The high-pressure torsion deformation at room temperature of extruded 6201 aluminum alloy with different revolutions (1, 2, 5, 10, 20 r) and the aging treatment at 175℃ for different time intervals (10,60 min) were carried out. The revolution of microstructure and hardness during deformation and aging was studied. The results show that with the increase of high-pressure torsion revolution, the grain size was refined from micrometer to submicrometer, the grains tended to become equiaxed grains, and the hardness of the alloy increased. When the high-pressure torsion revolution was greater than 10 r, the alloy microstructure was stabilized, the grain size was about 183 nm, and the hardness was basically unchanged. With the extension of aging time, the grains of the alloy by high pressure torsion of 10 r gradually grew and were coarsened, and after aging for 60 min, the average grain size of the sub-center position was about 523 nm; the alloy underwent obvious aging softening, and the hardness of the alloy decreased with increasing aging time.
Ultrasonic impact was conducted on the weld toe region of Q420qD/20MnMoNb thick plate dissimilar steel T-type welded joints with 18, 27 μm impact amplitudes, and the microstructure, residual stress and microhardness at the weld toe were analyzed under different impact amplitudes, and compared with those before ultrasonic impact. The results show that after ultrasonic impact, a plastic deformation layer, a lot of dislocations and a high compressive stress appeared on the surface layer of the weld toe, the grains were refined, and the hardness increased. When the ultrasonic impact amplitude increased from 18 μm to 27 μm, the depth of the plastic deformation layer increased from about 120 μm to 144 μm, the subcrystalline was further refined to about 25 nm in size, and the dislocation density increased to about 2.48×1014 m-2; the residual compressive stress on the surface increased to about 433 MPa and its affected depth increased to about 1 410 μm; the surface hardness increased to about 400 HV and the depth of hardening layer increased to about 900 μm.
The welding thermal simulation test was conducted on medium thick Q960 steel plate for hydraulic supports. The microstructure and hardness of the coarse-grained heat-affected zone under different cooling rates (0.5-100℃·s-1) were analyzed. The robot welding process was determined by Tekken test and welded joint mechanical performance tests. The results show that the simulated hardness of coarse-grained heat-affected zone could be stabilized below 420 HV when the cooling rate was lower than 80℃·s-1, namely the welding heat input was greater than 6.5 kJ·cm-1 under the preheated temperature of 100℃. The Q960 steel plate joint welded by robot welding had excellent cold crack resistance sensitivity under the welding heat input of 13.95 kJ·cm-1 and the preheated temperature of 100℃. The properties of robot welding Q960 steel plate joint could meet the standard requirements within the welding heat input of 13.02-15.18 kJ·cm-1 with ER96-G welding wire. The recommended welding parameters for robot welding were listed as follows:preheating at 100℃, welding current of 460 A, voltage of 33 V, welding speed of 60 cm·min-1 and wire feed rate of 8.5 m·min-1, and the welding deposition rate could reach 8 kg·h-1.
W-based composites doped with 0.5wt% and 2.0wt% Y2O3 were prepared. The microstructure of the composites was investigated. The deformation characteristics near ductile-brittle transition temperature region of the composites were analyzed by tensile tests at different temperatures (25-800℃). The results show that the two composites had a large number of dislocations formed by rolling deformation, and Y2O3 particles played a pinning role in dislocation motion. The composite containing 2.0wt% Y2O3 had smaller grain size and a lower ductile-brittle transition temperature. The semi-brittle behavior of the composite containing 2.0wt% Y2O3 occurred during tension at 300-400℃, and the dislocation density in the fracture area was between 3.8×1015-3.9×1015 m-2; during tension at 600-800℃, plastic deformation occurred, and the dislocation density increased to 6.2×1015-6.8×1015 m-2.
By taking the cross-linked polysiloxane as precursor, SiOC ceramics were prepared by flash sintering at different temperatures (730-780℃), applied electric field intensities (20-60 V·mm-1) and limiting currents (0.5-2.0 A), and the suitable process parameters were obtained. The microstructure, physical properties and thermal stability of the ceramics were studied. The results show that the temperature range for successfully preparing SiOC ceramics was 740-780℃, the limiting current range was 1.0-2.0 A, and the applied electric field intensity range was 30-60 V·mm-1; the test temperature was 660-620℃ lower than the traditional pyrolysis temperature (1 400℃), and the pyrolysis time was greatly shortened. As the applied electric field intensity, test temperature or limiting current increased, the SiC content in the ceramics increased, the SiO2 content decreased, the ceramic yield and bulk density decreased, and the linear change rate and apparent porosity increased. Compared with that of the traditional pyrolyzed SiOC ceramics at 1 400℃, the thermal stability temperature of flash-sintered SiOC ceramics increased by about 112℃, and with increasing applied electric field intensity, test temperature or limiting current, the thermal stability improved.
