Search

In this study, the microstructure evolution of laser solid forming (LSF) 34CrNiMo6 steel with different deposition thickness was investigated. The microstructure of 10 layers (1.35 mm) sample from bottom to top presented the martensite with a large amount of upper bainite, the mixture of martensite and lower bainite, and the mixture of martensite and retained austenite. When deposition thickness reached 20 layers (3.12 mm), the microstructure of the deposition zone consisted of a large amount of bainite (needle-like lower bainite and feather-like upper bainite) and a small amount of martensite. Besides, the number and size of the feather-like upper bainite gradually increased from the top to the bottom of the deposition, and the morphology of martensite changed from lath-shaped to island-shaped. Two types of carbides morphology existed in lower bainite at the top of 20 layers (3.12 mm) sample: a short rod-shaped with an angle of 55-60° from the main axis and a strip-shaped approximately parallel to the main axis. The carbides in lower bainite can be identified as Fe3C. [ABSTRACT FROM AUTHOR]

The building of Inconel 625 material was carried out using the selective laser melting method, and its fatigue crack growth property at ambient temperature was experimentally investigated. Compact‐tension specimens with different building orientations were utilized to determine the stress intensity factor threshold and fatigue crack growth rate curves at different stress ratios (R). The results indicated that the fatigue crack growth properties in the near threshold stress intensity factor and Paris regions were greatly affected by the loading factor, as well as the orientation of the alloy. The mechanism of fatigue crack growth at different stages was observed and discussed using scanning electron microscopy. Finally, based on the framework of the linear elastic fracture, a new and applicable effective driving force factor range was introduced to replace the traditional stress intensity factor range (ΔK) with good accuracy for all of the fatigue crack growth test data, considering both the stress ratio and orientation. [ABSTRACT FROM AUTHOR]

Four tools with taper (TA), triangular (TB), three-grooves (TC), and square (TD) pin-tip profiles were designed to friction stir weld (FSW) thick AA7075-T6 alloy plates aiming at improving the local and overall mechanical properties of the joints. The microstructure evolution, hardness, and mechanical properties of the FSW joints were studied by scanning election microscope (SEM), X-ray diffractometer (XRD), and transmission electron microscope (TEM). The results show that equiaxed grains in the bottom nugget zone (BNZ) obtained by using TC are noticeably refined compared to in the case of other tools. Also, finer and more dispersive precipitates are distributed in this BNZ, and higher contents of η phase and dislocations are observed. A similar trend is found in the middle of weld. The bottom slices welded by employing TC show superior strength and ductility with a highest ultimate tensile strength of 388 MPa, yield strength of 315 MPa, and elongation of 7.9 %, which are in agreement with the hardness profiles. This is owing to ultrafine grains, more high-angle grain boundaries, more dispersive and higher contents of precipitates, and optimum hardness distribution. Accordingly, the global mechanical properties of the overall joints prepared by using TC significantly improve. [ABSTRACT FROM AUTHOR]

Two-millimeter-thick 2060 Al-Li alloy plates were friction stir welded under a welding speed of 95-150 mm/min and rotation speed of 750-1500 rpm. The effects of welding speed and rotation speed on formation quality, microstructure, secondary phase particles' transformation, and mechanical properties of the joints were investigated. The results show that defect-free joints are produced for varying friction stir welding (FSW) parameters, and nugget size increases firstly and then decreases with increasing rotation speed or decreasing welding speed. The weld nugget zones (WNZs) have fine dynamically recrystallized grains, and the size decreases to 7.9 μm with increasing rotation speed to 1180 rpm or decreasing welding speed to 118 mm/min, while the grains are coarsened at 1500 rpm or 95 mm/min. A similar trend occurs in the transformation of secondary phase particles, whose size is the smallest in WNZ at 1180 rpm-118 mm/min. All joints exhibit softened zones where the hardness is the lowest, and the joints fracture from WNZs or heat-affected zones. The joints welded at 1180 rpm-118 mm/min perform the highest ultimate tensile strength of 495 MPa, yield strength of 380 MPa, and elongation of 10.2 %. With increasing rotation speed or decreasing welding speed, the strengths and elongation of the joints increase firstly and then decrease. [ABSTRACT FROM AUTHOR]

