Tensile properties and fracture behavior of partial squeeze added slow shot die-cast A356 aluminum alloy

In this contribution, effect of the microstructural characteristics on tensile properties and fracture behavior of partial squeeze added slow shot die-cast A356 alloy die casting in the as-cast and T6 heat-treated conditions was studied. The results show that, inferior tensile properties of the casting partial squeeze part were caused by the heterogeneity of α-Al cells with fragment, rosette, angular and globular shapes, while finer dendrites with smaller secondary dendrite arm spacing and more rounded silicon particles corresponded to higher tensile properties. After T6 treatment, tensile properties increased significantly, due to the spheroidization of silicon particle and consequently the reduction of stress concentration at silicon/eutectic matrix interface. Differences observed in the tensile fracture path were attributed to microstructural changes as well as morphological aspects of silicon phase.     

Fabrication of NiCr alloy parts by selective laser melting: Columnar microstructure and anisotropic mechanical behavior

NiCr alloy, because of its wide applications in electrical elements and dental field was widely studied in the past. In this work, NiCr cubes and tensile specimens were fabricated by using a new processing technique-selective laser melting (SLM). Microstructural and mechanical behavior characterization of SLM-processed NiCr components was performed. An unusual columnar microstructural architecture composed of 〈1 0 0〉 texture (corresponding to (2 0 0) plane) oriented the building direction was observed. Moreover, it was found that the columnar grain growth across the melt pools occurred during the SLM process and the growth trend became stronger with the decrease of the laser scanning speed. Associated with the microstructural characteristic, an anisotropic mechanical behavior at different reference planes (i.e., at the horizontal and vertical surfaces) was demonstrated for the samples fabricated using different processing parameters. The results showed that with increasing the laser scanning speed, the microhardness at the horizontal surface decreased, while at the vertical surface it increased; an increase of the yield strength (YS) and the ultimate tensile strength (UTS) was observed.     

Influence of the processing route on the microstructure of aluminum alloy A356 for thixoforming

Raw materials for thixoforming processes can be obtained by various routes. An analysis was made of the influence of five different processing routes on the production of thixotropic A356 alloy: 1) direct casting in a water-cooled mold; 2) direct casting as in (1), plus enhancement by electromagnetic stirring and grain refining; 3) as in (1), but enhanced by mechanical vibration; 4) as in (1), followed by one ECAP pass (Equal Channel Angular Pressing); and 5) using a commercial product as comparison. The alloy was characterized by both conventional and color microstructure for a comprehensive understanding of its structure prior to and after heating to 580 °C, which is the ideal processing temperature for this specific raw material in the semi-solid state. B&W (black and white) and polarized light color metallography of the microstructure were employed to better characterize these structures. Among the five tested routes, the samples prepared by the ECAP route presented the most suitable characteristics for thixoforming, showing smaller grain sizes (measured by polarized light color metallography), smaller globule size and greater sphericity (both measured by B&W metallography) in the semi-solid state. Interconnections in the residual solid fraction in the semi-solid state are usually visible only by polarized light color metallography. However, the use of both color and B&W metallography enabled the best morphological structures for thixoforming to be clearly identified, confirming the promising potential of the ECAP technique in processing raw materials for thixoforming.     

Significant improvement of semi-solid microstructure and mechanical properties of A356 alloy by ARB process

Accumulative roll bonding (ARB) process was used in this study as an effective method for manufacturing high-strength, finely-dispersed and highly-uniform A356 alloy. It was found that when the number of ARB cycles was increased, the uniformity of silicon particles in the aluminum matrix improved, the particles became finer and spheroider and therefore, the tensile strength (TS) and ductility of the samples improved. The microstructure of the manufactured A356 alloy after five ARB cycles indicated a totally modified structure such that it's TS and elongation values reached 269 MPa and 5.3% which were 2.6 and 2.5 times greater than those of the as-cast material, respectively. Also, the hardness value increased from 55.4 (for as-cast sample) to 100.2 HV (after the fifth cycle of ARB), and registered 81% increase.     

