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Ann Dokowsk,Łuksz˙Zrodowski,Monik Chlewick,Milen Korlnik,Bogusłw Admczyk-Cieˊslk,Jku Ciftci,Brtosz Moroˊnczyk,Mirosłw Kruszewski,Jku Jroszewicz,Driusz Kuc,WojciechˊSwi˛eszkowski,Jrosłw Mizer
a Faculty of Materials Science and Engineering,Warsaw University of Technology,141 Woloska,02-507 Warsaw,Poland
b Institute of Materials Engineering,Silesian University of Technology,Krasinskiego 8,40-019 Katowice,Poland
Abstract In this study,powder metallurgy methods were used to fabricate Mg-7.5Li-3Al-Zn alloys from repowdered extruded alloys.Extruded alloys were powdered using ultrasonic atomization,and then laser powder bed fusion(LPBF)and pulse plasma sintering(PPS)were used to consolidate the bulk materials.A comparison of the properties of the fabricated alloys with those of a conventionally extruded one was carried out using methods that characterized the microstructure and corrosion resistance.When compared to their conventionally extruded counterpart,LPBF and PPS materials exhibited refine microstructures with low enrichment in AlLi and coarse Al,Zn,Mn precipitates.The main drawback of the LPBF alloy,printed for the needs of this study,was its porosity,which had a negative effect on its corrosion.The presence of unrecrystallized particle boundaries in the PPS alloy was also unbeneficia with regard to corrosion.The advantage of the LPBF and PPS processes was the ability to change the proportion ofα(Mg)toβ(Li),which when the complete consolidation of the material is achievable,may increase the corrosion resistance of dual-structured Mg-Li alloys.The results show that powder metallurgy routes have a wide potential to be used for the manufacture of Mg-Li based alloys.© 2022 Chongqing University.Publishing services provided by Elsevier B.V.on behalf of KeAi Communications Co.Ltd.This is an open access article under the CC BY-NC-ND license(http://creativecommons.org/licenses/by-nc-nd/4.0/)Peer review under responsibility of Chongqing University
Keywords:Additive manufacturing;Laser Powder Bed Fusion(LPBF);Pulse Plasma Sintering(PPS);Corrosion;Mg-Li alloys.
The modern construction requirements of industries such as transportation focus on the reduction of their environmental impact and the lowering of energy costs.Such considerations have led to an increased interest in lightweight materials,such as magnesium alloys[1].They offer high relative strength and stiffness,good damping characteristics,high thermal and electrical conductivity,and(most importantly)the lowest weight among construction metals[2,3].However,their widespread use is restricted by two important factors[4-6]:low corrosion resistance leading to a rapid degradation of the elements within,and secondly,poor ductility and formability,which stem from the close-packed hexagonal structure restricting the number of independent slip systems in operation[7-10].The latter problem results in difficultie in obtaining more complex shapes using traditional manufacturing methods and raises the necessity to employ non-conventional techniques.Two types of approach that allow near-net-shape processing can be considered as alternative:non-conventional powder metallurgy[11-14]and additive manufacturing[6,15-22].Such techniques are extremely important in the biomedical industry where both a high structural integrity of the material and a wide freedom for customization to perfectly match anatomical geometries are desired[23].
