Page - 29 - in 3D Printing of Metals

Metals 2017,7, 2 (LSM)treatmentonthecorrosionbehaviourofmagnesiumalloys,ascorrosion isessentiallyasurface degradationprocess. LSMtreatmenthasbeeneffectiveinimprovingthecorrosionresistanceofAZand AMtypeofmagnesiumalloys, suchasAZ31,AZ61,AZ91,andAM60[122–128]. Improvedcorrosion resistanceof thesealloyswasmainlyattributed to thepronouncedrefinementofα-Mggrainsand uniformredistributionof intermetalliccompoundsafter laser treatment. In thecaseofmagnesium alloys, ithasbeenobservedthat,grainrefinementandhomogenizationofcompositionaldistribution candecrease thevolumefractionof theeffectivecathodes, therebylimitingthecellactioncausedby theaccumulationofcathodicphases [129]. Inaddition, thecorrosionresistancealso increasedwith theenrichmentofaluminiumcontent in the laser-meltedzonebecauseof theselectiveevaporation ofmagnesium,providingpassivecharacteristics to themeltedsurface [124]. Enrichmentofalloying elements canshift thecorrosionpotentialofα-Mgto thepositivedirection, lowering thecorrosion susceptibilityofα-Mgmatrix[130]. Furthermore, rapidlaser-meltingcouldincreasethesolidsolubility of alloying elements, such as Mn, Al, and Cr, promoting the formation of more protective and self-healingfilmsand thus limiting theoccurrenceof local galvanic corrosion [129]. It is expected that insightsgained fromhowLSMtreatment canenhance the corrosion resistanceofmagnesium alloys, couldbehelpful inunderstandingthemechanismsassociatedwithcorrosionbehaviourofSLM processedmagnesiumalloys. Inarecentstudy,Shuaietal. [49] investigatedthecorrosionbehaviourofSLM-processedZK60 alloy by performing immersion tests inHanks’s solution (pH 7.4± 0.1) at 37± 0.5 ◦C for 48 h. Hydrogenvolumeevolutionratesobservedwere in the rangeof0.006–0.0019mL·cm−2·h−1 as the laser energy inputvaried from420 J/mm3 to750 J/mm3. The lowesthydrogenevolutionvolume rateof0.006mL·cm−2·h−1wasobservedata laserenergy inputof600 J/mm3, forwhichamaximum densificationof97.4%wasachieved. Such lowvolumereleaserateofhydrogenachieved, iswithin the limitof0.01mL·cm−2·h−1 thatcanbetoleratedbythebodywithoutposinganyserious threat [131]. SLMprocessedZK60alloyshowedsignificantenhancement incorrosionresistancecomparedtocasted ZK60,whichhadahydrogenvolumeevolutionrateof0.154mL·cm−2·h−1 (~80 timeshigher)during immersion inHank’s solution [132]. However, the outcome fromShuai et al. [49] contradicts the findings fromstudiescarriedoutbyDaietal. [133,134]whoreportedthatSLM-producedTi-6Al-4V has an unfavourable corrosion resistance compared to its traditional counterparts. The reported contradictionabout theeffectsof lasermeltingonthecorrosionbehaviourofSLMprocessedmetal powdersmighthavebeenpossiblyengineeredbydifferentmechanismsofmicrostructureevolution andcorrosionreactions,occurringduring laserprocessingofdifferentmaterials. Theexactnatureof thecorrosionreactionsandtheassociatedmechanismsaffectingthecorrosionbehaviour forvarious metallicpowdersneedtobeexploredfurther in futurestudies. WhenZK60alloywassoakedinHank’ssolution, the followingreactionsoccurred[49]: Mg→Mg2++ 2e− (6) 2H2O+2e− → H2 ↑+2OH− (7) Mg2++ 2OH− →Mg(OH)2 (8) ThecorrosionofZK60 inHank’ssolutionaccompaniedareleaseofH2. Thereactionsweredriven by the relativepotentialdifferencebetween the relative anodeandcathode.As for theZK60alloy, homogenouslydistributedMg7Zn3 secondaryphaseservedasacathodeandformedagalvaniccouple with theMgmatrix resulting inmacroscopichomogenouscorrosioncharacteristics. Thecorrosionwas triggeredbythedissolutionof theα-Mgmatrixadjacent to intermetallic compounds. Theenhanced corrosion resistance observed in SLMprocessedZK60alloy canpartly be ascribed to the increase in the corrosion potential caused by enrichment of the solid solution of Zn (caused by increased “solute capture” effect), which possessed a higher corrosion potential (−0.76V) than that ofMg (−2.34V). Thus, increase in the corrosionpotential ofMgmatrix, namely, decrease in the relative potential difference, had a beneficial effect on reducing the relative anodic and cathodic reaction 29