Effect of wire-arc directed energy deposition on the microstructural formation and age-hardening response of the Mg-9Al-1Zn(AZ91)alloy

In recent years,wire-arc directed energy deposition(wa DED),which is also commonly known as wire-arc additive manufacturing(WAAM),has emerged as a promising new fabrication technique for magnesium alloys.The major reason for this is the possibility of producing parts with a complex geometry as well as a fine-grained microstructure.While the process has been shown to be applicable for Mg-Al-Zn alloys,there is still a lack of knowledge in terms of the influence of the WAAM process on the age-hardening response.Consequently,this study deals with the aging response of a WAAM AZ91 alloy.In order to fully understand the mechanisms during aging,first,the as-built condition was analyzed by means of high-energy X-ray diffraction(HEXRD)and scanning electron microscopy.These investigations revealed a finegrained,equiaxed microstructure with adjacent areas of alternating Al content.Subsequently,the difference between single-and double-step aging as well as conventional and direct aging was studied on the as-built WAAM AZ91 alloy for the first time.The aging response during the various heat treatments was monitored via in situ HEXRD experiments.Corroborating electron microscopy and hardness studies were conducted.The results showed that the application of a double-step aging heat treatment at 325℃with pre-aging at 250℃slightly improves the mechanical properties when compared to the single-step heat treatment at 325℃.However,the hardness decreases considerably after the pre-aging step.Thus,aging at lower temperatures is preferable within the investigated temperature range of 250-325℃.Moreover,no significant difference between the conventionally aged and directly aged samples was found.Lastly,the specimens showed enhanced precipitation kinetics during aging as compared to cast samples.This could be attributed to a higher amount of nucleation sites and the particular temperature profile of the solution heat treatment.

In vitro corrosion resistance of a Ta2O5 nanofilm on MAO coated magnesium alloy AZ31 by atomic layer deposition

Micro-arc oxidation(MAO)coating with outstanding adhesion strength to Mg alloys has attracted more and more attention.However,owing to the porous structure,aggressive ions easily invaded the MAO/substrate interface through the through pores,limiting long-term corrosion resistance.Therefore,a dense and biocompatible tantalum oxide(Ta2O5)nanofilm was deposited on MAO coated Mg alloy AZ31 through atomic layer deposition(ALD)technique to seal the micropores and regulate the degradation rate.Surface micrography,chemical compositions and crystallographic structure were characterized using FE-SEM,EDS,XPS and XRD.The corrosion resistance of all samples was evaluated through electrochemical and hydrogen evolution tests.Results revealed that the Ta2O5 film mainly existed in the form of amorphousness.Moreover,uniform deposition of Ta2O5 film and effective sealing of micropores and microcracks in MAO coating were achieved.The current density(icorr)of the composite coating decreased three orders of magnitude than that of the substrate and MAO coating,improving corrosion resistance.Besides,the formation and corrosion resistance mechanisms of the composite coating were proposed.

Crack Types, Mechanisms, and Suppression Methods during High-energy Beam Additive Manufacturing of Nickel-based Superalloys: A Review

Nickel-based superalloys have been widely used in aerospace fields,especially for engine hot-end parts,because of their excellent high-temperature resistance.However,they are difficult to machine and process because of their special properties.High-energy beam additive manufacturing(HEB-AM)of nickel-based superalloys has shown great application potential in aerospace and other fields.However,HEB-AM of nickel-based superalloys faces serious cracking problems because of the unique characteristics of superalloys,and this has become the most significant bottleneck restricting their application.In this review,the current research status related to the types,formation mechanisms,and suppression methods of cracks in nickel-based superalloys produced by HEB-AM is described.The initiation and propagation mechanisms of cracks and their multiple influencing factors are also analyzed and discussed.Then,several possible research directions to solve the cracking problems in nickel-based superalloys produced by HEB-AM are outlined.This review provides an in-depth and comprehensive understanding of the cracking problem in AM nickel-based superalloys.It also provides valuable references for AM crack-free nickel-based superalloy components.

High Energy Micro-arc Alloying of ZM5 Mg Alloy by Mg-Y Electrode under Liquid

High-energy micro-arc alloying (HEMAA) is a consumable electrode,micro-welding process which uses electrical pulses that are typically three orders of magnitude shorter than in other pulse welding processes.Pulse durations of a few microseconds combined with pulse frequencies in the 0.1 to 2-kilohertz range thus allow substrate heat dissipation over ～99% of the duty cycle while heating only about 1%.The cooling rates may approach 105 to 106 ℃/s,depending on material.This obtained coating can produce unique corrosion and tribological benefits.Substrates require no special surface preparation and nearly any metal,alloy or cermets can be applied to metal surfaces.This paper details experimental results when alloying Mg base alloy ZM5,using Mg-Y electrodes with a water-based dielectric fluid.The morphology and the composition of the alloying was analyzed by scanning electron microscope(SEM) and energy dispersive X-ray analysis(EDX).At the discharge energies employed,a thin recast layer thickness and rougher layers were formed on the surface.All layers were in general discontinuous.EDX analysis showed that the Y transferred from the solid electrodes to the workpiece surface.