Powder metallurgy and additive manufacturing routes offer attractive alternatives to conventional manufacturing of Mg alloys,and researchers to date have prepared and investigated the properties of the following Mg-based alloys:Mg-Al-Zn(AZ31,AZ61,AZ91)[24-26],Mg-Zn-Zr(mostly ZK60)[27-29],and those containing REE,such as WE43 or AE42[30,31].Generally,powder sintering methods exhibit many advantages in terms of microstructure modification such as the limited formation of a strong texture and the prevention of unwanted changes to the materials,such as unfavorable grain growth or the formation of deleterious intermetallic phases[32,33].The decisive factors with regard to the development of the aforementioned processes for the preparation of the Mg-based alloys are achieving superior mechanical properties and an improvement in the corrosion resistance of Mg alloys when compared to their traditionally cast counterparts[33,34].For example,it was shown that AZ91D produced by wire and arc additive manufacturing exhibited higher corrosion resistance than the equivalent cast alloy[16].Similarly,laser powder bed fusion(LPBF)produced ZK60 that was more corrosion resistant than a traditionally cast alloy[35].Since there are also disadvantages in the form of significan degradation and the early loss of mechanical stability of the produced materials,Kopp et al.[36]proposed suitable post-processing methods for the preservation of the long-term stability of mechanical properties modifie by heat treatment or plasma electrolytic oxidation of WE43 fabricated through laser powder bed fusion.Yang et al.[37]used additive manufacturing to prepare Mg-based composite with mesoporous bioglass,which shows enhanced corrosion properties in terms of orthopedic application.
In view of the microstructural considerations,the key issue in the design and production of Mg-based alloys is a full understanding and control of the properties of the powders used for consolidation[17].Secondly,strict environmental restrictions drive researchers and technologist to look for methods to reuse post-production waste materials.Therefore,the concept behind this study was to explore the possibilities of reusing traditionally extruded Mg-Li based alloys through atomization of the base alloy and reconsolidation it,using novel manufacturing methods.To the best of our knowledge this is the firs study regarding the fabrication of Mg-based alloys with Li as the main alloying element using LPBF and PPS.The main goal of this paper was to describe the differences in the corrosion resistance of the dualstructured Mg-7.5Li-3Al-Zn alloy fabricated by traditional extrusion and that produced by powder consolidation methods.This is the firs study showing the possibility of the fabrication of dual structured Mg-Li alloys using powder metallurgy methods.
2.1.Material fabrication
2.1.1.Reference material
The reference material was a dual-structured Mg-7.5Li-3Al-1Zn alloy prepared utilizing traditional casting and conventional extrusion.For casting,the AZ31B alloy was smelted,and 7.5 wt.% high-purity Li was added.The alloy was cast,extruded to a dimension ofØ20 mm,and subsequently annealed at 350 °C for 1 h.The microstructure and corrosion performance of the alloy was previously described by our research group in reference[38].As stated in this work,Li addition decrease the density and increase formability leading to the higher number of methods that can be used for plastic deformation of Mg-Li alloys.All these aspects result in fabrication of the Mg-Li alloys with higher plasticity and strength than traditional Mg-based alloys.
2.1.2.Atomization
The extruded Mg-7.5Li-3Al-1Zn was powdered by ultrasonic atomization using a rePowder device(AMAZEMET,Poland).57.7 g of the alloy was melted in a graphite crucible at a heating rate of 150 K/min up to 1023 K and kept for 2 min at this temperature under an Ar atmosphere.Subsequently,the molten alloy was treated with a Ti6Al4V sonotrode,which vibrated at 40 kHz average frequency and 15μm amplitude.As a result of a 40 s atomization process,52 g of powder was produced.The powder was collected under an inert atmosphere and after sieving through a 100μm sieve,a total mass of 48 g of powder was inserted into the laser melting machine.
2.1.3.Powder metallurgy methods(LPBF and PPS)
LPBF 3D printing was carried out using a Realizer 50(DMG MORI,Japan)laser melting machine equipped with a 120 W continuous wave 1064 nm laser.The process was carried out under an inert atmosphere of Ar 5.0 on a pure Mg build plate without preheating.After a preliminary study of the parameters,a sample of 15×15×3 mm was prepared with a 25μm powder layer,100μm hatch distance,80 mm/s scanning speed,and 15 W laser power without hatch rotation between the layers.During the process,extensive fuming was observed.
A second batch of samples was produced using powder prepared in the same way as for LPBF.The powder was consolidated using PPS,which is a novel current-assisted powder metallurgy technique(details may be found in reference[39]).The consolidation process was carried out under vacuum(5·10-3Pa)at 350 °C for 5 min under a pressure of 50 MPa.