Influence of peak temperature during simulation and real thermal cycles on microstructure and fracture properties of the reheated zones

The objective of this paper is to study the influence of the second peak temperature during real and simulated welding on properties of the subcritically (S), intercritically (IC) and supercritically (SC) reheated coarse grained heat affected (CGHAZ) zones. The X80 high strength pipeline microalloyed steel was subject to processing in a double-pass tandem submerged arc welding process with total heat input of 6.98 kJ/mm and thermal cycles to simulate microstructure of reheated CGHAZ zones. This involved heating to a first peak temperature (T_P1) of 1400 °C, then reheating to different second peak temperatures (T_P2) of 700, 800 and 900 °C with a constant cooling rate of 3.75 °C/s. Toughness of the simulated reheated CGHAZ regions were assessed using Charpy impact testing at 0 °C, −25 °C and −50 °C. The microstructure of the real and simulated reheated CGHAZ regions was investigated using an optical microscope and field emission scanning electron microscope. Morphology of the martensite/austenite (MA) constituent was obtained by the use of a field emission scanning electron microscope. The blocky and connected MA particles, along prior-austenite grain boundaries, act as a brittle phase for the initiation site of the brittle fracture. Charpy impact results indicated that IC CGHAZ had less absorbed energy with higher transition temperature and hardness. The SC CGHAZ region showed higher absorbed impact energy with lower hardness. Design of multipass weld joints with less IC CGHAZ regions can result in a higher toughness property.     

Predictive equations of the tensile properties based on alloy hardness and microstructure for an A356 gravity die cast cylinder head

One of the main problems in the design of complex Al–Si cast components is the wide variety of mechanical properties in different regions of the castings which is due to the wide range of solidification microstructures, related to the local solidification conditions. There are many papers available on the widely used A356/A357 Al–Si–Mg alloys, however, most experimental data on their tensile or fatigue properties are generally obtained from specimens cast separately or produced under controlled laboratory conditions, that are extremely different from those of industrially cast components. Moreover, most of these data often relate the mechanical properties to only one microstructural parameter, such as solidification defects or secondary dendrite arm spacing, and do not take their simultaneous effect into consideration. For all these reasons, the main problem, in the design phase, is the lack of knowledge of the true local mechanical properties in complex-shaped castings, which often means a conservative approach is necessary, with a consequent increase in thickness and therefore in weight. The aim of this research was to study a complex A356 gravity die cast cylinder head, in order to verify the range of variability of the main microstructural parameters and tensile properties, using specimens directly machined from the casting. The component was heat treated at the T6 condition, and the effect of the delay between quenching and aging on the alloy hardness was also evaluated. Simple experimental equations have been proposed, able to successfully predict the local tensile properties in the casting, when only the most important microstructural parameters and alloy hardness are known. These equations allow the designer to predict the local tensile behaviour without any tensile tests; moreover they can also link the post-processing results of the casting simulation software to the pre-processing phase of the structural ones, with an approach of co-engineered design.     

Influence of temperature on impact fracture behavior of an alloy steel

In this paper, the influence of temperature on impact toughness and fracture behavior of alloy steel (AISI Classification 8320) is presented and discussed. Impact toughness decreased with a decrease in test temperature. The extrinsic influence of temperature on impact toughness–fracture resistance relationships is rationalized in light of the conjoint and mutually interactive influences of intrinsic microstructural features, local stress states and macroscopic fracture behavior.     

Effect of precipitation on post-heat-treated Inconel 625 alloy after friction stir welding

This study evaluated the mechanical properties of friction stir welded and post-heat-treated Inconel 625 alloy. Friction stir welding (FSW) was performed at rotation and traveling speeds of 200 rpm and 100 mm/min, respectively; heat treatment was carried out after welding at 700 °C for 100 h in vacuum. As a result, the application of FSW on Inconel 625 alloy led to the grain refinement in the stir zone, which resulted in increase in mechanical properties than those of the base material. Especially, applying heat treatment after FSW led to the improvement of mechanical properties of the welds; microhardness and tensile strength increased by more than 30% and 50%, respectively, as compared to FSW alone.     

Effect of heat treatments on mechanical properties and fracture behavior of a thixocast A356 aluminum alloy

The effect of different heat treatments (T5 and T6) on mechanical properties, fracture behavior and damage evolution of A356 Thixocast aluminum alloy have been examined in detail in the present work. Tensile tests of the material have been performed in the as cast and as treated conditions in order to observe the different fracture behavior in consequence of the heat treatments. Optical and scanning electron microscopy techniques have been used to characterize the microstructure and fracture surfaces of the specimens. Finally, the precipitation processes of the material have been analyzed by hardness and electrical conductivity measurements and EDS analysis has been used to characterize the different phases in the as-thixo and as-treated conditions.     

