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.

Microstructure evolution and age-hardening response in Mg-Sn-Sm alloys under a wide range of Sm/Sn ratio

The microstructure evolution and age-hardening response for different Sm/Sn ratios(0-2.55,in wt.%)of Mg-Sn-Sm alloys were investigated.The second phase formation in as-cast alloys and the Mg_(3)Sm precipitates formed in aged alloys were characterized using XRD,FESEM and HAADF-STEM with EDS techniques.Results indicate that the Sm/Sn ratio has a great influence on the phase constitution,α-Mg grain size and age-hardening response.With the increment of Sm/Sn ratio,Mg_(41)Sm_(5) and thermally stable MgSnSm phases precipitate.When the Sm/Sn ratio is about 1.19,the secondary dendrite arm spacing ofα-Mg grains significantly decreases.Furthermore,the alloy with Sm/Sn ratio up to 2.55 exhibits the highest age-hardening response,the hardness value increases from 52 HB at solution-treated condition to 74 HB at peak-aged condition(ageing at 220 ℃ for a short time of 4 h).This is attributed to the large volume fraction of needle-like Mg_(3)Sm precipitates formed in theα-Mg matrix during ageing treatment,which results in a significant precipitation strengthening effect.

Age-hardening behavior,corrosion mechanisms,and passive film structure of nanocrystalline Al-V supersaturated solid solution

The effect of age-hardening on microstructure,hardness,and corrosion of an Al-5at.%V alloy,produced us-ing high-energy ball milling and subsequent cold compaction,has been investigated.The alloy exhibited a grain size below 100 nm and extremely high solid solubility of V in Al(3.1 at.%).The age-hardening was carried out at 150,200,and 250℃.The peak-aged condition of 150℃ demonstrated the highest hardness-transpired from grain refinement,precipitation,and solid solution hardening.The corrosion re-sistance of the Al-5at.%V alloy was studied as a function of aging conditions.The peak-aged condition retained the corrosion resistance while it deteriorated in the over-aged condition.Nonetheless,the cor-rosion resistance of the ball-milled Al alloys in all the aging conditions was superior to that of pure Al.The passive film structure and origin of corrosion were studied using scanning/transmission electron mi-croscopy(S/TEM).The high corrosion resistance of the alloy was attributed to the V enrichment at the film/metal interface and deposition of V on the cathodic phases,which suppresses the dissolution of Al within the pit and therefore promotes repassivation in the early stages of corrosion.

Structures and formation mechanisms of dislocation-induced precipitates in relation to the age-hardening responses of Al-Mg-Si alloys

In the slightly deformed Al-Mg-Si alloys,dislocation-induced precipitates are frequently observed,and they usually line up,forming sophisticated precipitation microstructures.Using atomic-resolution electron microscopy in association with hardness measurements,we systematically investigated these precipitates in relation to the age-hardening responses of the alloys.Our study reveals that the majority of dislocation-induced complex precipitates are actually short-range ordered while long-range disordered polycrystalline precipitates and multiphase composite precipitates,including polycrystalline U2 precipitates,B’/U2,B’-2/U2,B’/B’-2/U2 and’/U2 composite precipitates.It is suggested that the formation of these complex precipitates is mainly owing to a high nucleation rate and rapid growth of different precipitate phases parallel to the associated dislocation lines.Since dislocation-induced precipitates consume more Mg than Si from the matrix and have a high formation kinetics,they will have different impacts on the matrix precipitation in different types of Al-Mg-Si alloys.Our results further demonstrate that for the"normally-β"-hardened"alloy,their formation leads to a coarser precipitate microstructure in the matrix,whereas for the"normally-β’-hardened"alloy,their formation reverses the precipitation pathway in the matrix,resulting in a reduced age-hardening potential of the former alloy and an improved age-hardening potential of the latter alloy.

