Dimple Fracture Research Articles

Characterization of hot extrusion and heat treatment on mechanical properties in a spray formed ultra-high strength Al-Zn-Mg-Cu alloy

To obtain the optimal mechanical properties, several processes including spray forming technology, two passes hot extrusion and substantial heating treatments were operated on an Al-Zn-Mg-Cu-Zr alloy. The characterization methods used in this study include tensile tests, X-ray diffraction, differential scanning calorimetry and microstructure observations by optical microscope, scanning and transmission electron microscopy. The typical characteristics were the refined grain, homogeneous precipitates and porosity in the as-deposited pre-forms. The ultimate tensile strength, yield strength, hardness and elongation improved significantly after two passes hot extrusion. A good combination of ultimate tensile strength at 704 MPa and elongation at 9.7% were achieved under peak-aged treatment. The best ultimate tensile strength was obtained at 732 MPa after re-aging and regression. It was main dimple fracture in the extruded and aging treated specimens while brittle fracture for the as-deposited. The secondary phases were main MgZn2 and Al3Zr dispersoids throughout all the states. The contributions of grain boundaries, dislocations, solid-solution and precipitates on the yield strength were analyzed using a quantitative equation.

Influence of duplex ageing on secondary α precipitates and mechanical properties of the near β-Ti alloy Ti-55531

This paper presents the results of studies on the microstructure and mechanical properties of a near β-Ti alloy processed with different ageing approaches. Influences of intragranular ultrafine α precipitates and grain boundary α layer on mechanical properties in Ti-55531 alloy were analyzed. The results indicate that the precipitation microstructure of the duplex aged alloy exhibits a uniform size and acicular α precipitated orientation shows an angle of approximately 60°. While the morphology of secondary α precipitates in single-step aged samples appear the irregular distribution and heterogeneous size. With the final ageing time increasing, the morphology of intragranular α precipitates and grain boundary α phase brings out a distinct coarsening. The single-step aged alloy can yield a better combination of the ultimate tensile strength (1265 MPa) and ductility (9%). However, the ultimate tensile strength of the duplex aged alloy even reaches 1368 MPa but it breaks at the stage of elastic deformation. The fracture mode of Ti-55531 alloy changes from predominantly dimple fracture in single-step aged samples to predominantly faceted and cleavage type fracture in duplex aged samples causing by the differences of intragranular α precipitates and the interface between grain boundary α layer and intragranular microstructure.

The microstructures, mechanical properties, and temperature distributions in nodular cast iron friction-welded joint

The paper focuses on an experimental topic relating to the field of friction welding process of a nodular cast iron. The microstructures, phase transformation, temperature distributions, microhardness, and tensile test are all studied within the framework of the paper in question. Maximum temperature measurements in the axial center and periphery of the analyzed joints were equal to 950 and 840 °C, respectively. Both temperature and increasing temperature gradient at the axial center were higher than those at the periphery. The maximum tensile strength of the examined friction-welded nodular cast iron joints was 53% of that of the parent metal. The welding region was composed of deformed graphite nodules, coarse pearlite, proeutectoid ferrite, and acicular martensite. Highly deformed graphite nodules were distributed along the weld interface due to the material flow in the thermo-mechanically affected zone (TMAZ). In the central zone, graphite displayed a striped configuration and ferrite transformed into a martensite structure. In the peripheral region, graphite surrounded by martensite kept the form of individual granules. Maximum hardness at the interface in the TMAZ and the heat-affected zone reached 603 HV and 345 HV, respectively. The executed microstructure analysis showed that the cracks started occurring mostly at the interface of the deformed graphite nodules and then spread through the grain boundaries of metal matrix. The fracture surface appearance showed a cleavage fracture in the peripheral zone and a little dimple fracture around graphite nodules in the central zone.