2.2.Microstructural characterization
Scanning electron microscopes(SEM,Hitachi SU8000 and Hitachi SU70)were used to show the shape,distribution and microstructure of the powder particles.To show the microstructure of the particles,powder was mixed with an acrylic resin,and the specimen was milled in Ar+beam using ion-beam milling system(Hitachi IM4000).The particle size distribution(PSD)was measured using a Horiba Partica LA-950 laser scattering particle size distribution analyzer.
To analyze the phase composition of the alloys,X-ray diffraction(XRD,Bruker D8 Advance)operating at 40 kV and 40 mA with Cu Kαradiation was utilized.The results were recorded by stepwise scanning 2θfrom 10°to 120°,with a step size of 0.02° and a count time of 10 s per step.Based on the registered patterns,by comparing the integrated intensities of the diffraction peaks from each of the known phases,a semi-quantitative determination of their relative concentration was carried out.The relative concentration of Li in the investigated alloys was measured by atomic absorption spectroscopy(AAS,GBC 932 Plus)with an air oxidizing flam and a wavelength of 670.8 nm.
The samples’microstructure was observed with the Hitachi SU8000 SEM using tunneling contrast on the ion-milled surfaces.The ion milling was carried out utilizing a low-energy Ar+ion-beam milling system(Hitachi IM4000)over 4 h.To avoid surface oxidation(due to the high activity of Li),observations were taken immediately after sample preparation.The TEM observations were carried out using JEOL JEM 1200EX microscope with an acceleration voltage of 120 kV.Thin foils were prepared using a Gatan Model 691 Precision Ion Polishing System(PIPS)with a beam voltage of 3 V inclined to the sample surface at angle of 10°.The samples were scanned using a microfocused X-ray tomographic system(MicroXCT-400,Xradia-Zeiss),at 40 kV and 200μA.For each sample,1200 projection images were recorded with an exposure time of 5 s and a magnificatio objective of 4×.The volume was reconstructed using the supplied manufacturer software and was then exported to Avizo Fire(Thermo Fisher Scientific for further 3D image analysis and porosity evaluation.The voxel size was the same for all samples(5×5×5μm).
In order to investigate the mechanical properties,the test of microhardness was performed.Vickers microhardness testing was conducted under a load of 200 g using Innovatest Falcon 500 Micro/Macro Vickers Tester.10 points were measured on each material.
2.3.Corrosion performance
2.3.1.Electrochemical testing
The corrosion potential under open-circuit conditions,electrochemical impedance spectroscopy(EIS),and potentiodynamic measurements were carried out in naturally aerated,quiescent 0.01 M NaCl solution using an Autolab PGSTAT 302N potentiostat equipped with three electrodes:Pt as the counter electrode,Ag/AgCl as the reference electrode,and the measured sample as the working electrode.The corrosive medium was freshly prepared using analytical grade reagents and distilled water.The corrosion potential was recorded for 1 h,then EIS measurements and potentiodynamic tests were performed.The EIS measurements were recorded in a frequency range from 0.01 to 10,000 Hz.The potentiodynamic tests were conducted from 0.5 V below EOCPto 1.5 V vs.Ref with a scan rate of 5 mV.To ensure the reproducibility of the results,the data for each sample was collected three times.Nova 2.1.5 software was used to fi the data.To give a further view on the corrosion processes that occurred on the surface of the sample,high resolution SEM observations were carried out after immersion under open-circuit conditions(Hitachi SU8000,Japan).Two kinds of observation were performed:on the surfaces with the corrosion products as they were formed,and on the surfaces with chemically removed corrosion products.The corrosion products were removed by immersion in 4% nitric acid for 5 s as per references[40,41].
Table 1Li concentration measured by AAS(wt.%)and Zn concentration measured by EDX(wt.%).
