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1.
In the present work, the influence of γ eutectic phase on the cavitation behavior of wrought AZ31 magnesium alloy has been studied through applying a set of low strain rate hot tensile tests. The tensile tests were executed in two temperature range of 300–425 °C and 450–500 °C; i.e. somewhat below and higher eutectic melting temperature of γ precipitates (∼437 °C). The hot compression characteristics of the experimental alloy were also considered to assist explaining the related deformation mechanisms. The results indicated that a large amount of cavities originates from the γ second phases, specially located on grain boundaries. A sharp transition was realized from higher (>3%) to lower cavity area fraction (<0.02%) between 450 °C and 500 °C. The latter was attributed to the effect of γ liquid phase on stress relaxation through accommodating the grain boundary sliding phenomena. In addition, the current work explores the room temperature mechanical properties of tensile deformed specimens using shear punch testing method.  相似文献   

2.
From a study of interfacial behavior at deformations up to 60%, a mechanism is proposed for pressure welding of aluminum strips by rolling at warm and cold conditions, where faying surfaces were first degreased and scratch-brushed. According to this mechanism, the scratch-brushed layers fractured coherently after entering roll gap at a reduction of approximately 21%, which is regarding to the threshold deformation for roll bonding of commercial pure (CP) aluminum strips, and some small cracks perpendicular to the rolling direction formed. As deformation proceeds and roll pressure increases, these cracks quickly expanded into fissures. This process allowed the bond to be established between the underlying metals, termed virgin metals, of base metal, which were extruded through the cracks and fissures at the scratch-brushed regions. Moreover by increasing the rolling temperature, threshold deformation required for bond formation decreases.  相似文献   

3.
The effects of 2 wt.% rare earth element addition on the microstructure evolution, thermal stability and shear strength of AZ91 alloy were investigated in the as-cast and annealed conditions. The as-cast structure of AZ91 consists of α-Mg matrix and the β-Mg17Al12 intermetallic phase. Due to the low thermal stability of this phase, the strength of AZ91 significantly decreased as the temperature increased. The addition of rare earth elements refined the microstructure and improved both thermal stability and high-temperature mechanical properties of AZ91. This was documented by the retention of the initial fine microstructure and ultimate shear strength (USS) of the rare earth elements-containing material after long-term annealing at 420 °C. The improved stability and strength are attributed to the reduction in the volume fraction of β-Mg17Al12 and retention of the thermally stable Al11RE3 intermetallic particles which can hinder grain growth during the annealing process. This behavior is in contrast to that of the base material which developed a coarse grain structure with decreased strength caused by the dissolution of β-Mg17Al12 after exposure to high temperature.  相似文献   

4.
The fracture behaviour of the aluminium alloy AA7075-T651 is investigated for quasi-static and dynamic loading conditions and different stress states. The fracture surfaces obtained in tensile tests on smooth and notched axisymmetric specimens and compression tests on cylindrical specimens are compared to the fracture surfaces that occur when a projectile, having either a blunt or an ogival nose shape, strikes a 20 mm thick plate of the aluminium alloy. The stress state in the impact tests is much more complex and the strain rate significantly higher than in the tensile and compression tests. Optical and scanning electron microscopes are used in the investigation. The fracture surface obtained in tests with smooth axisymmetric specimens indicates that the crack growth is partly intergranular along the grain boundaries or precipitation free zones and partly transgranular by void formation around fine and coarse intermetallic particles. When the stress triaxiality is increased through the introduction of a notch in the tensile specimen, delamination along the grain boundaries in the rolling plane is observed perpendicular to the primary crack. In through-thickness compression tests, the crack propagates within an intense shear band that has orientation about 45° with respect to the load axis. The primary failure modes of the target plate during impact were adiabatic shear banding when struck by a blunt projectile and ductile hole-enlargement when struck by an ogival projectile. Delamination and fragmentation of the plates occurred for both loading cases, but was stronger for the ogival projectile. The delamination in the rolling plane was attributed to intergranular fracture caused by tensile stresses occurring during the penetration event.  相似文献   

5.
A new testing procedure, employing transverse load was adopted to investigate the high cycle fatigue behaviour of low pressure cast AZ91 magnesium alloy. The tests were conducted with an electro dynamic shaker system by employing specimens fabricated as per ASTM standard. SN plot was generated from the test results and compared with that of gravity cast AZ91 alloy tested in identical ambience. The influence of transverse load on the fatigue behaviour of these alloys is discussed. As fatigue cracks were found to have initiated in pores in most of the tested samples, pores were assumed as initial cracks as per linear fracture mechanics and the critical stress intensity amplitude (Kcr) was estimated. Structure–fatigue property correlations are discussed using fractographs. Mean stress effect on the fatigue properties and effects of alloying constituents are also discussed.  相似文献   