Influence of Al Addition Upon the Microstructure and Mechanical Property of Dual-Phase 9Cr-ODS Steels

With Al addition, dual-phase oxide dispersion strengthened (ODS) steels consisting of martensite and ferrite are fabricated by spark plasma sintering. It is found that Al addition has a negligible effect on martensite lath size, while the amount and size of ferrite grains are related to the Al content. M23C6 (M = Fe, Cr) carbides have been identified within the ferrite grains or along ferrite boundaries. With increasing Al concentration, more fine Y–Al–O oxide nanoparticles are formed. Upon annealing treatment, homogeneous and refined distribution of ferrite grains is obtained, which may involve the particle-stimulated nucleation of recrystallization caused by the large sized M23C6. As Al is increased from 0.05 to 0.1 wt%, the tensile strength of the annealed steel is decreased, as well as its ductility. For the annealed 9Cr-ODS steel containing 0.1 wt% Al, in tensile loading the large sized M23C6 along ferrite boundaries would facilitate the cracking along boundaries between the hard annealed ferrite and soft annealed martensite, producing the mixed fracture of dimple and intergranular fracture.

Effect of Powder Recycling on the Fracture Behavior of Electron Beam Melted Alloy 718

Abstract Understanding the effect of powder feedstock alterations during multicycle additive manufacturing on the quality of built components is crucial to meet the requirements on critical parts for aerospace engine applications. In this study, powder recycling of Alloy 718 during electron beam melting was studied to understand its influence on fracture behavior of Charpy impact test bars. High resolution scanning electron microscopy was employed for fracture surface analysis on test bars produced from virgin and recycled powder. For all investigated samples, an intergranular type of fracture, initiated by non-metallic phases and bonding defects, was typically observed in the regions close to or within the contour zone. The fracture mode in the bulk of the samples was mainly moderately ductile dimple fracture. The results show a clear correlation between powder degradation during multi-cycle powder reuse and the amount of damage relevant defects observed on the fracture surfaces. In particular, samples produced from recycled powder show a significant amount of aluminum-rich oxide defects, originating from aluminum-rich oxide particulates on the surface of the recycled powder.

Experimental and numerical investigations of interface characteristics of copper/steel composite prepared by explosive welding

Copper-steel composite is an attractive candidate for cooling staves applied in the blast furnace because of their lower cost and high deformation resistance ability. However, systematic researches on the application effect of composites are insufficient. In this work, a novel copper-steel composite stave was developed by explosive welding process. The experimental and numerical investigations were carried out on the interface of copper steel composite. The results showed that copper-steel composite cooling stave possesses the similar cooling capacity and higher anti-deforming capability than copper cooling staves. The interface is characterized by a wave structure and melted zones in the vortex region are surrounded by intensely deformed bulk materials. The grain structure in the interface indicated some grains were elongated in the direction of vortex. Refined grains and nano-sized grains of 56 nm were formed in the melted zones. The smaller nanocrystalline iron grains are dispersed uniformly at the grain boundary of the copper matrix and in the vicinity of the nanometer iron grains were tiny copper grains. The higher hardness in the deformed regions, superior local properties of the interface owing to nanocrystalline grains and typical dimple fracture confirm the good metallurgical bonding of the copper-steel composite prepared by explosive welding.

Effects of Surface-Modified MgO Nanoparticles on Inclusion Characteristics and Microstructure in Carbon Structural Steel

An innovative approach involving chemical modification of the surface of MgO nanoparticles (NPs) for steelmaking and application of NPs to carbon structural steel has been investigated. The results show that the inclusions in the test steels were completely converted to MgAl2O4 spinel or MnS complex inclusions. The mean inclusion size decreased with increasing NP content from 0.01% to 0.03%, but increased at 0.05% because of NP aggregation. Addition of NPs increased the amount of intragranular ferrite and prevented polygonal ferrite formation, thereby enhancing the impact toughness. Impact tests showed that the dimple fractures in steel with 0.05% NP content were deeper than those in the other samples because the MgAl2O4 inclusions were larger. The surface-modified MgO NPs had a major effect on the inclusion characteristics and microstructure of carbon structural steel.

