Microstructure and thermal expansion behavior of spray-deposited Al–50Si

The evolution of microstructure and coefficient of thermal expansion (CTE) of the Al–50Si (wt.%) alloy manufactured by spray deposition followed by hot isostatic pressing (HIP) are systematically investigated. The results indicate that the microstructure of the deposited alloy is composed of primary Si with average size of 12.5 ± 0.1 μm and α-Al. The CTE of the deposited alloy is higher than the corresponding alloy produced by casting due to the high solid solubility of Al in Si. After HIP, the CTE is lower than the parent as-deposited alloy owing to the high solid solubility of Si in Al. The residual thermal stress results in a higher CTE during the second heating as a result of the CTE mismatch between the Al matrix and the primary Si particles. Furthermore, the measured CTE value is in good agreement with the Turner model after complete densification by HIP at 843 K.     

Hot tearing mechanisms of B206 aluminum–copper alloy

In this study, the mechanisms of hot tearing in B206 aluminum alloy were investigated. Castings were produced at three mold temperatures (250 °C, 325 °C and 400 °C) and with two levels of titanium (0.02 wt% and 0.05 wt%) to investigate the effects of cooling rate and grain refinement. A constrained-rod casting mold attached to a load cell was used to monitor the contraction force during solidification and subsequently determine the onset temperature of hot tearing in B206. The corresponding onset solid fraction of hot tearing was estimated from the solid phase evolution of α-Al in B206 using in situ neutron diffraction solidification analysis. Hot tears were found to occur at solid fractions ranging from 0.81 to 0.87. Higher mold temperatures significantly reduced hot tearing severity in B206 but did not alter the onset solid fraction. In contrast, additions of titanium to B206 were effective at eliminating hot tears by transforming the grain structure from coarse dendrites to finer and more globular grains. Finally, in situ neutron diffraction solidification analysis also successfully determined the solid phase evolution of intermetallic Al₂Cu during solidification, which in turn, provided a better understanding of the role of Al₂Cu in the development of hot tears in B206.     

Research on the microstructure,fatigue and corrosion behavior of permanent mold and die cast aluminum alloy

Permanent mold (PM) and high pressure die cast (HPDC) AlMg5Si2Mn are employed to investigate the microstructure, fatigue strength and corrosion resistance. Results indicated that the mechanical properties (R_m, R_0.2 and δ) of HPDC specimens (314 MPa, 189 MPa and 7.3%) are significantly better than those of PM specimens (160 MPa, 111 MPa and 2.5%) due to the finer grain size and less cast defects. Fatigue cracks of PM samples dominantly initiated from shrinkage pores and obscure fatigue striations are observed in crack growth region. Corrosion and pitting potentials of PM and HPDC AlMg5Si2Mn alloy are around −1250 mV, −760 mV and −1220 mV, −690 mV respectively. Numerous pits are observed around the grain boundaries because the corrosion potential of Mg₂Si is more anodic than that of α-Al matrix. In addition, the superior corrosion resistance of HPDC samples can be attributed to the fine grain size and the high boundary density which improved the formation of oxide layer on the surface and prevented further corrosion.     

Effects of processing parameters on microstructure and mechanical properties of squeeze-cast Mg–12Zn–4Al–0.5Ca alloy

In this paper, a new magnesium alloy Mg–12Zn–4Al–0.5Ca (ZAX12405) was prepared by squeeze casting. The effects of processing parameters including applied pressure, pouring temperature and dwell time on the microstructure and mechanical properties of squeeze-cast ZAX12405 alloy were investigated. It was found that squeeze-cast ZAX12405 alloy exhibited finer microstructure and much better mechanical properties than gravity casting alloy. Increasing the applied pressure led to significant cast densification and a certain extent of grain refinement in the microstructure, along with obvious promotion in mechanical properties. Lowering the pouring temperature refined the microstructure of ZAX12405 alloy, but deteriorated the cast densification, resulting in that the mechanical properties firstly increased and then decreased. Increasing the dwell time promoted cast densification and mechanical properties just before the solidification process ended. A combination of highest applied pressure (120 MPa), medium pouring temperature (650 °C) and dwell time (30 s) brought the highest mechanical properties, under which the ultimate tensile strength (UTS), yield strength (YS) and elongation to failure (E_f) of the alloy reached 211 MPa, 113 MPa and 5.2% at room temperature. Comparing with the gravity casting ZAX12405 alloy, the UTS and E_f increased 40% and 300%, respectively. For squeeze-cast Mg–12Zn–4Al–0.5Ca alloy, cast densification was considered more important than microstructure refinement for the promotion of mechanical properties.     