2.3.2.Hydrogen evolution method
The evolved hydrogen was collected during immersion tests.To measure the hydrogen gas,samples polished with up to #4000 SiC paper were placed in a beaker and connected to a burette.Hydrogen was collected in the burette through a PVC pipe installed above the beaker equipped with a dropping funnel.The corrosion rate was calculated as follows[42]:
where:Δm is the mass loss(g)calculated from hydrogen evolution,s is the surface area(m2),t is the time of exposure(days).To determine the standard deviation of the measured data,three parallel samples were immersed from each material.
3.1.Particle size and distribution
The shape and chemical analysis of the powder were analyzed by SEM/EDX and AAS.As shown in Fig.1a,the particles of the repowdered alloy were spherical,and no agglomerates were observed.The morphology of a single particle is shown in Fig.1b,and an EDX analysis revealed that the particle was composed of Mg,Al,and Zn(Fig.1d and e).AAS results confirme that no Li evaporation and/or oxidation was observed during atomization of the extruded ingot,and it was found to be around 7.4 wt.%;Zn depletion did not take place either during atomization(Table 1).The phase composition of the powder,namelyα(Mg),β(Li),AlLi,and MgLi2Al,is depicted in Fig.2a.The cross-sectional SEM observations of the powder showed that pores were present in the outer regions of the particles(Fig.1c),and the microstructure of the particle was composed ofα(Mg)andβ(Li)with the 80% ofα(Mg)and 19% ofβ(Li),Table 2.AlLi particles were also visible in the microstructure of the powder(Fig.1f).For sample consolidation,a powder with a size from 29 to 200μm was used with the frequency shown in Fig.3.The average diameter of the particles was 70μm.10% of the powder particles had a size below 48μm,while D90is a parameter indicating that 90% of the particles had a size below 95μm.
Fig.1.SEM of powdered Mg-7.5Li-3Al-Zn alloy:a)the overall image of the powder,b)single particle image,c)the microstructure of the powder,d)EDX analyses made in area A,e)magnifie area from the panel b),f)magnifie area of the powder microstructure.
Table 2Relative phase composition(%)of the powder,PPS and LPBF alloys calculated based on XRD measurements.
3.2.Microstructure of the manufactured samples
As shown in Fig.4a,the extruded Mg-7.5Li-3Al-Zn alloy exhibited a dual-phase microstructure where the dark phase wasβ(Li)and the light oneα(Mg).The results published in our previous study[38]showed that the relative volume fractions ofα(Mg)andβ(Li)were calculated to be 65% to 35%,respectively,and coarse white precipitates marked as P1,rich in Al,Zn,and Mn,were formed in the alloy.We also found AlLi,MgLi2Al,and Mg17Al12in the extruded alloy.The LPBF-and PPS-produced alloys also had dualphase structures;however,the shape,size,and distribution of the phases changed due to the various processing routes(compare Fig.4a-c).It is worth to mention that the Li concentration in both alloys remained at the same level as indicated in the powder(Table 1).The microstructure of the 3Dprinted sample appeared as refine unevenly shaped grains,with the dark areas ofβ(Li)surrounded by theα(Mg)matrix(Fig.4b).Irregularly shaped pores(Fig.4b)were also clearly distinguished in this microstructure.Coarse white precipitates P1 and the presence of tiny white AlLi areas were revealed(confirme by combined SEM and XRD,magnifie Figs.4b and 2c,correspondingly).The PPS sintered alloy exhibited a microstructure similar to that presented by Dvorsky et al.[31]for WE43 sintered by spark plasma sintering.Individual particles were clearly visible,and the areas between them were not fully recrystallized(shown as“particle interface”in Fig.4c).Inside each particle,β(Li)surrounded byα(Mg)could be observed(magnifie Fig.4c).In the same Fig.4c,irregularly shaped white coarse precipitates enriched with Al,Zn,and Mn were observed[38],and tiny AlLi precipitates could also be easily recognized(confirme by XRD,Fig.2b).During consolidation MgLi2Al probably solutionized into the solid solution,therefore they were not detected in the PPS alloy.The relative concentration of theα(Mg)toβ(Li)changed depending on the different processing routes,being 85% to 13% for the PPS alloy,and 58% to 39% for the LPBF-printed material(Table 2).It can also be observed that together with the changing processing route the microstructure refinemen occurred with the highest refinemen obtained for LBPF sintered sample(Fig.4).With the increasing microstructure refinement microhardness of the alloys also changed,and it increased from 93 HV0.2for the extruded sample,to 114 HV0.2for the PPS and 130 HV0.2for the LPBF-sintered ones(Fig.5).