6.
The increased demand for plastic mould steels in pre-hardened condition has drawn the attention to this specific type of steel. As a result, more investigations are performed to understand microstructure and properties. In this work, the microstructures of two pre-hardened plastic mould steels, one quench-tempered (Uddeholm Impax HH) and the other continuously cooled (Uddeholm Nimax), are studied in delivery condition by means of different microscopy techniques and are linked to their production procedure. The results show that the quench-tempered material contains large amounts of M3C carbides formed within the martensite plates as well as at the lath- and prior austenite grain boundaries. A few coarser Cr-rich M7C3 carbides have also been found. In comparison, the microstructure of the continuously cooled material consists of mainly bainite with much lower density and finer cementite particles. The hardness is with ∼40 HRC more or less constant over the cross section of both materials.  相似文献   

7.
T-shape side pressing experiment is a sort of friction test which, recently, is employed for evaluation of friction for bulk metal forming processes. One of important advantages of this experiment, compared with other friction tests such as the ring compression test, is the occurrence of appropriate surface enlargement during the deformation of the specimen. This paper is concerned with experimental and numerical studies on this test, when it is used for some magnesium alloys such as AZ31 and AZ80. Based on the experimental results, it was found that the friction sensitivity of T-shape experiment increased when the die edge radius decreased or the test temperature or ram velocity increased. Good repeatability of this test was also observed during experimental part of this research work. Finally, employing the flow curves gained from the compression tests and friction factors obtained from the T-shape experiments for the finite-element simulations of this test, resulted in a very good agreement between the numerical and experimental load curves.  相似文献   

8.
A combined experimental–numerical study of tailor-made blanks (TMBs) with dissimilar alloys is presented in this paper. The selected adhesive is AF163-2 K (from 3 M). Metal sheets are made from two dissimilar aluminum alloys, namely 2024-T3 and 7075-T6. The 7075-T6 sheet acts as a base sheet on top of which a 2024-T3 sheet is bonded. While the thickness of the base sheet (7075-T6) is the same (=2.0 mm) for all specimens, the thickness of the upper sheet (2024-T3) varies between 0.5 mm and 2.5 mm. The bonded sheets are machined to the standard dog bone shape. In addition, a thickness difference either perpendicular to the loading direction (transverse specimens) or parallel with it (longitudinal specimens) is created by milling. The tensile tests are conducted, while measuring the three-dimensional deformations of the sheets using digital image correlation technique. It is shown that, in contrast with the TMBs with similar-alloys (both sheets made from 2024-T3), the upper sheet does not delaminate during the test. The failure mode is therefore metal fracture. This is an important conclusion, because it improves the in-plane straining limits of adhesively-bonded TMBs. A Finite Element Method (FEM) model of the tensile tests is created. The adhesive damage and metal failure are, respectively, modeled using cohesive interface and distributed continuum damage theories. It is shown that the FEM model can explain the behavior observed in the experiments. The lower ductility of the base sheet together with competition between two failure mechanisms, namely metal failure and delamination, are found responsible for the observed behavior.  相似文献   

9.
The effects of tool rotational speed and probe length on lap joint quality of friction stir welded 2-mm AZ31B-H24 magnesium alloy were investigated in terms of welding defects, microstructure and mechanical properties. Tensile shear load initially increases with increasing tool rotational speed but decreases with further increase. However, the tensile shear load always increases with increasing probe length. The highest shear strength is obtained using a set of welding parameters resulting in a downward hooking defect at the maximum stress location of the top sheet. Sound lap joints with low distortion, lack of cavity, minor kissing bond and preferable hooking defects, and high tensile shear strength were successfully obtained, indicating the great potential of friction stir welding technique for magnesium alloys.  相似文献   

10.
Isothermal forging of a rib–web shape in AZ31B magnesium alloy in the rolling direction was conducted at speeds of 0.01–10 mm s−1 in the temperature range of 300–500 °C with the purpose of validating the results of materials models involving kinetic analysis and processing map. The process was also simulated using finite element method DEFORM to obtain the local values of strain and strain rate. Forging parallel to the rolling direction in the range 375–550 °C and 0.0003–0.3 s−1 under the conditions of dynamic recrystallization (DRX) resulted in a symmetrical cup-shape while at other conditions an elliptical boat-shape was produced with the major axis coinciding with the transverse direction and the minor axis aligned with the normal direction. This anisotropy of flow has been attributed to the strong basal texture in the rolled plate and the dominance of prismatic slip at lower temperatures. In the DRX domain on the other hand, pyramidal slip dominates along with cross-slip as the recovery mechanism, which destroys the initial texture and restores the symmetry of flow. The grain size variation for forgings done in the DRX domain validated the predictions of the material models.  相似文献   