Microstructure, mechanical properties and post-weld heat treatments of dissimilar laser-welded Ti2AlNb/Ti60 sheet

Dissimilar joining of Ti2AlNb and Ti60 rolled sheet was performed by laser beam welding. The microstructures and mechanical properties of as-welded and post-weld heat-treated Ti2AlNb/Ti60 joints were investigated by microstructure observation and mechanical properties testing. The fusion zone (FZ) of the as-welded joint consisted of B2/β matrix and small S2 balls. The heat-affected zones HAZ-Ti2AlNb and HAZ-Ti60 for the as-welded joint were characterized as α2 + B2/β + O and α′ + α + β + S2, respectively. The tensile strength and elongation of as-welded sheet were 597.5 MPa and 6.2% at 650 °C, respectively. Most of the plastic deformation was taken place at the side of Ti60 alloy. With the increase in the heat treatment temperature, the anchoring strength between each other of B2/β grains was enhanced by the precipitated lamellar α′ and O grains at the FZ. The depth of the fracture dimples increased with the heat treat temperature increasing. The heat-treated welding joint at 970 °C presented a good balance of the tensile strength and plasticity, where the tensile strength and the elongation were enhanced to 669.7 MPa and 7.1% at 650 °C, respectively.

Grain boundary engineering approach to improve hydrogen embrittlement resistance in Fe[sbnd]Mn[sbnd]C TWIP steel

This investigation aimed to improve hydrogen embrittlement (HE) resistance of Fe17Mn−0.8C (wt %) TWIP steels by use of grain boundary engineering approach. The degree of enhancement in HE resistance by grain boundary engineering was compared with those of Al-added TWIP steels (Fe17Mn−0.8C1Al and Fe17Mn−0.8C2Al). By applying grain boundary engineering, the special boundaries that shared a high fraction of lattice points between neighboring grains were increased from 48% to 59%, and the morphology of grain boundaries was changed to ragged type. After hydrogen charging, fracture strength and reduction of area were clearly degraded in all four samples (Base, GBE, 1Al, and 2Al), revealing a transition of fracture mode from dimple fracture to intergranular fracture. However, the alloy with higher fraction of special boundaries had higher resistance to hydrogen-induced intergranular embrittlement. The HE resistance of GBE was obviously improved to a level similar to that of Al-added TWIP steels. Important implication of this work is that HE resistance can be greatly enhanced by controlling the distributions of grain boundary characters (GBCs) without adding Al. This was attributed to not only the higher fraction of special boundaries, but also the loss of continuity of random boundaries.

Enhanced Impact Toughness at Ambient Temperatures of Ultrafine-Grained Al-26 wt.% Si Alloy Produced by Equal-Channel Angular Pressing

Overcoming general brittleness of hypereutectic Al-Si alloys is in urgent need for expanding their application in automotive, aerospace and construction industries. A unique phenomenon was observed that bulk ultrafine-grained Al-26 wt.% Si alloy, produced by severe plastic deformation via equal-channel angular pressing, exhibited higher toughness at the impact temperature of − 196 ~ 100 °C than the coarse-grained casting alloy. The improvement in impact toughness at all testing temperatures was mainly due to the homogeneous ultrafine-grained structure with the breakage of brittle primary silicon crystals, which generated more and deeper fracture dimples that consumed much higher fracture energy. It indicates the advantage of bulk ultrafine-grained Al-Si alloys and spurs their application interest at various ambient temperatures.

Tensile Deformation Temperature Impact on Microstructure and Mechanical Properties of AISI 316LN Austenitic Stainless Steel

Uniaxial tensile tests were conducted on AISI 316LN austenitic stainless steel from − 40 to 300 °C at a rate of 0.5 mm/min. Microstructure and mechanical properties of the deformed steel were investigated by optical, scanning and transmission electron microscopies, x-ray diffraction, and microhardness testing. The yield strength, ultimate tensile strength, elongation, and microhardness increase with the decrease in the test temperature. The tensile fracture morphology has the dimple rupture feature after low-temperature deformations and turns to a mixture of transgranular fracture and dimple fracture after high-temperature ones. The dominating deformation microstructure evolves from dislocation tangle/slip bands to large deformation twins/slip bands with temperature decrease. The deformation-induced martensite transformation can only be realized at low temperature, and its quantity increases with the decrease in the temperature.