Fig.3.PSD obtained using laser-scattering particle size distribution analyzer.
Fig.5.Microhardness profile of the extruded,PPS and LPBF-sintered alloys.
Fig.4.SEM microstructure observations of the a)extruded alloy,b)LPBF-produced alloy,and c)PPS alloy.
Different manufacture methods led to the creation of the microstructures with significan differences clearly distinguishable in nanoscale.TEM observations revealed that nanosized precipitations exist in both,LPBF and PPS fabricated samples,with the higher number of precipitates with the size of around 150 nm formed in the material fabricated by LPBF(Fig.6a and c).There were many stacking faults in the LPBF processed material(marked by the red arrows in Fig.6b),while formation of complex precipitates was observed in the PPS sintered sample(marked by the red arrows in Fig.6d).The greyscale confirme that within one location of such complex precipitate,various chemical composition could be distinguished(inset in Fig.6d).Unfortunately,due to small size of the precipitates we were not able to perform diffraction.
Based on the XRD and SEM results,it can be stated that the precipitates formed in the alloys did not diverge qualitatively,but their quantitative distribution changed.TheμCT measurements allowed the density and distribution of the coarse precipitates to be defined as shown in Fig 7.Those precipitates had a size between 5 and 30μm.This information combined with the SEM observations allowed us to identify the coarse precipitates with P1 in Fig.4.The density of these precipitates formed in the traditionally extruded material was very low,being 30 precipitates per mm3(Fig.7a).When compared to the traditionally produced alloy,a higher density of the P1 precipitates was observed in the PPS alloy,and the highest number of precipitates was formed in the LPBF alloy(N=310/mm3).Their distribution in the matrix of both alloys was nearly random.
One of the challenges faced during the production of Mgbased alloys using LPBF is minimizing porosity[43,44].Therefore,an important issue in this research was to analyze the porosity of each of the produced materials.We did not observe any pores formed in the extruded and PPS alloys,but a high number of closed pores were formed in the LPBFproduced alloy(Fig.8).The calculated volume fraction of the pores was 3.51%,while their average size was found to be 28.3μm with a standard deviation of 18.2μm.We also calculated the maximum size of the pore,which was established to be 151.6μm.The fina structure of the alloy resembled a sponge,with the pores being unreticulated.It must be mentioned that the porosity level of 3.51% in Mg-7.5Li-3Al-Zn was very high when compared other LPBF-prepared Mg alloys,such as AZ61 with a porosity of 0.8%[45],while for AZ31 a porosity level of less than 0.5% was achieved[44].
3.3.Corrosion behavior
The changes in open-circuit potential(EOCP)with respect to immersion time are given in Fig.9a.The lowest values of EOCP(around-1.42 V/Ref)were characteristic for the extruded Mg-7.5Li-3Al-Zn alloy;the EOCPof this alloy was fairly stable during the entire immersion time.The LPBF and PPS alloys exhibited higher values of EOCP,which were comparable,oscillating just below-1.35 V/Ref.The potentiodynamic curves shown in Fig.9b were typical for alloys that undergo active dissolution,with a slight shift in the curve of the extruded alloy towards more negative values of potential.The parameters of corrosion potential(Ecorr)and corrosion current density(icorr)extrapolated using the Tafel method are depicted in Table 3.The lowest icorrof 0.2 mA/cm2was calculated for the extruded alloy,while the other two alloys both exhibited an icorrof 0.5 mA/cm2.As proved by Birbilis et al.[46],because of the negative difference effect,potentiodynamic tests by themselves cannot describe the corrosion rate of Mg-based alloys in an appropriate manner.Therefore,to have a wider view of the corrosion rates of the analyzed materials,corrosion rates were calculated based on hydrogen evolution.According to the results shown in Table 4,the samples produced via powder metallurgy methods had higher corrosion rates than the conventionally extruded alloy.The highest corrosion rate of 3.7×104g/(m2day)was calculated for the LPBF-printed alloy,a lower value of 2.5×104g/(m2day)was found for the PPS materials,and the lowest value was for the extruded alloy,2.3×103g/(m2day).
