•NiCrAlCoY-Al coatings were fabricated on Ti-6Al-4V substrate.•Gradient composite structure was formed after mechanical alloying treatment.•The high-temperature oxidation resistance of the coating was investigated.•The oxidation process of the coating at evaluated temperature was elucidated.In the present work, NiCrAlCoY-Al gradient composite coatings were successfully fabricated on Ti-6Al-4V alloy substrate via high-energy mechanical alloying method. The microstructures and elemental and phase composition of the coatings have been studied. Effects of both NiCrAlCoY-Al powder ratio and milling time on the fabrication of composite coating were investigated. The NiCrAlCoY-Al weight ratio 65:35 was considered the optimal powder ratio to fabricate the coating at the selected milling parameters. The high-temperature cyclic oxidation tests were performed at 850 °C and 750 °C. The fabricated sample using 65:35 wt ratio and 4 h milling time showed the best oxidation resistance due to the low mass gain, rutile TiO2 undetected surface, and densified composite cross-section morphology. The decreased oxygen content of the oxidized cross-section line scanning results revealed the improvement of oxidation resistance. The oxidation process of the as-fabricated coating had been divided into five steps. Three protective Al2O3 oxide layers together with other alloy compounds could prevent the further diffusion of oxygen. The coating could enhance the oxidation resistance of the substrate.

•Friction stir processing in water is used to produce Al5083/Ti composite.•The external water has inhibitive effect on the growth of recrystallized grain and the Al/Ti interfacial reaction.•The fine grain structures obtained by FSP in water can significantly improve the strength.•The excellent Al/Ti interface bonding can avoid great loss in ductility.The hard metallic particle as reinforcements can improve the strength of Al matrix without severely sacrificing its ductility. However, it is technically challenging to fabricate metallic particle reinforced aluminum matrix composites (AMCs) for easy reactivity of Al with most metals. In the present work, friction stir processing (FSP) was used to disperse titanium particles into 5083 Al matrix to fabricate AMCs. Especially, FSP was performed in water to better inhibit the Al/Ti interfacial reaction and the recrystallized grain growth. The effects of the external water on the microstructure and mechanical properties of FSPed AMCs were investigated. Results showed that the external water significantly lowered peak temperature and increased cooling rate, which not only rapidly reduced material flow, but also effectively inhibited grain growth, thereby creating finer grains about 1 μm. Also, an excellent Al/Ti interfacial bonding consisting of an element solid solution was obtained in both FSPed composites. Tensile test results indicated that water-FSPed AMCs possessed the higher yield strength (265 MPa) and ultimate tensile strength (423 MPa) compared with the 5083Al matrix (YS: 198 MPa, UTS: 301 MPa) and the air-FSPed AMCs (YS: 236 MPa, UTS: 383 MPa). Moreover, an appreciable amount of ductility (14.9%) was retained despite of their evidently increased strength.

•Friction stir welding (FSW) assisted by electric arc heating was used to join AA6061 aluminum alloy to E235A steel.•The temperature distribution of the work-piece was investigated under the conditions of preheating and no preheating.•The flow of the material was investigated under the conditions of preheating and no preheating.•The temperature distribution of the work-piece was significantly changed by the preheating treatment.•The flow velocity of the material improved significantly with preheating treatment.Material flow and temperature distribution during welding are important factors that affect the properties of friction stir welding (FSW) joints. The scope of this investigation is to evaluate the effect of preheating treatment on the temperature distribution and material flow of aluminum alloy and steel joints produced by FSW. In this investigation, the finite volume model of butt FSW was established on basis of the FLUENT software. The flow of material during the FSW was simulated, and the temperature distribution in the FSW process was obtained by experiment and simulation. The effects of preheating treatment on the thermal history, temperature distribution, and material flow during FSW were elucidated in detail. The effects of preheating on the tensile strength and failure modes of the specimens were also discussed. The simulation shows that the preheating treatment increased the peak temperature of the steel, and also reduced the large temperature difference between the steel and the aluminum alloy in the high temperature stage. Simulation results, including temperature distribution and thermal history, were corresponded with the experimental results. The preheating treatment improved the flow velocities of the plastic materials and reduced the difference in flow velocities of the two materials. The preheating treatment could reduce the influence of the difference between the two materials on the forming of the welded joint and improve the property of the welded joint.Download high-res image (209KB)Download full-size image