Electron beam welding of laser additive manufacturing Ti–6.5Al–3.5Mo–1.5Zr–0.3Si titanium alloy thick plate

Individually fabrication parts by laser additive manufacturing (LAM) and then jointing them together through electron beam welding (EBW) is a viable way for manufacturing large components with reduction of internal stress. For investigating the microstructure and mechanical property of EBW joint along longitudinal and transverse direction in LAMed component, two LAMed Ti–6.5Al–3.5Mo–1.5Zr–0.3Si plates were successfully welded without defects. Results show that the microstructure of base metal (BM) is a typical basket-weave morphology that exhibits lamellar α within β matrix. In heat affected zone (HAZ), the part of primary α transforms to β with the some very fine lamellar αs precipitates out. Due to the fast solidification rate, a large number of acicular α′ forms in fusion zone (FZ), leading to the highest microhardness. All tensile samples fail in BM region with the fracture type of intergranular dimpled fracture. Compared with the T-joint, the L-joint shows higher ultimate tensile strength and yield strength, but lower elongation and reduction of area due to the morphology of columnar grains and the strong texture of β<010> parallel to the deposition direction.

Effects of Cu/Mg ratio on the microstructure, mechanical and corrosion properties of Al-Li-Cu-Mg-X alloys

The effects of the Cu/Mg ratio on the microstructure, properties and fracture morphology of Al-Li-Cu-Mg-X (X = Mn, Zr, Zn) alloys were investigated. The results showed that the modification of the alloy composition with different Cu/Mg ratios significantly corresponds to various microstructures, which affects the corrosion resistance, hardness and tensile properties. For instance, when the Cu/Mg ratio varies from 0.83 to 4.33, the ultimate tensile strength of the Al-Cu-Li-Mg-X alloys gradually increases from 267.2 to 366.6 MPa. Corrosion tests revealed that increasing the Cu/Mg ratio markedly enhances the corrosion resistance of the alloy. The constitution and morphology of the precipitates are also found to vary with the Cu/Mg ratio. The main precipitates are δ'(Al3Li) and the minor TB(Al7Cu4Li) phase when the Cu/Mg ratios are 1 and 0.83. By increasing the Cu/Mg ratio to 2.35, the θ'(Al2Cu) phase can be detected, while only the T1(Al2CuLi) phase is detected in the alloy with the highest Cu/Mg ratio. The combinative hardening of the T1, TB, θ' and δ' precipitates is responsible for the improvement in the mechanical properties and corrosion resistance of the alloy with the highest Cu/Mg ratio. During the tensile testing, the failure mode is converted from a typical shallow dimple fracture to an intergranular-dominated mixed one with the decrease of the Cu/Mg ratio.

Strain Rate‐Dependent Constitutive and Low Stress Triaxiality Fracture Behavior Investigation of 6005 Al Alloy

In order to study the dynamic and fracture behavior of 6005 aluminum alloy at different strain rates and stress states, various tests (tensile tests at different strain rates and tensile shearing tests at five stress states) are conducted by Mechanical Testing and Simulation (MTS) and split‐Hopkinson tension bar (SHTB). Numerical simulations based on the finite element method (FEM) are performed with ABAQUS/Standard to obtain the actual stress triaxialities and equivalent plastic strain to fracture. The results of tensile tests for 6005 Al show obvious rate dependence on strain rates. The results obtained from simulations indicate the feature of nonmonotonicity between the strain to fracture and stress triaxiality. The equivalent plastic strain reduces to a minimum value and then increases in the stress triaxiality range from 0.04 to 0.30. A simplified Johnson‐Cook (JC) constitutive model is proposed to depict the relationship between the flow stress and strain rate. What is more, the strain‐rate factor is modified using a quadratic polynomial regression model, in which it is considered to vary with the strain and strain rates. A fracture criterion is also proposed in a low stress triaxiality range from 0.04 to 0.369. Error analysis for the modified JC model indicates that the model exhibits higher accuracy than the original one in predicting the flow stress at different strain rates. The fractography analysis indicates that the material has a typical ductile fracture mechanism including the shear fracture under pure shear and the dimple fracture under uniaxial tensile.