Fatigue behavior and modeling of short fiber reinforced polymer composites including anisotropy and temperature effects

Effects of anisotropy and temperature on cyclic deformation and fatigue behavior of two short glass fiber reinforced polymer composites were investigated. Fatigue tests were conducted under fully-reversed (R = −1) and positive stress ratios (R = 0.1 and 0.3) with specimens of different thicknesses, different fiber orientations, and at temperatures of −40 °C, 23 °C, and 125 °C. In samples with 90° fiber orientation angle, considerable effect of thickness on fatigue strength was observed. Effect of mold flow direction was significant at all temperatures and stress ratios and the Tsai–Hill criterion was used to predict off-axis fatigue strengths. Temperature also greatly influenced fatigue strength and a shift factor of Arrhenius type was developed to correlate fatigue data at various temperatures, independent of the mold flow direction and stress ratio. Micromechanisms of fatigue failure at different temperatures were also investigated. Good correlations between fatigue strength and tensile strength were obtained and a method for obtaining strain–life curves from load-controlled fatigue test data is presented. A fatigue life estimation model is also presented which correlates data for different temperatures, fiber orientations, and stress ratios.     

Effect of stress ratio on VHCF behavior for a compressor blade titanium alloy

Effect of stress ratio on fatigue properties of a titanium alloy (TC-17) in the high cycle fatigue (HCF) and very high cycle fatigue (VHCF) were investigated by electromagnetic and ultrasonic fatigue testing. The S–N curves at R = −1, 0.1, 0.5 and 0.7 at 110 Hz and 20 kHz were obtained and discussed. The effects of frequency on fatigue strength was also investigated. It was concluded that the fatigue strength with 50% fatigue failure probability at R = 0.1, 0.5 and 0.7 is lower to the Goodman line and shows a bilinear decreasing trend. Cleavage fracture of primary grains in the surface and interior initiation zone were observed. The formation of the facets induced by the basal or prismatic slips of the H.C.P grains decreased the fatigue strength with variation in mean stress.     

Micromechanisms of fatigue crack growth in cast aluminium piston alloys

The fatigue crack growth behaviour in as-cast and hot isostatically pressed (HIP) model cast aluminium piston alloys with hypoeutectic Si compositions of 6.9 wt% and 0.67 wt% has been investigated. The HIP alloys showed slightly improved fatigue crack growth resistance. Analysis of the crack path profiles and fracture surfaces showed that the crack tends to avoid Si and intermetallic particles at low ΔK levels up to a mid-ΔK of ∼7 MPa√m. However, some particles do fail ahead of the crack tip to facilitate crack advance due to the interconnected microstructure of these alloys. At higher levels of ΔK, the crack increasingly seeks out Si and intermetallic particles up to a ΔK of ∼9 MPa√m after which the crack preferentially propagates through intermetallic particles in the 0.67 wt%Si alloy or Si and intermetallics in the 6.9 wt%Si alloys. It was also observed that crack interaction with intermetallics caused crack deflections that led to roughness-induced crack closure and possibly oxide-induced crack closure at low to mid-ΔK. However, crack closure appears unimportant at high ΔK due to the large crack openings and evidenced by the fast crack growth rates observed.     