11.
AA2219 aluminium alloy square butt joints without filler metal addition were fabricated using gas tungsten arc welding (GTAW), electron beam welding (EBW) and friction stir welding (FSW) processes. The effect of three welding processes on fatigue crack growth behaviour is reported in this paper. Transverse tensile properties of the welded joints were evaluated. Microstructure analysis was also carried out using optical and electron microscopes. It was found that the FSW joints are exhibiting superior fatigue crack growth resistance compared to EBW and GTAW joints. This was mainly due to the formation of very fine, dynamically recrystallised grains and uniform distribution of fine precipitates in the weld region.  相似文献   

12.
This paper describes the design and examination of W/Ni double interlayer to produce a joint between SiC and ferritic stainless steel. Diffusion bonding was performed by a two steps solid state diffusion bonding process. Microstructural examination and mechanical properties evaluation of the joints show that bonding of SiC to steel was successful. EDS and XRD analysis revealed that W5Si3 and WC were formed at SiC/W interface. The diffusion products at W/Ni interface, Ni-rich solid solution Ni(W) or intermetallic compound Ni4W, was found to be dependent on the second step joining temperature. Neither intermediate phases nor reaction products was observed at Ni/steel interface for the joints bonded at the temperature studied. The average tensile strength of 55 MPa which is insensitive to the second step process was measured for as-bonded SiC/steel joint and the failure occurred at SiC/W interface. The hardness near the various bonded interfaces was also evaluated.  相似文献   

13.
In micro-forming process, the prediction of deformation behavior is difficult as the conventional material constitutive model is no longer valid when the part geometry is scaled down to micro-level. This is caused by the so-called “size-effect”. It is thus necessary to study the size effect and how it affects the deformation behavior in micro-forming process. In this research, a material constitutive model was established based on micro-compression test and its applicability was then studied. To facilitate the research, a flexible tooling set for micro-extrusion was designed and developed first. A modified micro-double cup extrusion test was proposed and the corresponding Finite Element Method (FEM) simulation was conducted. Through experiment and simulation, a set of deformation load curves were generated so as to provide a reference for calibration of flow stress–strain curve in modeling of micro-extrusion process. The applicability of the calibrated flow stress–strain curve was finally validated by the experimental and simulation results of micro-forward extrusion. It is therefore believed that the flow pattern, the material surface constraint and the material deformation mode are critical in determination of material flow stress curve. Furthermore, it was found that the change of cup height ratio of the extruded part is not caused solely by the change of friction when the part size is in micro-scale. The material flow stress significantly affects the cup height ratio. These findings provide a basis in understanding of micro-extrusion process.  相似文献   

14.
The demand on micro-parts is significantly increasing in the last decade due to the trend of product miniaturization. When the part size is scaled down to micro-scale, the billet material consists of only a few grains and the material properties and deformation behaviors are quite different from the conventional ones in macro-scale. The size effect phenomena occur in micro-scale plastic deformation or micro-forming and there are still many unknown phenomena related to size effect, including geometry and grain size effects. It is thus critical to investigate the size effect on deformation behavior, especially for the fracture behavior in micro-scale plastic deformation. In this research, tensile test was conducted with annealed pure copper foils with different thicknesses and grain sizes to study the size effects on fracture behavior. It is found that flow stress, fracture stress and strain, and the number of micro-voids on the fracture surface decrease with the decreasing ratio of specimen size to grain size. Based on the experimental results, dislocation density based models which consider the interactive effect of specimen and grain sizes on fracture stress and strain are developed and their accuracies are further verified and validated with the experimental results obtained from this research and prior arts.  相似文献   

15.
Simultaneous effects of notch and texture on strengthening mechanisms of rolled thin sheets of commercially pure titanium were investigated. The presence of notch led to the restriction of deformation systems and different fracture behaviors compared to un-notched specimens. The loss of material’s ability to accommodate plastic deformation at the notch tip with increase in rolling reductions changed the notch strengthening phenomenon to the notch weakening one. At medium levels of deformation, due to the simultaneous development of a triaxial stress state and strong basal texture at the notch tip, a new strengthening mechanism which is called “notch-texture strengthening mechanism” led to a significant enhancement of tear strength. However, the lack of stress triaxiality in un-notched tensile specimens and a strong basal texture component in other notched specimens reduced the impact of strengthening. It was found that the restriction of deformation systems due to the c-axis compression condition at the notch tip was responsible for this strengthening mechanism.  相似文献   