AerMet100 ultra-high strength steel plates with a thickness of 2 mm were welded using a CO laser welding system. The influences of the welding process parameters on the morphology and microstructure of the welding joints were investigated, and the mechanical property of the welding joints was analyzed. The experimental results showed that the fusion zone of welding joint mainly consisted of columnar grains and a fine dendrite substructure grew epitaxially from the matrix. With the other conditions remaining unchanged, a finer weld microstructure was along with the scanning speed increase. The solidification microstructure gradually transformed from cellular crystal into dendrite crystal and the spaces of dendrite secondary arms rose from the fusion line to the center of the fusion zone. In the fusion zone of the weld, the rapid cooling caused the formation of martensite, which led the microhardness of the fusion zone higher than that of the matrix and the heat affected zone. The tensile strength of the welding joints was tested as 1 700 MPa, which was about 87% of the matrix. However, the tensile strength of the welding joints without defects existed was tested as 1832 MPa, which was about 94% of the matrix. [ABSTRACT FROM AUTHOR]

The plunge speed of the tool was divided into two plunge speeds, including pin- and shoulder-plunging speeds, for a detailed study of the plunging process in friction stir spot welding of A6061-T6 sheets. The effect of the pin- and shoulder-plunging speeds on hook geometries and mechanical properties was investigated. The results showed that the shoulder-plunging speed had an obvious effect on the hook geometry and tensile shear load, but the pin-plunging speed had almost no effect. The effective bond width ( W) and effective sheet thickness ( T) used to describe the hook geometry were important factors for determining the tensile shear load and fracture mode. Two fracture modes were observed: tensile/shear mixed fracture and shear fracture. The largest tensile shear load was obtained when the joint failed in the tensile/shear mixed fracture. [ABSTRACT FROM AUTHOR]

Inconel 718 superalloys deposited by laser solid forming (LSF) were heat treated with solution treatment, intermediate heat treatment (IHT) and two-stage aging treatment in sequence (SITA heat treatment). The effect of IHT temperature on microstructure, tensile property and notch sensitivity of LSFed Inconel 718 superalloy at 500 °C were investigated. As-deposited columnar grains have transformed to equiaxed grains and the grains were refined due to the recrystallization during the SITA heat treatment. It is found that the size and amount of δ phase dispersed at grain boundaries decreased with the increasing of IHT temperature, and δ phase disappeared when the IHT temperature reached 1 020 °C. The ultimate tensile strength (UTS) and yield strength (YS) of smooth samples increased to a maximum when the IHT temperature reached 980 °C and then decreased slightly to a minimum when the IHT temperature was 1 000 °C, and followed by slight increasing again till the IHT temperature reached 1 020 °C, resulted from the competition of precipitation strengthening effect of γ″ and γ′ phase and the grain boundary weakening effect caused by the gradual disappearance of δ phase with increasing the IHT temperature. The notch sensitivity factor ( q) decreased but still greater than 1 as the IHT temperature increased, which is attributed to the decrease of the size and amount of δ precipitation. [ABSTRACT FROM AUTHOR]

Recrystallization, which is often occurred in heat treated deformed materials, was examined in laser solid formed (LSFed) Inconel 718 alloy. Its influence on microstructures and mechanical properties of LSFed Inconel 718 alloy were investigated. Microstructure observation indicated that as-LSFed grains were refined and columnar grains were substituted by recrystallized equiaxed grains after recrystallization. The grain refinement mechanism was analyzed and three recrystallized grain nucleation models were found, i.e. the grain-boundary migration nucleation model, the sub-grain coalescence nucleation model and the twinning related nucleation model. The dependence of recrystallized grain size on residual thermal stress distribution was also analyzed. Higher complete recrystallization temperature, longer incubation time and larger recrystallized grain sizes were discovered when comparing with the traditional static recrystallization of deformed materials. Tensile testing results show that the anisotropy in tensile properties of LSFed Inconel 718 alloy is eliminated by the recrystallization process to a certain extent. [ABSTRACT FROM AUTHOR]

In the process of laser-based direct energy deposition, the particle concentration distribution and geometric characteristics of powder flow play an important role in laser–powder interaction and powder utilization, and they affect the forming quality and accuracy. In the current study, based on the geometry information of a powder nozzle and the divergence angles of a powder jet at the nozzle outlet, the geometric profile of a powder stream is analyzed. A set of formulas for calculating the geometric characteristics of the powder stream is derived based on an analytic geometry method. The influence of each parameter on the geometric characteristics of the powder stream is further studied using single-factor and sensitivity analyses. Validation is performed by comparing the results from the presented analytical expressions with those from experiments and/or simulations in published papers. The analytical formulas provided in this paper are simple and practical, providing a theoretical foundation for the control of powder flow and related processes in the forming process. [ABSTRACT FROM AUTHOR]