Development and evaluation of a magnesium–zinc–strontium alloy for biomedical applications — Alloy processing,microstructure, mechanical properties,and biodegradation

A new biodegradable magnesium–zinc–strontium (Mg–Zn–Sr) alloy was developed and studied for medical implant applications. This first study investigated the alloy processing (casting, rolling, and heat treatment), microstructures, mechanical properties, and degradation properties in simulated body fluid (SBF). Aging treatment of the ZSr41 alloy at 175 °C for 8 h improved the mechanical properties when compared to those of the as-cast alloy. Specifically, the aged ZSr41 alloy had an ultimate tensile strength of 270 MPa, Vickers hardness of 71.5 HV, and elongation at failure of 12.8%. The mechanical properties of the ZSr41 alloy were superior as compared with those of pure magnesium and met the requirements for load-bearing medical implants. Furthermore, the immersion of the ZSr41 alloy in SBF showed a degradation mode that progressed cyclically, alternating between pitting and localized corrosion. The steady-state average degradation rate of the aged ZSr41 alloy in SBF was 0.96 g/(m²·hr), while the pH of SBF immersion solution increased. The corrosion current density of the ZSr41 alloy in SBF solution was 0.41 mA/mm², which was much lower than 1.67 mA/mm² for pure Mg under the same conditions. In summary, compared to pure Mg, the mechanical properties of the new ZSr41 alloy improved while the degradation rate decreased due to the addition of Zn and Sr alloying elements and specific processing conditions. The superior mechanical properties and corrosion resistance of the new ZSr41 alloy make it a promising alloy for next-generation implant applications.     

Effect of electroceramic particles on damping behaviour of aluminium hybrid composites produced by ultrasonic cavitation and mechanical stirring

In this study, electroceramics PBN and PLZT along with SiC were included in Al–3.96 wt.% Mg (A514.0) master alloy. Ultrasonic cavitation (UST) and mechanical stirring (MS) were employed to improve wettability and dispersion during casting. Two composite systems were produced: PBN system (5 wt.% PBN + 1 wt.% SiC and 15 wt.% PBN + 1 wt.% SiC) and the PLZT system (follows the same designation). The influence of fabrication method on the microstructures, particle distribution and wettability as well as electroceramic impact on dynamo-mechanical properties of prepared composites were investigated. Optical microscope (OM) and scanning electron microscope (SEM) results indicate that the processing technique was effective as it promoted wettability and homogeneous dispersion of particles throughout the Al matrix. Dynamic mechanical analysis (DMA) study of the composites demonstrated that the addition of the functional particles to the Al alloy matrix improved damping capacity (Tan δ) at 200 °C. The composites exhibited an increase in Tan δ of 24.3 ± 0.3% and 91.4 ± 0.2% for 5 and 15 wt.% PBN + 1 wt.% SiC and an increase of 19.7 ± 0.5% and 42.5 ± 0.3% for 5 and 15 wt.% PLZT + 1 wt.% SiC, respectively, when compared to the aluminium alloy matrix.     

Fatigue strength of sharp V-notched specimens made of ductile cast iron

In the present, the high cycle fatigue strength of notched ductile cast iron is investigated. Experimental tests under axial loading, has been carried out on sharp V-notched specimens taken from heavy section casting considering nominal load ratios (R = 0). All specimens, taken from a heavy section casting, are characterized by a notch tip radius less than 0.1 mm, a notch depth of 10 mm and a notch opening angle = 90°. In order to evaluate the influence of chunky graphite morphology on fatigue life, fatigue tests were carried out also on a second set of specimens without that microstructural defect.Metallurgical analyses were performed on all the samples and some important microstructural parameters (nodule count and nodularity rating, among others) were measured and compared. It was found that a mean content of 40% of chunky graphite in the microstructure (with respect to total graphite content) does not influence significantly the fatigue strength properties of the analysed cast iron.     

Fatigue fracture of friction-stir processed Al–Al3Ti–MgO hybrid nanocomposites

This paper presents experimental results on the fatigue properties of Al-matrix nanocomposites prepared by the friction stir processing (FSP) technique. An Al–Mg alloy (AA5052) with different amounts (∼2 and 3.5 vol%) of pre-placed TiO₂ nanoparticles were FSPed up to 6 passes to attain homogenous dispersion of nano-metric inclusions. Microstructural studies by electron microscopic and electron back scattering diffraction (EBSD) techniques showed that nano-metric Al₃Ti (50 nm), TiO₂ (30 nm), and MgO (50 nm) particles were distributed throughout a fine-grained Al matrix (<2 μm). Consequently, a significant improvement in the tensile strength and hardness was attained. Uniaxial stress-controlled tension–tension fatigue testing (R = 0.1) were utilized to evaluate the fatigue behavior of the prepared nanocomposites. The results were compared with the un-processed (annealed) and FSPed alloy without pre-placing TiO₂ particles. It was found that FSP of the aluminum alloy increased the fatigue strength (at 10⁷ cycles) for about 28% and 32% compared with the annealed specimen when the concentration of the reinforcing particles was 2 and 3.5 vol%, respectively. Fractographic analysis determined a ductile fracture behavior with deep-equiaxed dimples for the annealed and FSPed alloy. The facture surface of the nanocomposites revealed a combined ductile–brittle fracture mode with finer dimples. The mechanism of the fatigue fracture and the role of nano-metric inclusions were elaborated.     