On the failure of low carbon steel resistance spot welds in quasi-static tensile–shear loading

Failure behavior of low carbon steel resistance spot welds in quasi-static tensile–shear test is investigated. Microstructure, hardness profile and mechanical performance of the spot welds were studied. Results showed that spot welds are failed in two distinct failure modes: double-pullout and interfacial failure modes. There is a critical fusion zone size beyond which, pullout failure mode is guaranteed. Metallographic examination showed that failure is a competitive process between shear plastic deformation of weld nugget and necking of the base metal. In pullout failure mode, only the grain pattern of the base metal changes significantly and that of the fusion zone and heat affected zone remains unchanged. Strain localization was occurred in the base metal due to its low hardness. Moreover, the experimental results showed that increasing the holding time which increases the hardness of the fusion zone did not affect the peak load. It was concluded that in the pullout failure mode, the strength of the spot welds is not affected by the fusion zone strength. Fusion zone size proved to be the most important controlling factor for the spot welds’ mechanical performance in terms of peak load and energy absorption.     

Influence of the annealing cooling rate on the microstructure evolution and deformation behaviours in the cold ring rolling of medium steel

Pre-heat treatment is a necessary step of cold ring rolling that leads to different microstructure evolution processes and different macroscopic deformation behaviours of rings in the cold ring rolling process. In this paper, the cold ring rolling process of 1045 steel with different annealing cooling rates is studied because the annealing cooling rate is the factor with the greatest influence on the result of the pre-heat treatment. By subjecting 1045 steel to different annealing cooling rates, it is found that, within the experimental range, the grain size of the rings becomes smaller and the lamellar spacing of pearlite decreases as the annealing cooling rate increases, resulting in a stronger and tougher material. The ABAQUS finite element (FE) software was employed to simulate the cold ring rolling process. The simulations indicated that increasing the annealing cooling rate causes the stress in the rolled rings to increase, the strain to decrease and the cementite lamellae to become more fractured.     

Effect of calcium addition on the microstructure and compressive deformation behaviour of 7178 aluminium alloy

Aluminium 7178 alloys containing 1% calcium are used to study the effect of calcium addition on their microstructure and compressive deformation behaviour. The compressive deformation behaviour of aluminium alloy containing 1% calcium is studied at varying strain rates (10⁻²–10/s). The material is prepared using stir casting technique. The yield stress, flow stress and elastic limit are measured from the true stress–strain graph. The strain rate sensitivity and strain-hardening exponent was also determined for each material at different strain rates. Its microstructural characterization reveals that Ca particles act as grain refiners for primary base alloy and helps in improving the strength of the virgin alloy. An empirical relationship has been proposed to predict the flow curve of the alloys as a function of strain and strain rate.     

Effect of deformation temperature on microstructure evolution in aluminum alloy 2219 during hot ECAP

The effect of deformation temperature on microstructure evolution during equal channel angular pressing (ECAP) was studied in a coarse-grained aluminum alloy 2219 in a wide temperature interval from 250 to 475 °C. The structural changes taking place during ECAP up to strains of 12 are classified into the following three stages irrespective of deformation temperatures: i.e. (1) an incubation period for formation of the embryos of deformation bands (DBs) at low strains; (2) development of large-scale DBs followed by grain fragmentation at moderate strains; (3) rapid development of new grain at high strains. Microstructure development in stages 1 and 2 is hardly influenced by temperature, while that in stage 3 is most significantly affected at higher temperature. An increase in the pressing temperature leads to decreasing the volume fraction of new grains and increasing the average grain size in stage 3. This can be attributed to relaxation of strain compatibility between grains due to frequent operation of dynamic recovery and grain boundary sliding at higher temperature. The mechanism of grain refinement is discussed in detail.     

Acoustic emission and fracture behavior of GFRP woven laminates at cryogenic temperatures

This paper describes an experimental and analytical study on fracture and damage behavior of GFRP woven laminates at cryogenic temperatures. CT (compact tension) tests were carried out at room temperature, liquid nitrogen temperature (77 K) and liquid helium temperature (4 K) to evaluate the critical values of the fracture mechanics parameters. During the CT tests, AE (acoustic emission) method was implemented. AE signals can identify the critical load at which gross failure occurs. A FEA (finite element analysis) was also applied to calculate the fracture mechanics parameters. The failure criteria (Hoffman criterion and maximum strain criterion) or the damage variable based on the continuum damage mechanics was incorporated into the model to interpret the experimental measurements and to study the damage distributions within the specimen. Several methods of calculating J-integral are discussed.     