A double reinforced layer on an aluminum alloy surface was produced using friction stir processing (FSP) by adding 34CrNiMo6 powder into Al (AA2024) substrate for better wear resistance and gradient transitions. The microstructures of the composites were analyzed using scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). The phase composition was examined by X-ray diffraction (XRD). The results show that the double reinforced layer of the Al13Fe4 intermetallic compound could be successfully fabricated via FSP. The volume fraction of Al13Fe4 in the double reinforced layer was higher than in the single reinforced layer due to adding 34CrNiMo6 powder and reinforced twice, and the Al13Fe4 particles were dispersed more homogeneously in the double reinforced layer. The interfaces between the double and single reinforced layer had a good metallurgical bond. The microhardness of the double reinforcement layer was significantly increased. Compared with the AA2024 substrate, the microhardness of the double and single reinforced layers increased five- (576 HV) and two-fold (254 HV), respectively. [ABSTRACT FROM AUTHOR]

Repairing damaged parts using proper repairing methods has become an important means to reduce manufacturing and operational costs and prolong the service life of 34CrNiMo6 steel structures. In the conventional fusion repairing method, welding wire and powder are often used as filling materials. Filling materials are often expensive or difficult to find. Some metallurgical issues (such as solidification crack, higher distortion) were also found with these methods. At the same time, most of the equipment that requires welding wire and powder is expensive. In this study, a new method based on friction stir processing (FSP) was successfully employed to repair 34CrNiMo6 steel, using a block as filling material. Filling blocks are much cheaper than conventional fusion repair consumables. As a result of solid-state repair, this method can also avoid the metallurgical issues of fusion repair. The microstructure and mechanical properties of the repaired samples were investigated using OM (Optical Microscope), SEM, EDS (Energy Dispersive Spectroscopy), XRD, and a Vickers hardness electronic universal tensile tester. The results showed that 34CrNiMo6 steel was successfully repaired by this method, with no defect. Tensile tests showed that the maximum ultimate strength (UTS) was 900 MPa and could reach 91.8% of that of the substrate. The fracture mode of the tensile samples was ductile/brittle mixed fracture. Hence, the repairing method based on FSP appears to be a promising method for repairing castings. [ABSTRACT FROM AUTHOR]

Alloy sheets of type 2060 aluminum–lithium were welded by laser beam welding (LBW) filled with ER2319 Al-5.6Cu wire. Microstructural observations showed the uneven distribution of columnar grains, equiaxed grains and equiaxed dendrite grains in the weld. The θ′(Al2Cu) phase and other phases precipitated in the weld. The θ′(Al2Cu) phase centrally distributed at the grain boundaries. During the immersion corrosion, the pitting corrosion first occurred and then gradually expanded and transformed to intergranular corrosion and exfoliation corrosion. The electrochemical corrosion test showed a higher corrosion tendency of the base metal and heat-affected zone for the lower corrosion potential, but the corrosion current density of the weld was relatively larger. The segregation of Cu, Mg and other elements at the grain boundary aggravated the occurrence of intergranular corrosion. [ABSTRACT FROM AUTHOR]

Laser forming repair (LFR) technology has considerable potential in high strength steel structure repair. 34CrNiMo6 steel has been widely used in high-value components, and it is imperative to repair these damaged components. In this study, two different thicknesses of repaired layers are deposited on the 34CrNiMo6 wrought substrate with five layers and 20 layers via LFR technology. The microstructure, phases, microhardness, and tribological properties are analyzed using optical microscopy, scanning electron microscopy, X-ray diffraction, Vickers hardness testing, and dry sliding wear testing. These results show that the 34CrNiMo6 repaired layers were successfully deposited on the substrate. The microstructure of the laser-repaired layers in the five-layer sample included bainite and retained austenite. For the 20-layer sample, the microstructure in the top of the repaired layers was still bainite and retained austenite, whereas that in the bottom of the repaired layers was transformed into ferrite and cementite. The average coefficients of friction of repaired layers is not significantly different from the substrate. The wear rate of the five LFR layers, 20-layer LFR, and substrate samples were 12.89 × 10−6, 15 × 10−6, and 23.87 × 10−6 mm3/N·m, respectively. The laser forming repaired samples had better wear resistance compared to the substrate. The wear mechanism of laser forming repaired samples is abrasive wear; whereas that of the substrate is abrasive wear and fatigue wear. [ABSTRACT FROM AUTHOR]