•Mechanical alloying method was applied to form Al–B4C composite coating.•The effects of annealing treatment on the coating were investigated.•The mechanical properties of the coating were investigated.•The oxidation behavior of the coating was analyzed and discussed.An Al-B4C coating was synthesized on Ti–6Al–4V alloy substrate with Al and B4C powder mixture by means of mechanical alloying. The coating had a B4Cp/Al metal matrix composite structure. The effects of the annealing treatment on the microstructure, the mechanical properties and the high-temperature oxidation resistance of the coating were investigated. The annealed coating had a bi-layer structure. The inner layer was an Al3Ti layer and the outer layer consisted of Al3BC, AlB2 and residual Al and B4C. The adhesion behavior of the coating decreased after annealing treatment. The microhardness of the coating was improved to > 4 times of the substrate hardness because of the formation of ceramic and intermetallic phases. The coatings could protect the substrate under dry sliding conditions. The oxidation resistance of the coating was improved after annealing. In the annealed coating, on the one hand, the B4C particles were surrounded by Al3BC, which could prevent the direct oxidation of B4C during oxidation; on the other hand, the thick Al3Ti layer could act as an oxygen barrier to retard the inward diffusion of oxygen. The annealing treatment could improve both the surface mechanical properties and the high-temperature oxidation resistance of the Al–B4C coating.

•The distribution of temperature during lap FSW of dissimilar aluminum alloys was described.•The temperature change along the depth direction was not continuous and had saltation.•The influences of the position, time and material properties on the temperature field were discussed.A Computational Fluid Dynamics (CFD) based modified three-dimensional model was developed to predict the distribution of temperature during lap friction stir welding (FSW) of dissimilar aluminum alloys AA2024 and AA7075. Volume of fluid approach was used and the lap FSW process was modeled as a steady-state visco-plastic turbulent flow past a rotating tool. The experimental records of transient temperature at several specific locations were used for checking the temperature field gained by numerical simulation. The temperature distribution, variations in temperature and thermal history of the processed zone were investigated. Results indicate that there is an obvious asymmetric temperature distribution between the advancing side (AS) and the retreating side (RS). The temperature distribution and variation of each observation point was different. The temperature of the observation points rose rapidly but decreased slowly. In particular, the temperature distribution was not continuous due to the difference of the material properties, and the temperature changes had saltation at the interface of upper and lower plates.During the FSW process, the temperature distribution and variation of observation points are affected by position, time and material properties. Especially due to the physical properties of two kinds of materials of different results in the change of temperature along the z direction is not continuous, and the temperature changes have saltation at the interface of upper and lower plates.

•Chrome–copper composite coatings were prepared on pure Cu substrate.•The copper content has effects on roughness and adhesion of coatings.•A multilayered structure was formed after treatment in copper-rich powder.•The fabricated coatings could largely strengthen the substrate surface.Cr–Cu coatings with a composite structure were fabricated by means of mechanical alloying. Powders with various Cr–Cu ratios were used to fabricate coatings. Microstructures and chemical compositions of as-synthesized coatings were analyzed by SEM and EDS. Phases and microstructural features of ball milled powders were identified by XRD and SEM. The results indicated that large amount of initial Cu powder addition led to the excess of pure Cu after Cu–Cr mechanical alloying, which prevented particles from being fractured and thus resulted in the increase of surface roughness. Coatings with a multilayered structure could be detected in the cross-section micrographs. The formation mechanism of the coating was discussed. Adhesion behaviors of the coatings were determined through the combination of scratch tests and SEM equipped with EDS. Mechanical properties such as microhardness and friction and wear resistance were also tested. The test results provided an evidence of the surface reinforcement of as-synthesized coatings. Taking into consideration surface roughness, coating adhesion and mechanical properties including microhardness and friction and wear resistance, the optimal Cr–Cu ratio of starting powder was about 13:7 at the selected mechanical alloying processing parameters.