By taking cubic boron nitride powder, titanium powder and aluminum powder as raw materials, high content polycrystal cubic boron nitride (PcBN) was prepared by in-situ synthesis method at high temperatures (1 400-1 600℃) and an ultra-high pressure (6.5 GPa). The effects of sintering temperature on the mechanical properties, microstructure and cutting property of PcBN were investigated. The results show that the prepared PcBN all consisted of BN, TiN, TiB2 and AlN phases. With the increase of sintering temperature, the number of pores in PcBN decreased, the density increased, the microhardness and flexural strength increased first and then decreased, and the flank wear loss of PcBN tool for cutting run length of 10 km decreased first and then increased. At the sintering temperature of 1 550℃, PcBN had the best comprehensive properties with the largest flexural strength of 969 MPa, the largest microhardness of 40.7 GPa, and the smallest flank wear loss of 0.26 μm.
With Ti3AlC2 powder prepared by pressureless sintering at different temperatures (1 300,1 350,1 400℃) and copper powder as raw materials, Ti3AlC2/Cu composites were prepared by powder metallurgy. The tribological properties of composites with different content of Ti3AlC2 (mass fraction of 5%, 10%, 15%, 15%) under dry friction condition and in distilled water and seawater environments. The results show that the optimum pressureless sintering temperature of Ti3AlC2 powder was 1 350℃, and the mass fraction of Ti3AlC2 was 96%. With increasing Ti3AlC2 content, the relative density of the composite decreased, and the hardness increased first and then decreased, and reached the highest value (about 120 HV) when the mass fraction of Ti3AlC2 was 15%. Under dry friction condition, the friction coefficient and wear rate of the composite decreased significantly with increasing Ti3AlC2 content. The wear rates of composites in distilled water and seawater environments were lower than those under dry friction condition; the friction coefficient in distilled water environment was higher than that under dry friction condition, and in seawater environment was lower than that in distilled water environment and slightly higher than that under dry friction condition.
The stress corrosion cracking and corrosion behavior of 316LN austenitic stainless steel were studied by slow strain rate tensile and electrochemical methods in different corrosive media (3wt% NaCl solution, 6wt% FeCl3 solution) at different temperatures (25, 50℃) through comparing with those of 316L austenitic stainless steel. The results show that in Cl- containing solution, the stress corrosion sensitivity of 316LN steel was lower than that of 316L steel. 316LN steel exhibited passivation-breakdown behavior in NaCl solution, while in FeCl3 solution, it presented the characteristics of active dissolution, and the impedance spectrum showed a single capacitive reactance arc in both. The higher the temperature, the larger the self-corrosion current, the smaller the arc radius and the charge transfer resistance of 316LN steel. 316LN steel showed better corrosion resistance than 316L steel.
Ti(C, N)-based cermets were prepared by low-pressure sintering with AlxCoCrFeNi high-entropy alloy (x=0, 0.5, 1, molar ratio) as the binder, and the effects of AlxCoCrFeNi high-entropy alloy binder on the microstructure and high-temperature oxidation resistance of cermets were investigated. The results show that the prepared cermets were mainly composed of Ti(C, N) phase with face-centered cubic (FCC) structure and high-entropy alloy binder phase with FCC structure, and the transformation from body-centered cubic (BCC) structure phase to FCC structure phase occurred in Al0.5CoCrFeNi and AlCoCrFeNi high-entropy alloys during high-temperature sintering. The cermets all had a typical core-ring structure, and there were more pores in the cermets prepared with AlCoCrFeNi high-entropy alloy binder. After oxidizing at 1 000℃ for 6 h, the mass gains per unit area of cermets prepared with CoCrFeNi, Al0.5CoCrFeNi, and AlCoCrFeNi high-entropy alloy binders were 3.58, 2.95, 2.81 mg·cm-2, respectively, and the cermets prepared with AlCoCrFeNi high-entropy alloy binder had the best high-temperature oxidation resistance.
Laser shock peening was conducted on the surface of 45 steel specimens under different laser energy (4, 6, 8 J). The effects of laser energy on the microhardness, residual stress, microstructure and friction and wear properties of the specimens were studied. The results show that with the increase of laser energy, the degree of lattice distortion, compressive residual stress, microhardness and wear resistance of 45 steel were improved. Under the laser energy of 8 J, the surface (211) crystal plane full width at half maximum, residual compressive stress and microhardness of 45 steel increased to 3.5°, 500 MPa and 345 HV, and the friction coefficient and the wear mass loss decreased to 0.61 and 157 mg, respectively; the wear surface had the least peeling material, and the depth and width of the furrow were the smallest. The improvement of friction and wear properties of 45 steel was mainly due to the high-density dislocation and high-amplitude compressive residual stress induced by laser shock peening.
ZrO2-8Y2O3 (mass fraction/%) thermal barrier coating was prepared on DZ411 alloy substrate by atmospheric plasma spraying. The high temperature oxidation properties and the section micromorphology after oxidation of the thermal barrier coating were studied. The high temperature oxidation life prediction model was established and verified by experiments. The results show that the oxidation mass gain Δm and the thermal growth oxide layer thickness of the thermal barrier coating increased in a parabolic pattern during oxidation at 940, 1 030℃. The Δm increase rate of the thermal barrier coating at 1 030℃ was larger than that at 940℃. Compared with the experimental results, the predicted value of the themal growth oxide layer thickness by the established high temperature oxidation life prediction model was within the dispersion band of ±2 times, indicating that the established life model and the analysis method could predict the high temperature oxidation life of the thermal barrier coating well.