Fig.7.Plane image of the microstructure with the corresponding 3D image showing density of pores in a)extruded Mg-7.5Li-3Al-Zn b)LPBF-produced alloy,c)PPS alloy.
Fig.8.Pore distribution in LPBF-produced alloy(volume fraction of pores=3.51%,average size of the pores 28.3μm,stand.dev=18.2μm,max size of the pore=151.6μm).
Table 3The electrochemical parameters extrapolated using Tafel method based on results from potentiodynamic polarization tests recorded in 0.01 M NaCl.
Table 4Calculated corrosion rate based on hydrogen evolution method.
Fig.9.Results delivered from electrochemical measurements performed in naturally aerated 0.01 M NaCl a)Ecorr evaluation over 1 h,b)potentiodynamic polarization curves.
Fig.10.The results of EIS measurements performed in naturally aerated 0.01 M NaCl:a)Nyquist plots,b)EEC used for data fitting
Table 5Parameters delivered from fittin EIS data with the EEC presented in Fig.8b.
The Nyquist plots presented in Fig.10a confirme the results from the potentiodynamic polarization and hydrogen evolution.Based on the radius of the capacitive loop,it can be stated that the extruded alloy had the highest corrosion resistance,while the LPBF and PPS alloys both had significantl lower values.In order to interpret the EIS results,the equivalent electrical circuit(EEC)shown in Fig.10b was chosen.In this circuit,Rsis the solution resistance,while the charge transfer resistance and the electric double layer at the interface between the substrate and electrolyte are define by Rctand CPEdl,respectively.The inductive character of the loop at low frequency is described by RLand L,which stand for the resistance and the inductance related to the initiation of localized corrosion[47].Data fittin results are given in Table 5.Since Mg-Li alloys are very active and the strong hydrogen evolution reactions that occur during immersion can disturb the results of the corrosion rate measurements,it is worthwhile to compare the corrosion rates calculated from the EIS results to those obtained from the hydrogen evolution method.In the case of the EIS,the corrosion rate can be calculated based on the polarization resistance:
As proved by King et al.[48],Rpcan be accurately determined based on the impedance of the EEC(herein presented in Fig.8b):
Taking into consideration the EEC shown in Fig.8b,Rpis a sum of:
The measured values gave straightforward information about corrosion activity of the alloys,which followed the sequence:LPBF＞PPS＞extruded alloy(Table 6).These finding were highly consistent with the values of corrosion rate calculated from hydrogen evolution.
Fig.11.Characterization of corroded surfaces of the conventionally extruded Mg-7.5Li-3Al-Zn alloy after 1 h of immersion in 0.01 M NaCl:a)surface covered with corrosion products,b)surface without corrosion products,c)surface without corrosion products shown at higher magnification
Fig.12.Characterization of corroded surfaces of the LPBF-printed Mg-7.5Li-3Al-Zn alloy after 1 h of immersion in 0.01 M NaCl:a)surface covered with corrosion products,b)surface without corrosion products,c)surface without corrosion products shown at higher magnification
Fig.13.Characterization of corroded surfaces of the PPS Mg-7.5Li-3Al-Zn alloy after 1 h of immersion in 0.01 M NaCl:a)surface covered with corrosion products,b)surface without corrosion products,c)surface without corrosion products shown at higher magnification
Table 6Polarization resistance calculated based on EIS results.
