,and thus the cracks are inclined to nucleate at the bond interface between metal chips,which might account for the lower compression properties of the recycled billets after heat treatment with furnace cooling.

The cracks or holes were scarcely observed in the matrix adjacent to the bond interface during the isothermal compression process,which indicates the superior bond properties between metal chips in the consolidated billets through SPS.The bond interfaces were kinked and zigzagged during the isothermal compression process,which was corresponding to the kinking and zigzagging of the lamellar 14H-LPSO phase within theα-Mg grains.The oxide films at the bond interface were also fragmented during the hot compression process at elevated temperature and lower strain rate,and the finer DRX grains also formed at the vicinity of the bond interface,which might result in the lower peak stress of the recycled billets during the isothermal compression.

Fig.13.The schematic diagram of the bond interface and block interdendritic LPSO phases.

The schematic diagram of the bond interface and block interdendritic 18R-LPSO phases during the SPS,heat treatment and isothermal compression process was illustrated in Fig.13.The metal chips were machined from the Mg–Gd–Y–Zn–Zr alloy containing interdendritic and lamellar LPSO phase,and were effectively bonded during the SPS process.The oxide films of RE element formed at the bond interface between metal chips,and were mainly from the surface oxidation of the metal chips during the turning machining and SPS process.The furnace cooling facilitates the precipitation of the lamellar 14H-LPSO phase across theα-Mg grains,even theα-Mg matrix in the vicinity of the bond interface between metal chips.The dislocations are inclined to accumulate at the lamellar 14H-LPSO phases adjacent to the bond interface,which results in the lower CFS after heat treatment with furnace cooling.However,the lamellar 14H-LPSO phases were hardly observed in the vicinity of the bond interface after heat treatment with air cooling.The DRX grains preferably nucleate at the coarse deformed grain boundaries or the vicinity of the bond interface during the hot compression at elevated temperature and lower strain rate.The block interdendritic 18RLPSO phases were effectively broken into rod-shaped 18RLPSO phases,and the oxide films at bond interface were also zigzagged and fragmented at elevated temperature and lower strain rate.

5.Conclusions

The cast Mg–Gd–Y–Zn–Zr alloy metal chips were successfully recycled through the spark plasma sintering process in this paper.The effects of the heat treatment with different cooling method on the recycled billets were also investigated.Furthermore,the evolution of the bond interface between the metal chips during the isothermal compression process was also studied.The main conclusions are listed as follows:

(1)The sintering temperature significantly affect the microstructure and mechanical properties and sintering duration slightly affects the microstructure and mechanical properties of the recycled billets.The fully densified recycled billets with superior mechanical properties were successfully fabricated through SPS at 500°C for 10min,which was ascribed to the combined effects of the mechanical and joule heating from spark pulsed current.The nano-sized amorphous oxidation of RE element also formed at bond interface between metal chips.

(2)The compression yield stress,ultimate compression strength and compression failure strain of the recycled billets via SPS at 500°C for 10min were 217MPa,467MPa and 20.4%,which was superior than that of the cast billets(181MPa,405MPa and 19.5%).The formation of the nano-sized RE oxidation at bond interface can effectively inhibit the dislocation movement during compression,which results in the elevated compression strength of the SPS recycled billets.

(3)The recycled billets after solid solution treatment with air cooling presents better mechanical properties compared with the furnace cooling.The furnace cooling facilitates the precipitation of the intragranular lamellar 14H-LPSO phase across theα-Mg grains,even theα-Mg matrix adjacent to the bond interface between metal chips,which results in inclination of the dislocation accumulation and cracks nucleation at the bond interface.

(4)The cracks or holes were scarcely observed in theα-Mg matrix adjacent to the bond interface during the isothermal compression,which indicates the superior bond properties between metal chips and deformation consistency performance withα-Mg matrix in the recycled consolidation billets via SPS.The oxide films at the bond interface between metal chips were zigzagged and fragmented during isothermal compression.

Conflict of Interest

The authors declared that they have no conflicts of interest to this work.

Acknowledgment

This project is supported by National Natural Science Foundation of China(Grant No.51875127).