A multi-layer perceptron neural network prediction model between the process parameters of CO2 gas shielded welding and the weld geometry (melting width and depth) was established, and the mathematical analytic formula of the model was determined based on the welding test data training the model. The virtual simulation model of weld morphology was established by analyzing the characteristics of weld section and surface morphology. The weld morphology prediction and virtualization simulation system was developed by python programming. The results show that the maximum deviation for predicting melting width with the established multi-layer perceptron neural network prediction model was 0.097 mm with a model fitting coefficient of 0.999 269, and that for predicting melting depth was 0.051 mm with a model fitting coefficient of 0.999 567. The mathematical model of weld section morphology with melting depth and melting width as input variables and the mathematical model of surface morphology with melting width as input variables were established.
The dynamic mechanical properties of dual-phase high strength steels HC340/590DP and HC700/980DP for vehicles were studied by room temperature tensile tests at different strain rates (0.001-500 s-1). The flow stress-strain curves of the two steels were fitted by Johnson-Cook model, Swift Hockett/Sherby model and the modified model obtained by introducing Swift Hockett/Sherby model into Ludwik model, and the fitting results of the three constitutive models were compared and analyzed. The results show that with increasing strain rate, two steels both exhibited the phenomenon of strengthening and plasticizing. The average fit values of Johnson-Cook model, Swift Hockett/Sherby model, and modified Ludwik model were 0.950, 0.999, 0.997, respectively. The modified Ludwik model not only had the coupling characteristics of stress at each strain rate, but also maintained the high fitting accuracy; it could accurately describe the dynamic rheological behaviors of dual-phase high strength steels for vehicles.
The processes of additive manufacturing and welding are both complex processes with multiple physical field coupling, and it is difficult to directly observe the evolution of the microstructure of the molten pool by experimental methods. With the rapid development of computational material science and numerical models, it is possible to study the microstructure evolution during solidification by numerical simulation. Several commonly used microstructure simulation methods are compared and analyzed, among which the phase field method has a unique advantage in the accuracy of grain morphology simulation. The application status of phase field method in the microstructure simulation in additive manufacturing and welding fields is reviewed, and the research direction in future is prospected.
The metal powder prepared by gas atomization has small particle size, uniform composition, high sphericity and good fluidity, and the method has become the main preparation method of 3D printing spherical metal powder. However, gas atomization is a complex process of multi-phase flow interaction, and the change of subtle factors may lead to the change of powder characteristics, which has a decisive impact on the performance of 3D printed parts. The effects of metal superheat, gas-liquid flow rate, atomizing medium type, atomizing gas pressure and temperature, atomizer nozzle configuration and delivery tube geometry in the process of gas atomization on powder characteristics are reviewed. The development direction of 3D printing metal powder materials and preparation technology is also prospected.
An irregular porous structure of Voronoi polygon was constructed by Rhino software, and the porous TC4 titanium alloy was prepared by selective laser melting (SLM). The effects of laser power (180, 200, 220 W), scanning speed (1 200, 1 600, 2 000 mm·s-1) and scanning spacing (80, 100, 120 μm) on the alloy microstructure were investigated. The results show that with the increase of laser power, the decrease of scanning speed or the increase of scanning spacing, the number and size of micropore defects in solid part of SLM formed porous TC4 titanium alloy decreased, and the relative density increased; the scanning speed was the main reason affecting the formation of defects. Under laser power of 220 W, scanning speed of 1 200 mm·s-1 and scanning spacing of 120 μm, the titanium alloy had the fewest micropore defects with relative density of 99.2%. There were equiaxed crystals and columnar crystals parallel to the substrate plate surface in the section near the pores of the porous structure, while the microstructure far away from the pores was mainly composed of β columnar crystals, and inside the columnar crystals there was primary acicular martensite arranged in parallel at ±45° to its major axis. With the decrease of laser power, the increase of scanning speed or the decrease of scanning spacing, the width of β columnar crystal and the length of primary martensite decreased; the scanning spacing had a greater effect on the microstructure.
316L stainless steel samples were prepared by selective laser melting under the condition of large layer thickness with powder thickness of 0.15 mm. The effects of laser power (750-850 W), scanning speed (800-1 000 mm·s-1) and scanning spacing (0.06-0.10 mm) on the relative density, microstructure and properties of 316L stainless steel were studied, and the selective laser melting process was optimized. The results show that with the increase of laser power, scanning speed or scanning spacing, the relative density of the formed samples with large layer thickness all increased first and then decreased. When the laser power was 800 W, the scanning speed was 900 mm · s-1, and the scanning spacing was 0.08 mm, the tensile properties of the formed sample were the best, and the tensile strength and the percentage elongation after fracture were 682.7 MPa and 33.4%, respectively, which were close to those of the small layer thickness formed sample. Meanwhile, the microhardness was 224.77 HV, and the microstructure was composed of equiaxed grains with size of 0.4-1.0 μm; the free-corrosion potential in 3.5wt% NaCl solution reached -0.69 V, showing the best corrosion resistance.