Effect of tandem submerged arc welding process and parameters of Gleeble simulator thermal cycles on properties of the intercritically reheated heat affected zone

The effects of real and Gleeble simulated double pass thermal cycles on the properties of the intercritically reheated coarse grained heat affected zones in X80 microalloyed pipeline steel has been investigated. The Gleeble simulated process involved heating the X80 steel specimens to the first peak temperature of 1400 °C and then reheating to the second peak temperature of 800 °C, with different cooling rates. The size and area fraction of martensite/austenite (M/A) constituents were obtained by a combination of field emission scanning electron microscopes and image analysis software. In addition, misorientation was characterized by electron back-scatter diffraction analysis. It is clear that the intercritically thermal cycles have a significant effect on morphology of M/A constituents. The M/A constituent’s size, such as mean diameter and length, are important factors influencing Charpy impact properties of thermally simulated intercritically reheated heat affected zones. The simulated thermal cycles of the intercritically reheated region in the high heat input tandem submerged arc welding processes, showed extremely poor Charpy impact absorbed energy. The intercritical reheated thermal cycles with lower heat input value showed higher Charpy impact absorbed energy due to a decrease in the prior-austenite grain and M/A particle size.     

Effect of tool pin profile on microstructure and mechanical properties of friction stir welded AZ31B magnesium alloy

In this investigation the effect of friction stir welding pin geometry on the microstructure and mechanical properties of AZ31B magnesium alloy joints is studied. The considered pin geometries are simple cylindrical, screw threaded cylindrical and taper. The joints are friction stir welded at different traverse and rotational speeds. Microstructures of the joints are examined using the optical and scanning electron microscopes. Also, the tensile properties and hardness of the joints are measured. The results show that taper and screw threaded cylindrical pins produce defect free joints. In addition, the taper pin results in finest microstructure and highest mechanical properties. Furthermore, it is found that rotational speed has a more significant role on the final microstructure and mechanical properties of the joints, compared to the traverse speed.     

Effect of extrusion ratio on mechanical and corrosion properties of AZ31B alloys prepared by a solid recycling process

Some AZ31B magnesium alloy bars were prepared by a solid recycling process with different extrusion ratios. A reference specimen was processed by extruding an as-received AZ31 ingot. The microstructures, mechanical and corrosion properties of AZ31B magnesium recycled specimens were investigated. With increasing extrusion ratio, the yield strength, tensile strength and yield ratio increases. The reliability of the recycled alloy is poorer than the reference specimen. The corrosion rates of recycled AZ31B magnesium specimens increase immersed in both alkaline and neutral 4% NaCl solution with a decrease extrusion ratio. The corrosion resistance of recycled AZ31B magnesium specimens is improved with increasing pH of immersed solution. The recycled specimens show superior corrosion resistance than reference specimen.     

Influence of notch severity on the impact fracture behavior of aluminum alloy 7055

In this paper, the conjoint influence of notch severity and test temperature on the impact behavior of an Al-Zn-Mg-Cu alloy 7055 in the T7751 microstructural condition is presented and discussed. Notch angles of 45°, 75° and 90° were chosen for a standard charpy impact test specimen containing two notches. For a given angle of the notch the increase in dynamic fracture toughness, with test temperature, is most significant for the least severe of the notches, i.e. 45°. At a given test temperature, the impact toughness of the T7751 microstructure decreased with an increase in notch severity. An increase in notch severity resulted in essentially Mode I dominated fracture at all test temperatures. The influence of localized mixed-mode loading is minimal for the alloy has low dynamic toughness. The impact fracture behavior of the alloy is discussed in light of alloy microstructure, mechanisms governing fracture and the deformation field ahead of a propagating crack.     

半固态A356铝合金浆料的充填行为及组织分布

采用流变压铸方法研究了低过热度浇注和弱电磁搅拌制备的半固态A356铝合金浆料的充填行为和组织分布,结果表明：采用该技术制备的半固态A356铝合金浆料,其组织形态优良,经过感应均热后,浆料内部的温度场分布均匀,初生α—Al晶粒更圆整．半固态A356铝合金的浆料温度、压射比压和冲头速度对浆料的充填行为有较大的影响．较高的浆料温度、压射比压和冲头速度都有利于半固态铝合金浆料的充填．在本文的实验条件下,合适的浆料温度为585-595℃,压射比压为15-25MPa,冲头速度为0．072-0．12m／s．得到的流变压铸件的组织分布均匀,无明显的固液相偏析．