Enhanced age-hardening behavior in Al–Cu alloys induced by in-situ synthesized TiC nanoparticles

The influence of in-situ synthesized TiC nanoparticles on age-hardening behavior of Al–Cu alloys was investigated in Al–4.5 Cu–1.5 TiC alloy. It was found that TiC nanoparticles decrease the peak-age time effectively, from about 20 h for Al–4.5 Cu alloy decreasing to about 12 h for the Al–4.5 Cu–1.5 TiC. Mechanical property test shows that the age-hardening effect has been improved by the TiC nanoparticles. The increment of yield strength before and after aging is about 84 MPa for Al–4.5 Cu, while, it reaches to about113 MPa for the Al–4.5 Cu–1.5 TiC. After aging heat treatment, precipitates have been observed both in matrix and around TiC nanoparticles. Due to the difference of coefficient of thermal expansion between TiC and Al, high density dislocations in the Al–4.5 Cu–1.5 TiC were generated during water quenching after solution. Dislocations play a role of diffusion path for Cu atoms during aging, which reduces the peak-age time. Alpha-Al lattice distortion resulted from lattice mismatch of TiC/Al interface induces the precipitation of θ' phase around TiC nanoparticles, which increases the number density of θ' and improves the age-hardening effect. This finding is supposed to be also applicable to alloy systems of Al–Cu–Mg,Al–Cu–Mg–Li, Al–Cu–Mg–Ag, etc.

Microstructural evolution and age-hardening behavior of quasicrystal-reinforced Mg-Dy-Zn alloy

Microstructural evolution and age-hardening behavior of Mg-2 Dy-6 Zn(at%)alloy during solid-solution and aging treatment were investigated.The microstructure of as-cast alloy is composed of a-Mg,Mg3 DyZn6(Ⅰ)phase,Mg3 Dy2 Zn3(W)phase,Mg(Zn,Dy)phase and a small amount of Mg0.97Zn0.03 phases.After solid-solution treatment(480℃,12 h),all the I phases and most W phases dissolve into a-Mg matrix and the remainder W phases transform into Mg(Dy,Zn)phase and MgDy3 phase.During aging treatment,I phase and small amounts of W phases co-precipitate from α-Mg matrix,respectively.The alloy exhibits a peak hardness of HV 77.5 at 200 ℃ for 8 h.The excellent age-hardening behavior of alloy is mainly attributed to the co-precipitation strengthening of I and W phases.

Fatigue properties of age-hardened AI alloy 2017-T4 under ultrasonic loading

Fatigue properties of age-hardened Al alloy 2017-T4 under ultrasonic loading frequency （20 kHz） were investigated and compared with the results under conventional loading of rotating bending（50 Hz）.The growth of a crack retarded at about 500μm in surface length under ultrasonic loading,while at about 20μm under rotating bending.Although striations being a typical fracture mechanism were observed under conventional loading,most of fracture surface was covered with many facets under ultrasonic loading.These facets were also observed under rotating bending in nitrogen gas.The difference in growth mechanism depending on the loading frequency and the retardation of a crack growth under ultrasonic loading may be caused by the environment at the crack tip due to high crack growth rate under ultrasonic loading.

Hardening effect and precipitation evolution of an isothermal aged Mg-Sm based alloy

The age-hardening behavior and precipitation evolution of an isothermal aged Mg-5Sm-0.6Zn-0.5Zr(wt.%) alloy have been systematically investigated by means of transmission electron microscopy(TEM) and atomic-resolution high-angle annular dark field scanning transmission electron microscopy(HAADF-STEM). The Vickers hardness of the present alloy increases first and then decreases with ageing time. The sample aged at 200 ℃ for 10 h exhibits a peak-hardness of 90.5 HV. In addition to the dominant β_(0)’ precipitate(orthorhombic,a = 0.642 nm, b = 3.336 nm and c = 0.521 nm) formed on {11-20}α planes, a certain number of γ’’ precipitate(hexagonal, a = 0.556 nm and c = 0.431 nm) formed on basal planes are also observed in the peak-aged alloy. Significantly, the basal γ’’ precipitate is more thermostable than prismatic β_(0)’ precipitate in the present alloy. β_(0)’ precipitates gradually coarsened and were even likely to transform into β_(1) phase(face centered cubic, a = 0.73 nm) with the increase of ageing time, which accordingly led to a gradual decrease in number density of precipitates and finally resulted in the decreased hardness and mechanical property in the over-aged alloys.