•A novel method of chemical activation was used to make surface modification for TA2 substrate.•TA2 modified surface was typical of a flower-like structure and fully covered by the Ce-rich phase.•Formation mechanisms of electrodeposited coatings on the activated TA2 surface were validated.•Superior tribological properties of Ni–CeO2 coatings were attributed to the Ce-rich abrasive products.In order to remedy the poor tribological properties and low micro-hardness for pure titanium (TA2) materials, the Ni–CeO2 nanocrystalline coating was deposited from a Watts–Nickel electrolyte on the modified surface of TA2 substrate. In the present study, an available method of chemical activation was innovatively employed to make surface modification for TA2 substrate, in which the modified surface was characteristic of a rough surface and fully covered by the Ce-rich conducting phase for better electrodeposition. In order to study the effect of surface pretreatments on the modified TA2 surface, the surface roughness profiles and average roughness values were carried out using optical profilometry on the measured surfaces. A predictive modeling of TA2 surfaces before and after surface pretreatments was compared and successfully validated for better understanding of the formation mechanisms of electrodeposited Ni coatings reinforced with CeO2 nanoparticles. To clarify the beneficial effects of CeO2 addition on surface morphologies, phase composition, and textural evolution, the as-received nanocrystalline coatings were evaluated using various analytical techniques such as XRD, FE-SEM/EDX, and TEM. In addition, the scratch tests performed with an acoustic emission (AE) detector were conducted for the determination of interfacial adhesion between the as-deposited coating and the modified surface of TA2 substrate. Tribological properties and micro-hardness of investigated specimens were also examined. Experimental results revealed that this innovatively chemical activation was more favorable for increasing interfacial adhesion. The existence of well-distributed CeO2 phase that precipitated along the defective grain boundaries in the coating was contributed to make particulate reinforcement, thereby achieving better structural densification for Ni–CeO2 coatings. Meanwhile it exhibited superior tribological properties of the Ni–CeO2 coatings as compared to those of pure nickel and TA2 substrate, which was mainly attributed to the Ce-rich abrasive products that acted as a self-lubricating phase to make the effect of solid lubricants on worn surfaces.

In the present work, a novel method of ultrasonic-assisted double-pulsed electro deposition was innovatively used to prepare nanocrystalline Ni coatings from a modified Watts–Nickel electrolyte without and with the addition of CeO2 nanoparticles. The dynamic oxidation kinetics of pure nickel and the Ni–CeO2 coatings was conducted in air at 650 and 850 °C. To clarify the beneficial effects of CeO2 addition on surface morphologies, phase composition and microstructural evolution, the as-received coatings were investigated using various analytical techniques of FE-SEM, XRD, XPS, and HR-TEM observations. In addition, both EDX maps and TG–DSC scans were also carried out for further evaluating the effective bonding effect that generated from the Ce-rich precipitated phase on the inhibition of coarsening growth and textural transformation. Meanwhile, an analytical model for predicting the growth mechanism of oxide scales was successfully validated for the comparison of their different oxidation behaviors. Experimental results illustrated that interfacial boundaries existed in nanocrystalline Ni coatings were favorable for embedding CeO2 nanoparticles to make dispersion strengthening. The existence of well-distributed CeO2 phase in the coating was beneficial to achieve finer-grained size of Ni grains for improvements of structural densification. It exhibited super oxidation resistance of the Ni–CeO2 coatings as compared to that of pure nickel, which was mainly attributed to the Ce-rich oxide scales that acted as a passive layer to fully cover on the coating surface. Furthermore, the precipitation of the Ce-rich phase along high-angle grain boundaries would make great contributions to promote the bonding interactions with high-energy defective regions for effectively completing those cracking boundaries, thereby achieving denser microstructure attached with excellent thermodynamic stability of the Ni–CeO2 coatings.