Effect of Strain Rate on Tensile and Fracture Behavior of Ultrafine grained Al6061 processed through Cryorolling and Warm rolling

The present study is focused on investigating strain rate sensitivity of UFG Al 6061 subjected to different rolling conditions and heat treatment. The different conditions used in this study were: (i) Solution treatment, (ii) Cryo-rolling + Warm rolling (CR+WR), (iii) Cryo-rolling + Warm rolling + Peak Ageing (CR+WR+PA), and (iv) Cryo-rolling + Peak Ageing (CR+PA). The rolled specimens were tested for tensile and K1c (three point bend test) at three different strain rates used in tensile tests were (0.6x10-3 /s, 1.0x10-2 /s and 5.0x10-2 /s) and three point bend tests were conducted at ram velocity of 1 mm/min, 20 mm/min and 100 mm/min. Among all tests, CR+PA samples showed good tensile strength of 257 MPa and fracture toughness of 50 MPa(m)1/2. Increased strength is attributed to the formation of ultrafine grains and high dislocation density. The tensile samples takes less load at higher strain rate when compared to lower strain rate. However, in three point bend test, no appreciable change in load taken by specimen at higher strain rates was observed compared to lower strain rate. Which indicates that loading rates selected were not high enough. The variation in strength at higher strain rate is supported by fractography studies, where failure mechanism has changed from dimple fracture to cleavage at higher strain rates.

Effect of vanadium-alloying on hydrogen embrittlement of austenitic high-nitrogen steels

The effect of hydrogen on tensile behavior and fracture mechanisms of V-alloying and V-free high-nitrogen austenitic steels was evaluated. Two steels with the chemical compositions of Fe-23Cr–17Mn–0.1C–0.6N (0V-HNS) and Fe-19Cr–22Mn–1.5V–0.3C–0.9N (1.5V-HNS) were electrochemically hydrogen-charged in NaCl water-solution for 100 hours. According to X-ray diffraction analysis and TEM researches, V-alloying promotes particle strengthening of the 1.5V-HNS. Despite differences in chemical compositions, namely, carbon and nitrogen concentrations, a solid solution hardening is similar for both steels because of precipitate-assisted depletion of austenite by interstitial atoms (carbon and nitrogen) in 1.5V-HNS. For hydrogen-free state, the values of the yield stress and the tensile strength are higher for particle-strengthened 1.5V-HNS as compared to 0V-HNS. Hydrogen-charging increases both the yield stress and the tensile strength of the steels, but hydrogen-assisted fracture micromechanisms are different for 0V-HNS and 1.5V-HNS. Hydrogen-charging drastically reduces a total elongation in 0V-HNS but provides insufficient embrittlement in 1.5V-HNS. Hydrogen-assisted brittle layers form on lateral surfaces of the specimens, and the widths and fracture micromechanisms in them are different for two steels. For 0V-HNS, surface layers of 84 μm in width possess transgranular brittle fracture mechanism (quasi-cleavage mode). For 1.5V-HNS, the brittle surface layers (31 μm width) destroy in intergranular brittle fracture mode. The central parts of steel specimens show dimple fracture similar to hydrogen-free steels. The possible reasons for different hydrogen-induced effects in 0V-HNS and 1.5V-HNS are discussed.