EFFECT OF Cr ADDITION ON MICROSTRVCTURES AND MECHANICAL PROPERLIES OF β-NiAl ALLOYS

The effect of Cr addition on the microstructures and mechanical properties of NiAl inermetallic alloys containing 4 to 8 mol% Cr has been characterized. It is shown that Cr is not only an efficient soild solution strengthener in NiAl, it is also an efficient precipitation Strengthener by fine dispersion of α-Cr particles. Transmission electron microscope observations have revealed that fine spherical particles of α-Cr appeared homogeneously in the NiAl matrix by aging at temperatures around 1073K after solution annealing at 1563K. The aging behavior was investigated by microhardness measurements and the temperature dependence of the yield stress of NiAl containing α-Cr particles was investigated by compression tests over the tempemture range from 298 to 1173K. NiAl alloys happened appreciably by the precipitation of α-Cr particles.Dislocations bypass the particles during deformation and typical Orowan loops were observed surrounding the α-Cr particles after deformation. Although the dispersion Of α-Cr particles increases the yield strength of NiAl at intermediate temperutre, the strength decreases appreciably at higher temperatures.

Effects of Solution Treatment Time and Sr-Modification on Microstructure and Mechanical Property of Al-Si Piston Alloy

The purpose of this paper was to investigate the effects of solution treatment time and Sr-modification on the microstructure and property of the Al-Si piston alloy.It was found that as-cast microstructures of unmodified and Sr-modified Al-Si alloys consisted of a coarse acicular plate of eutectic Si,Cu_3NiAl_6 and Mg_2Si phases in theα-Al matrix but different in size and morphology.Both size and inter-particle spacing of Si particles were significantly changed by increasing the solution treatment time.After a short solution treatment,the coarse acicular plate of the eutectic Si appears to be fragmented.Fully modified microstructure of Sr-modified alloy can reduce the solution treatment time compared to unmodified alloy.The maximum of a peak hardness value is found in the very short solution treatment of both Al-Si piston alloys.Compared to 10 h solution treatment,the solution treatment of 2-4 h is sufficient to achieve appropriate microstructures and hardness. The short solution treatment is very useful to increase the productivity and to reduce the manufacturing cost of the Al-Si piston alloys.

MICROSTRUCTURES AND MECHANICAL PROPERTIES OF β-NiA1CONTAINING α-Fe PRECIPITATES

The microstructures of B2-ordered NiA1 containing α-Fe precipitates have been investigated in terms of transmission electron microscopy. Fine precipitation of α-Fe (bcc structure) occurs in NiA1 by aging around 973K. The aging behavior was investigated by microhandness measurements and the temperature dependence of the yield strength of precipitate- comaining B2- ordered NiA1 was investigated by compression tests over the temperature range of 673-1273K. The fine precipitation of α-Fe was found to enhance greatly the low and intermediate temperature yield strength. When the deformation temperature is over 1073K, the strength of precipitate- containing NiA1 was comparable to ternary solid solution hardening NiA1-Fe. Dislocations bypass the particles during deformation and typical Orowan loops were observed surrounding the or-Fe particles after deformation.

Effect of ageing treatment on microstructures,mechanical properties and corrosion behavior of Mg-Zn-RE-Zr alloy micro-alloyed with Ca and Sr