•Coatings with amorphous microstructure were prepared on Ti–6Al–4 V alloy substrate.•The composite layer and amorphous layer made up the coatings.•The fabricated coatings could largely strengthen the substrate surface.•The formation mechanism of coatings with two structured layers was presented.A high-energy planetary ball mill was applied to fabricate Ti–Cu–Al coating on Ti–6Al–4 V alloy substrate. The microstructure and elemental and phase composition of coatings were studied. The surface SEM images showed that the rough but densified coating surfaces, the cross-section microstructures as well as the related EDS line scan results illustrated different structures in the coatings, and the XRD results as well as TEM and corresponding SEAD pattern provided evidences of the existence of amorphous phase in the coatings. Mechanical properties including microhardness and friction and wear behavior were tested respectively. The microhardness testing results indicated that the fabricated coatings could largely strengthen the substrate surface. And the friction and wear resistance were tested with a ball-on-block sliding configuration, with the testing results showing decrease in both friction coefficients and wear weight loss after mechanical alloying treatment. The analysis and testing results also showed that a proper increase in the applied milling time favored the improvement in the thickness, surface smoothness, densification level, microstructural and chemical homogeneity as well as amorphization degree of the deposited coating. On the basis of two significant action mechanisms associated with a planetary ball mill, a reasonable mechanism was presented for the formation of amorphous phase contained coatings on Ti–6Al–4 V alloy substrate.

After the process optimization experiments according to a series of principles stated in the paper, the TiC particle-reinforced surface composites were successfully fabricated via friction stir processing (FSP) method on the Ti–6Al–4V alloy substrates, using a surface-reservoir reinforcement placement method for embedding TiC powder into the substrate. A relatively uniformed dispersion and an ultra-refined average size of the introduced TiC reinforcements were obtained. The reinforcement behaviors of dispersion and refinement were investigated in consideration of the plasticized Ti-matrix material-flow characteristics during FSP. Nano-sized martensite-α′ phase and ultra-refined TiC particles were found in the TiCp/Ti–6Al–4V surface composites. The average micro-hardness of the produced surface composites was 680Hv0.2. The thicknesses of hardening surface layer with the hardness upon 650Hv0.2 reached 900 μm. It was further discussed on the hardening mechanisms including ceramic particle strengthening, nano-sized refinement pinning effect and matrix martensite-α′ influence.

•Enlarged boundaries provided much adsorption space for embedding CeO2 nanoparticles.•The precipitated Ce-rich phase was beneficial to make a bonding link between boundaries.•Improved corrosion resistance was attributed to the formation of insoluble corrosion products.This paper presents an experimental study of the effects of CeO2 addition on the microstructure and corrosion behavior of nanocrystalline Ni coatings. A novel method of ultrasound-assisted pulsed electrodeposition was used to prepare the coatings from a modified Watts-nickel electrolyte. Surface morphologies and micro-structural evolution of the coatings before and after aging treatments were characterized using E-SEM and TEM observations. Furthermore, electrochemical techniques attached with the XRD and XPS analysis were applied to study their corrosion behavior in a 5 wt.% HCl aqueous solution. The underlying mechanisms of corrosion inhibition resulted from the Ce-rich passive layer were discussed in detail. Experimental results indicated that the existence of well-distributed CeO2 phase in the coatings promoted dispersion strengthening through a proper aging treatment at 650 °C for 4 h, thereby achieving an effective bonding link between interfacial boundaries for denser microstructure. According to the measured results from polarization curves and the AC-impedance measurements, it exhibited superior corrosion resistance of the Ni-CeO2 coatings as compared to pure nickel. During the long-term static immersion tests in an acid 5 wt.% HCl aqueous solution, a small amount of Ce3 +/4 + ions was released from the dissolution–precipitation of a Ce-rich phase and served as the corrosion inhibitors. Furthermore, some insoluble corrosion products resulted from the process of electrochemical reactions were favorable for precluding the corroded surface from the corrosive attack and diffusion behavior by Cl− ions.