3.4.Characterization of corroded surfaces
The SEM observations of corroded Mg-7.5Li-3Al-Zn alloys produced via different methods are shown in Figs.11-13.Figs.11a,12a,and 13a present the surfaces of the samples covered with corrosion products after 1 h of immersion in aerated 0.01 M NaCl.The least intense corrosion products were formed on the surface of the extruded samples(Fig.11a).Thick layers of oxides were formed on the surfaces of the LPBF and PPS samples,with a more compact layer being formed on the LPBF-printed alloy than on the PPS one.
To obtain a better view on the corrosion mechanisms proceeding on the alloys’surfaces,the corrosion products were removed.Clearly,micro-galvanic corrosion betweenα(Mg)andβ(Li)was a dominant mechanism in the case of the extruded alloy(Fig.11b and c),withβ(Li)being preferentially dissolved during immersion.A different situation was observed for the LPBF-printed alloy,where there was no distinguishable corrosion mechanism between the various phases;nevertheless,the printed alloy exhibited the lowest corrosion resistance among all the investigated materials.This weak corrosion resistance of the LPBF-printed alloy was a result of its high porosity.Inside pores accumulated reactions change the pH of the solution,and local chemical variations cause rapid corrosion of these areas[49,50].A very similar corrosion resistance was observed for the PPS alloy,where corrosion proceeded along unrecrystallized particle boundaries forming oxides,while micro-galvanic corrosion betweenα(Mg)andβ(Li)could also be seen.
In this study,a new approach for the manufacture of Mg-Li alloys was investigated;the microstructure and corrosion behavior of conventionally extruded Mg-7.5Li-3Al-Zn alloy were compared with those produced by powder-metallurgy techniques(LPBF and PPS).This research was carried out to elucidate the microstructure-dependent corrosion mechanisms proceeding in the dual-structured Mg-Li alloys produced from recycled powders,and the results of this work shows that powder metallurgy routes have a wide potential to be used for the manufacture of Mg-based alloys.Nevertheless,for the production of uniform,fully recrystallized,and pore-free materials,the parameters of the LPBF and PPS methods need to be further optimized and investigated.
As a result of consolidation powdered Mg-7.5Li-3Al-Zn alloy,bulk materials with various microstructures were produced.When compared to their conventionally extruded counterpart,both alloys exhibited refine microstructures with small AlLi and coarse Al,Zn,Mn enriched precipitates.The main drawback of the LPBF-printed alloy was its porosity,which had a detrimental effect on its properties.The presence of unrecrystallized particle boundaries in the PPS alloy was also unbeneficia from the point of view of corrosion.Nevertheless,the key differences observed in the microstructures of the formed alloys are important to understand,as the microstructure dictates the properties of Mg-based alloys.
As demonstrated in our previous study[38],the main corrosion mechanism in dual-structured Mg-7.5Li-3Al-Zn is the micro-galvanic corrosion processing betweenα(Mg)andβ(Li),where the most active species(as a result of its greater reactivity)isβ(Li).Corrosion in dual-phase structured Mg-Li alloys is clearly phase-size dependent,and when analyzing the corrosion mechanisms occurring in the Mg-7.5Li-3Al-Zn produced by LPBF and PPS,this statement is also accurate.Nevertheless,the results of this study showed that the corrosion mechanisms proceeding in these alloys were dependent not only on the variability of microstructural factors(qualitative factor),but also their number and distribution(quantitative factor).As shown,all the produced alloys were mainly composed ofα(Mg)andβ(Li).Not only did the relative volume fraction ofα(Mg)andβ(Li)in each of the alloy change,but also the size of the phases,and this result is the most important factor which should be discussed when analyzing the corrosion performance of the investigated alloys.As shown in our previous study[38],the volume fraction ofα(Mg)toβ(Li)plays a predominant role in the corrosion performance of the extruded alloy.The more uniform the proportion ofα(Mg)compared toβ(Li),the more uniform is the distribution of cathodic to anodic sites of corrosion.Consequently,if we were able to avoid porosity in the 3D-printed alloy,its corrosion resistance should surpass that of the conventionally extruded alloy.The volume ratios ofα(Mg)toβ(Li)were 58% to-39% and 85% to-13% for the 3D and PPS alloys,respectively.The change