16.
This is the first reported research into the tensile behavior of as-deformed Al–Zn–Mg–Cu alloy in the semi-solid state. Tensile tests of extruded 7075 aluminium alloy were carried out in the high temperature solid and semi-solid states. Based on the tensile results and microstructural examination, the tensile behavior can be divided into three stages according to the effect of liquid: one behaves in predominantly ductile character between 400 and about 520 °C (fl  0.31%), one is governed by both of solid and liquid between 520 and 550 °C (fl  2%), and almost completely dominated by liquid above ∼550 °C. A brittle temperature range (519–550 °C) is proposed, in which the as-deformed Al–Zn–Mg–Cu alloy exhibits large crack probability. An equation based on ultimate tensile stress and temperature is proposed.  相似文献   

17.
When the geometry of metal deformed part is scaled down to micro-scale, the understanding and prediction of micro deformation behaviour becomes difficult. This is because the conventional material deformation models are no longer valid in micro-scale due to the size effect, which affects the deformation behaviour in micro plastic deformation, and thus leveraging the traditional knowledge of plastic deformation from macro-scale to micro-scale is not meaningful. In this paper, the size effect on micro-scale plastic deformation and frictional phenomenon are investigated via micro-cylindrical compression test, micro-ring compression test and Finite Element (FE) simulation. The experimental results show the occurrence of various size-effect related deformation phenomena, including the decrease of flow stress and the increases of: (a) irrational local deformation, (b) the amount of springback, and (c) the interfacial friction stress with the decreasing specimen size. The research further verifies that the established surface layer models, with the identified surface grain, the internal grain properties and the measured friction coefficients, are able to predict micro deformation behaviour. The research thus provides an in-depth understanding of size effect on deformation and frictional behaviours in micro-scale plastic deformation.  相似文献   

18.
Microstructure and mechanical properties of friction stir weld joints of dissimilar Mg alloys AZ31 and AZ80 were investigated in the present work. Several different welding parameters were adopted in the study, and the effects of rotation speed and welding speed on the joint quality were discussed comprehensively. In addition, material arrangement which means that AZ31 alloy was at advancing side or at retreating side has significant influence on the joint formation, including the joint microstructure and mechanical properties. A few kinds of defects were observed when the improper parameters were taken in the experiment, and the reasons for generating these defects were revealed in this work. Sound joints with good mechanical properties could be easily obtained when AZ31 was at retreating side, but it was difficult to obtain the sound joint with the contrary material arrangement. These results suggest that the material with inferior plastic deformability should be set at the advancing side and the material with superior one should be set at the retreating side in order to get sound FSW joint of dissimilar Mg alloys.  相似文献   

19.
A study was carried out to investigate the effect of governing metal thickness (GMT) and stack orientation on weld quality and mechanical behaviour of resistance spot welded (RSW) AA5754 aluminium. Individual samples from 27 different joint stacks in three test geometries; lap-shear, coach-peel and cross-tension were evaluated for quasi-static and fatigue performance; micro examination was also conducted on some of the samples to assess weld quality. The results derived from over 1000 samples show that: the GMT has a significant effect on welding quality by controlling progression of weld nugget from under-developed to over penetrated. The GMT also determines the feasible quasi-static joint strength regardless of stacks in the three joint geometries tested, though the effect differs with respect to test geometry. The fatigue behaviour is dominated by the effect of GMT on attainable weld size, overall joint stiffness and stress concentration, providing good quality of weld nuggets is achieved. No notable effect of stack orientation on weld quality and joint strength was found with respect to the joint stack asymmetry and welding orientation to the electrodes. These fundamental relationships between weld qualities, joint strength, GMT and stack orientation for RSW of aluminium will have significant relevance to design and manufacturing communities.  相似文献   

20.
Plates of aluminum alloys 2219-T62 were joined in a butt joint by friction stir welding. The residual stresses on the top and bottom surfaces were measured using the hole-drilling strain-gauge method. In the test specimen, it was found that the residual stresses on the top surface peaked at about 171 MPa, while the value reached 243 MPa for the weld with tunnel defect and had the conventional “M” profile with tensile stress peaks in the heat-affected zone. Those attached on the bottom surface had the inverted “V” profile with tensile stress peaks in the weld centre and the corresponding value was 99.4 MPa. Meanwhile, with the increase of rotary speed, the longitudinal residual stress decreased on the top surface, but increased on the bottom surface.  相似文献   

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