Two different types of aircraft frame components, which had collapsed respectively in their former vibration-fatigue performance tests, were submitted for failure analysis. The two failed frames were both made of aerial material ZL205A, a high-strength cast Al–Cu–Mn–Ti alloy. According to a series of experimental procedures including visual observations, X-ray detections, fractography inspections, microstructure examinations, mechanical tensile tests, hardness measurements and fluorescent penetrating inspections, it was indicated that the fracture was attributed to fatigue cracks which were induced by casting porosity defects at the external surfaces of frames. Numerous fine fatigue striations presented in the vicinity of casting porosities. Especially, it was observed of a special appearance of latitude–longitude crossed fatigue striations on the fracture surface due to the coupled stresses supplied by the former multi-directional vibration tests. The overload fast-rupture regions on fracture surfaces suggested the typical cleavage fracture mode, which was characterized by a number of river patterns and cleavage steps. The intergranular spatial dendrite-shaped casting porosities largely contributed to the local stress-concentrations in matrix materials. Triangular grain boundaries induced by the former casting burnt implied that the intergranular melting phenomenon had occurred. Furthermore, the effect of groove-shaped structure at roots of spatial convex-bodies on the edge of casting porosity was especially analyzed. And the influence of the casting porosity size on fatigue cracks was briefly discussed.Highlights► Failure analysis for aircraft frame components was completed with multiple procedures. ► Characterizations of ZL205A casting defects were analysis. ► Fractography shown different types of fatigue striations propagating from porosities. ► Spatial dendrite structure of porosity played a special role during the fatigue damage.

The single-pass friction stir weld of aluminum 2219-T6 with weld-defects was repaired by overlapping friction stir welding technique. However, without any post weld heat treatment process, it was found that the phenomena of abnormal particle-coarsening of Al2Cu had occurred in the overlapping friction stir repair welds. The detecting results of non-destructive X-ray inspection proved that not only one group of repair FSW process parameters could lead to occurrence of the abnormal phenomena. And the abnormally coarsened particles always appeared on the advancing side of repair welds rather than the retreating side where the fracture behaviors occurred after mechanical tensile testing. The size of the biggest particle lying in the dark bands of ‘Onion-rings’ was more than 150 μm. After the related investigation by scanning electron microscope and X-ray energy spectrometer, three types of formation mechanisms were proposed for reasonably explaining the abnormal phenomenon: Aggregation Mechanism, Diffusion Mechanisms I and II. Aggregation Mechanism was according to the motion-laws of stir-pin. Diffusion Mechanisms were based on the classical theories of precipitate growth in metallic systems. The combined action of the three detailed mechanisms contributed to the abnormal coarsening behavior of Al2Cu particles in the friction stir repair weld.Highlights► Defective friction stir welds were repaired by overlapping FSW technique. ► Abnormal Al2Cu-coarsening phenomena were found in 2219-T6 friction stir repair weld. ► Three formation mechanisms were proposed for reasonable explanations.

The selective laser melting (SLM) of the TiH2–Ti blended powder was performed in the present work. Porous titanium scaffolds characterized by high porosity (∼ 70%), interconnected Ti walls and open porous structures with macroscopic pores (in a range of ∼ 200 to ∼ 500 μm) were successfully prepared at a laser power of 1000 W and a scan speed of 0.02 m/s. The effects of componential and processing conditions in terms of TiH2 content and scan speed on the microstructural development of porous titanium (porosity and pores size) were investigated. Reasonable mechanisms for pores formation during SLM apart from microstructural evolutions were proposed.