Effect of strain-rate on forming limit in biaxial stretching of aluminum sheet

High-speed biaxial stretching apparatus was developed, where the punch was driven by a drop weight. Effect of the strain-rate on the forming limit strain of thin aluminum A1050 and A5052 sheets was inspected for various strain ratios. Strain-rate was about 100 /s in the high-speed test. It was from 0.0003 to 0.1 /s in the low-speed test. Limit strain of A1050 was larger in high speed. The strain of A5052 in high speed was comparable with that in 0.1 /s, though there exists a slight difference under low-speed conditions. Multiple cracks were formed in high-speed stretching of A5052 attributed to the positive strain-rate sensitivity. Profile of the crack opening is rough in high speed. Growth of the ductile fracture dimple size and depth in microscopic view may play a role to increase the roughness. Sheet thickness at the crack becomes smaller in high-speed stretching, which accords with the tendency of the forming limit strain.

Investigating of the tensile mechanical properties of structural steels at high strain rates

St37 and St52 structural steel plates were tested in uniaxial tension at room temperature over various strain rates ranging from 0.001/s to 0.1/s. The yield stress, flow stress and fracture behavior of steels were analyzed. It was found that the strain rate has a strong effect on the tensile mechanical properties of St37 steel, while St52 has a less sensitive strain rate and that the yield strength of both steels exhibits a higher strain sensitivity rate than the other mechanical properties. An increase in the loading rate from 0.001/s to 0.1/s led to a %30 increase in the lower yield strength of St37 steel and an increase of %6 for St52. The equations were derived to express the yield stress behavior with the strain rate. The ductile dimple fracture was observed in static and dynamic conditions; however, increasing the strain rate resulted in a pronounced cleavage-type fracture in both steels. The St37 fracture strain decreased considerably by increasing the strain rate.

Ductilizing of cast hypereutectic Al–17%Si alloy by friction stir processing

In present research work, multi-pass friction stir processing was utilized for the as-cast Al–17%Si alloy. The multi-pass friction stir processing reproduced the aluminum (Al) matrix with a uniform distribution of the ultra-fine silicon (Si) particles. The multi-pass friction stir processing also resulted in the formation of refining cast microstructure with negligible porosity. The significant reduction in silicon particle size was measured and it reduced from 204 µm to 0.86 µm after the first pass and up to 0.30 µm after two pass friction stir processing. The frequency of fine Si particles was increased after two pass friction stir processing as compared to single pass friction stir processing. The engineering stress–strain curves revealed a significant enhancement in ductility and strength after one and two passes of friction stir processing as compared to the as-cast alloy. After one and two passes of friction stir processing, the ultimate tensile strength of as-cast alloy was enhanced by 24% and 31% and ductility was increased by 300% and 500%, respectively. The secondary electron micrograph of fracture surfaces of tensile specimens was taken before and after friction stir processing. The fractographs revealed the transformation from brittle mode to ductile dimples fracture after the multi-pass friction stir processing process.

Effect of Local Post Weld Heat Treatment on Tensile Properties in Friction Stir Welded 2219-O Al Alloy

To improve the formability of the aluminum alloy welds and overcome the size limitation of the bulk post weld heat treatment (BPWHT) on large size friction stir welded joints, a local post weld heat treatment method (LPWHT) was proposed. In this method, the resistance heating as the moving heat source is adopted to only heat the weld seam. The temperature field of LPWHT and its influence on the mechanical properties and formability of FSW 2219-O Al alloy joints was investigated. The evaluation of the tensile properties of FSW samples was also examined by mapping the global and local strain distribution using the digital image correlation methodology. The results indicated that the formability was improved greatly after LPWHT, while the hardness distribution of the FSW joint was homogenized. The maximum elongation can reach 1.4 times that of as-welded joints with increase the strength and the strain of the nugget zone increased from 3 to 8% when annealing at 300 °C. The heterogeneity on the tensile deformation of the as-welded joints was improved by the nugget zone showing large local strain value and the reason was given according to the dimple fracture characteristics at different annealing temperatures. The tensile strength and elongation of LPWHT can reach 93.3 and 96.1% of the BPWHT, respectively. Thus, the LPWHT can be advantageous compared to the BPWHT for large size welds.