Effects of ageing treatment on the microstructures,mechanical properties and corrosion behavior of the Mg-4.2Zn-1.7RE-0.8Zr-xCa-ySr[x=0,0.2(wt.%),y=0,0.1,0.2,0.4(wt.%)]alloys were investigated.Results showed that Ca or/and Sr additions promoted the precipitation hardening behavior of Mg-4.2Zn-1.7RE-0.8Zr alloy and shortened the time to reaching peak hardness from 13 h to 12 h.The maximum hardness of 77.1±0.6 HV for the peak-aged Mg-4.2Zn-1.7RE-0.8Zr-0.2Ca-0.2Sr alloy was obtained.The microstructures of peak-aged alloys mainly consist ofα-Mg phase,Mg_(51)Zn_(20) phase and ternary T-phase.The Zn-Zr phase is formed within theα-Mg matrix,and the Mg_(2)Ca phase is formed near T-phase due to the enrichment of Ca in front of the solid-liquid interface.Furthermore,fine short rod-shapedβ′1 phase is precipitated within theα-Mg matrix in the peak-aged condition.The peak-aged Mg-4.2Zn-1.7RE-0.8Zr-0.2Ca-0.2Sr alloy exhibits optimal mechanical properties with an ultimate tensile strength of 208 MPa,yield strength of 150 MPa and elongation of 3.5%,which is mainly attributed to precipitation strengthening.In addition,corrosion properties of experimental alloys in the 3.5wt.%NaCl solution were studied by the electrochemical tests,weight loss,hydrogen evolution measurement and corrosion morphology observation.The results suggest that peak-aged alloys show reduced corrosion rates compared with the as-cast alloys,and minor additions of Ca and/or Sr improve the corrosion resistance of the Mg-4.2Zn-1.7RE-0.8Zr alloy.The peak-aged Mg-4.2Zn-1.7RE-0.8Zr-0.2Ca-0.2Sr alloy possesses the best corrosion resistance,which is mainly due to the continuous and compact barrier wall constructed by the homogeneous and continuous second phases.

Residual stress and precipitation of Mg-5Zn-3.5Sn-1Mn-0.5Ca-0.5Cu alloy with different quenching rates

The effect of the quenching rate after solution treatment on the residual stress and precipitation behavior of a high strength Mg-5 Zn-3.5 Sn-1 Mn-0.5 Ca-0.5 Cu plate is studied.The simulation results show decreasing temperature gradient in the plate with decreasing quenching rate,which leads to weakened inhomogeneous plastic deformation and decreased residual stress.No dynamic precipitation on the grain boundary happens after either cold water cooling or air cooling,however,air cooling leads to dynamic precipitation of Mg-Zn phase on Mn particles around which a low-density precipitate zone develops after aging treatment.Moreover,the fine and densely distributed Mg-Zn precipitates observed after aging treatment of the cold water cooled alloy are replaced by coarse precipitates with low density for the air cooled alloy.Both the low-density precipitate zone near Mn particles and the coarsening of precipitates are the source of the decrease in hardness and tensile properties of the air cooled alloy.The residual stress drops faster than the hardness with decreasing quenching rate,which makes it possible to lower the residual stress without sacrificing too much age-hardening ability of the alloy.

PRECIPITATION BEHAVIOR OF Co PHASES IN B2-ORDERED(Ni,Co)Al COMPOUND

The precipitation behavior of Co phases in B2-ordered (Ni,Co)Al has been investigated in terms of transmission electron microscopy. Fine precipitation of fcc-Co occurs in (Ni,Co)Al by aging at temperature over 973K. The orientation relationship between the fee-Co precipitates and the B2-(Ni,Co)Al matrix follows the Kurdjumow-Sachs (K-S) orientation relation. But when the aging temperature is under 873K the Co precipitates have a hcp crystal structure. The orientation relationship between the hep-Co precipitates and the B2-(Ni,Co)Al matrix follows the Burgers orientation relation. (Ni,Co)Al is hardened appreciably by the fine precipitation of both the fee-Co and hep-Co phases. The temperature dependence of the yield strength of precipitate-containing BS-ordered (Ni, Co)Al was investigated by compression tests over the range of 298-1273K. The fine precipitation of Co phases enhances greatly the low and intermediate temperature yield strength. When the deformation temperature was over 87SK, the strength of precipitate-containing (Ni, Co)Al is comparable to ternary dual-phase (Ni,Co)Al+Ni3Al alloy.