This paper presents a detailed investigation into the influence of the rare earth (RE) oxide (La2O3) addition upon the densification and the resultant microstructural characteristics of the submicron WC–Co particulate reinforced Cu matrix composites prepared by direct laser sintering. It is found that the relative density of the laser sintered sample with 1 wt.% La2O3 addition increased by 11.5% as compared with the sample without RE addition. The addition of RE element favored the microstructural refinement and improved the particulate dispersion homogeneity and the particulate/matrix interfacial coherence. The metallurgical functions of the RE element in improving the sinterability were also addressed. It shows that due to the unique properties of RE element such as high surface activity and large atomic radius, the addition of trace RE element can decrease the surface tension of the melt, resist the grain growth coarsening and increase the heterogeneous nucleation rate during laser sintering.

•A bi-layer coating and a mono-layer coating were synthesized by mechanical alloying.•The microstructures of the coatings were carefully analyzed and compared.•The friction and wear resistances of the coatings were compared.•The high-temperature oxidation resistances of the coatings were compared.A bi-layer Al–Cr/Al–Si coating and a mono-layer Al–Cr–Si coating were respectively produced on the Ti–6Al–4V alloy substrate by mechanical alloying method. The bi-layer coating had an inner composite Al–Si layer and an outer composite Al–Cr layer, which was obtained by milling in Al–Si powder and Al–Cr powder successively. The mono-layer coating was a Sip and Crp reinforced Al matrix coating. The adhesion behavior, the microhardness, the friction and wear resistance, and the high-temperature oxidation resistance of the coatings were investigated and compared. The results indicated that the mono-layer coating had better adhesion behavior compared with the outer layer of the bi-layer coating. Both the mono-layer and the bi-layer coatings could improve the surface microhardness of the substrate. The mono-layer coating had higher surface microhardness. But the bi-layer coating had an inner cushioning layer to maintain the hardness and prevent its sharp decrease with the increase of coating depth. The mono-layer coating had better friction and wear resistance. The bi-layer coating had lower mass gain and oxidation rate compared with the mono-layer coating. A continuous Al3Ti layer formed during oxidation of the bi-layer coating, which could protected the substrate effectively at the elevated temperature. The mono-layer coating was severely oxidized and porous structure formed in the coating. The bi-layer coating had advantages in protecting the substrate at elevated temperature.

•Mechanical alloying method was applied to form Al–Si coating.•The as-synthesized coating had a composite structure.•The high-temperature oxidation resistance of the coating was investigated.•A multilayered structure formed after oxidation.•The oxidation process of the coating at elevated temperature was elucidated.An Al–Si coating was successfully fabricated on Ti–6Al–4V alloy substrate by means of mechanical alloying method. The coating was prepared with Al–33.3 wt% Si powder mixture. The coating had a composite structure and its thickness was about 350 μm. The mass gain and the oxidation rate of the substrate at 850 °C were largely decreased because of the as-synthesized coating. The oxidized coating had a multilayered structure, which included an Al diffused layer, a Ti5Si4 interlayer, an inner TiAl3 layer, an outer Ti–Al–Si alloy layer and an oxide layer from the inner substrate to the top coating surface. During oxidation, the coating was melted and gradually formed the ternary Ti–Al–Si alloy with the diffusion of Ti from the substrate to the coating. The Al diffused layer formed with the adequate interdiffusion of the coating and the substrate. The Ti5Si4 interlayer and the inner TiAl3 layer formed owing to their low formation energy at the elevated temperature. The outer Ti–Al–Si alloy layer mainly included Ti(AlxSi1-x)2 phase, which formed due to the decrease of Al after the formation of the TiAl3 layer and the oxide layer. The direct oxidation of the outer layer promoted the formation of a continuous oxide layer on the coating surface. These layers could retard and prevent the inward diffusion of oxygen at high temperature. The as-synthesized Al–Si coating could effectively improve the oxidation resistance of the Ti–6Al–4V substrate.