inα(Mg)toβ(Li)volume ratio is related to the colling rate of each method,where cooling rate during LPBF is significantl higher than colling rate of PPS[51,52].Additionally,the higher refinemen and more uniform distribution of theβ(Li)in theα(Mg)matrix should support the corrosion resistance of the alloy,and LPBF is a proven technique which uses laser melting and rapid solidificatio to directly form fine-graine microstructure[53].Nevertheless,the corrosion reactions occurring on the 3D-printed alloy were mainly affected by the porosity of the structure.Taking into consideration that corrosion inside the pores is increased by auto-catalytic processes because of the limited interchange of the solution inside the pores with that outside them,pores have a detrimental effect on the corrosive degradation of Mg alloys[54,55].Moreover,in the case of the 3D printed alloy,the real surface area,because of additional internal surface area inside the pores,is significantl larger that the exposed geometric surface area.If the parameters of LPBF could be properly optimized to prevent the growth of porosity in the investigated alloys,there is a strong indication that superior corrosion resistance,because of the significan refinemen of theα(Mg)andβ(Li)phases,may be achieved.The parameters for LPBF were chosen based on those actually used for AZ31 fabrication[53].Due to the changed alloy chemistry,especially the presence of volatile Li,further studies have to be performed to obtain a poreless structure.Metals such as Zn cause similar problems during laser processing,and specialized machines with high gas fl w rates should be used in further studies to address the issue of fuming and spatterrelated porosity[50].
In the PPS alloy,microgalvanic corrosion betweenα(Mg)andβ(Li)occurred,but it was also supported by significan corrosion at the particle interfaces.Unfortunately,the parameters chosen during material consolidation did not give full microstructure recrystallization,and unrecrystallized particleparticle interfaces were still observed.Such locations,because of their different energy compared to the inner parts of the particles,are prone to corrosion.Since the size of the particles’interfaces was observed to have a major influenc on the corrosion behavior of WE43[31],the relationship between the size of the powder particles and the corrosion resistance of Mg-7.5Li-3Al-Zn should be further investigated.
Due to various techniques of powder metallurgy used for the materials fabrication,different microstructural features were formed in the alloys produced by LPBF and PPS.Results showed that high density of stacking faults were created during LPBF,and the microhardness of this alloy was also the highest among the investigated materials.Both factors can indicate that by using LPBF there is a possibility to produce Mg-3Al-7.5Li-Zn with superior mechanical properties[56].
•Powder metallurgy methods can be beneficia in the fabrication of ultrafin and fine-graine Mg-7.5Li-3Al-Zn alloys from atomized powders.
•Dual-phase microstructures with various degree of refine ment and various volume ratio ofα(Mg)toβ(Li)were formed in the alloys depending on the manufacture route.The parameters chosen for PPS of the atomized powders did not provide full recrystallization of the microstructure,and unrecrystallized particle boundaries were still present in the alloy.
•The microstructure-dependent corrosion of the produced alloys was increased by the presence of pores in the LPBF alloy and by the unrecrystallized particle boundaries revealed in the PPS alloy.
•The advantage of the LPBF and PPS processes lies in changing the proportion ofα(Mg)toβ(Li),which when the complete consolidation of the material is achievable,may increase the corrosion resistance of dual-structured Mg-Li alloys.
•The high density of stacking faults can have detrimental effect in increasing microhardness of the Mg-Li materials produced using LPBF.
•The results show that powder metallurgy routes have a wide potential to be used for the manufacture of Mg-Li alloys.
Declaration of Competing Interest
The authors declare that they have no known competing financia interests or personal relationships that could have appeared to influenc the work reported in this paper.
Data for reference
Raw data were generated at Faculty of Materials Science and Engineering,Warsaw University of Technology.Derived data supporting the finding of this study are available from the corresponding author anna.dobkowska@pw.edu.pl on request.
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