HARDENING OF L1_2-ORDERED Co_3Ti BY PRECIPITATION OF FCC-Co PHASE

The Co3 Ti phase hardens appreciably by the fine precipitation of disordered fcc Co-rich phase upon aging after quenching from solution annealing temperature. Transmission electron microscope (TEM)observations revealed that the precipitates are platelet in shape, lying nearly parallel to the {100} planes of the L12-ordered matrix, and perfectly coherent with the matrix lattice at the beginning of aging. The high temperature strength increases appreciably with the fine precipitation of disondered Co-rich phase over the whole temperature range investigated. TEM observations of the deformed alloys after underaging revealed that saperdislocations are pinned by precipitates indicating an attractive interaction between dislocations and precipitates.

A hidden precipitation scenario of theθ’-phase in Al-Cu alloys

Al-Cu binary alloys are important and interesting industry materials.Up to date,the formation mechanisms of the key strengthening precipitates,namedθ’-phase,in the alloys are still controversial.Here,we report that for non-deformed bulk Al-Cu alloys theθ’-phase actually has its own direct precursors that can form only at elevated aging temperature(>ca.200℃).These high-temperature precursors have the same plate-like morphology as theθ’-phase precipitates but rather different structures.Atomicresolution imaging reveals that they have a tetragonal structure with a=0.405 nm and c=1.213 nm,and an average composition of Al_(5-x)Cu_(1+x)(0≤x<1),being fully coherent with the Al-lattice.This precursor phase may initiate with a composition of Al5 Cu and evolve locally towards Al_(4)Cu_(2)in composition,eventually leading to a consequent structural transformation into theθ’-phase(Al4 Cu2=Al2 Cu).There are evidences that because of their genetic links in structure,such a high-temperature precursor may transform to theθ’-phase without having to change their morphology and interface structure.Our study reveals a well-defined and previously hidden precipitation scenario for theθ’-phase to form in Al-Cu alloys at an elevated aging temperature.

Microstructure and mechanical properties of as-cast and heat treated Mg-15Gd-3Y alloy

The microstructure evolution and mechanical properties of Mg-15Gd-3Y alloy were investigated in the as-cast and heat treated conditions. The microstrucmre evolution from as-cast to cast-T4 states involved a-Mg solid solution＋Mg5（Gd,Y） phase→a-Mg supersaturated solid solution＋rare earths compound Mg3（Gdl.26,Y0.74）→a-Mg supersaturated solid solution＋rare earths compound Mg3（Gd0.Tas,Y1.255）. It showed that 480 ℃/4 h was the optimal solution treatment parameter. If the solution temperature was high or the holding time was long, such as 520 ℃/16 h, an overheating phenomenon would be induced, which had a detrimental effect on the mechanical properties. When ageing at 225 and 200℃, the alloy would exhibit a significant age-hardening response and great long-time-age-hardening potential, respectively. The best mechanical properties were obtained at the parameters of 480 ℃/4 h＋225 ℃/16 h, with the UTS of 257.0 MPa and elongation of 3.8%.

Improved Properties in Relation to Fine Precipitate Microstructures Tailored by Combinatorial Processes in an Al-Cu-Mg-Si Alloy

The 2xxx series Al alloys have been widely used in aerospace industry owing to their high strength,good plasticity and superior formability.To ensure a good control of shape,the quenched alloy sheets require a small pre-deformation before artificial aging.However,this pre-deformation considerably deteriorates the mechanical strength of the Al-3.0 Cu-1.8 Mg-0.5 Si(wt%)alloys due to the formation of unfavorable large-sized precipitates at dislocations.To tackle this issue,we designed a pre-aging process prior to the pre-deformation.The thermal-mechanical treatment,involving pre-aging,pre-deformation and subsequent aging,markedly enhanced the ultimate tensile strength up to 521 MPa compared to that(448 MPa)of the alloy without pre-aging.Microstructure characterization revealed that the fine precipitates(~2 nm)with a uniform dispersion were promoted within the Al matrix,which in turn partly suppressed the formation of the unfavorable large-sized precipitate(~100 nm).Our findings provide a new clue for designing stronger Al alloys with age-hardenability.