Microstructures,mechanical and cytocompatibility of degradable Mg–Zn based orthopedic biomaterials

The Mg–1Zn–xSr (x = 0.2, 0.5, 0.8 and 1 wt.%) alloys have been prepared by zone purifying solidification followed by backward extrusion (BE). The grain size was reduced and the hardness was improved with the increased concentration of strontium (Sr) after backward extrusion. The BE–Mg–1Zn–0.8Sr alloy was mostly composed of fine precipitates (MgZn and Mg₁₇Sr₂) and Mg matrix. At the same time, the mechanical properties of BE–Mg–Zn–Sr alloys were increased with the increment of strontium, which were strongly associated with fine average grain size and homogeneous secondary precipitates. The degradation rate is significantly increased when Sr content is over 0.8 wt.%. The homogenous degradation rate is achieved. The degradation products show good biocompatibility evaluated by MTT method using L929 cell line. It is demonstrated that the micro-alloying element of Sr is a potential approach to develop novel Mg–Zn based biomaterials.     

Structural characteristics and elevated temperature mechanical properties of AJ62 Mg alloy

Structure and mechanical properties of the novel casting AJ62 (Mg–6Al–2Sr) alloy developed for elevated temperature applications were studied. The AJ62 alloy was compared to commercial casting AZ91 (Mg–9Al–1Zn) and WE43 (Mg–4Y–3RE) alloys. The structure was examined by scanning electron microscopy, x-ray diffraction and energy dispersive spectrometry. Mechanical properties were characterized by Viskers hardness measurements in the as-cast state and after a long-term heat treatment at 250 °C/150 hours. Compressive mechanical tests were also carried out both at room and elevated temperatures. Compressive creep tests were conducted at a temperature of 250 °C and compressive stresses of 60, 100 and 140 MPa. The structure of the AJ62 alloy consisted of primary α-Mg dendrites and interdendritic nework of the Al₄Sr and massive Al₃Mg₁₃Sr phases. By increasing the cooling rate during solidification from 10 and 120 K/s the average dendrite arm thickness decreased from 18 to 5 μm and the total volume fraction of the interdendritic phases from 20% to 30%. Both factors slightly increased hardness and compressive strength. The room temperature compressive strength and hardness of the alloy solidified at 30 K/s were 298 MPa and 50 HV 5, i.e. similar to those of the as-cast WE43 alloy and lower than those of the AZ91 alloy. At 250 °C the compressive strength of the AJ62 alloy decreased by 50 MPa, whereas those of the AZ91 and WE43 alloys by 100 and 20 MPa, respectively. The creep rate of the AJ62 alloy was higher than that of the WE43 alloy, but significantly lower in comparison with the AZ91 alloy. Different thermal stabilities of the alloys were discussed and related to structural changes during elevated temperature expositions.     

A high strength and ductility Mg–Zn–Al–Cu–Mn magnesium alloy

A high strength Mg–8.0Zn–1.0Al–0.5Cu–0.5Mn (wt.%) magnesium alloy with outstanding ductility was developed using a common casting technique and heat treatment. The microstructure of the as-cast alloy is composed of α-Mg, MgZn, MgZnCu and Al–Mn phases. After the solution treatment and subsequent two-step aging treatment, the yield strength (YS), ultimate tensile strength (UTS) and elongation of the alloy at peak hardness reach 228 MPa, 328 MPa and 16.0% at room temperature, respectively. The comprehensive mechanical properties of the alloy are superior to almost all other high performance casting Mg alloys.     

Structure property relation of hybrid biocomposites based on jute,viscose and polypropylene: The effect of the fibre content and the length on the fracture toughness and the fatigue properties

In the present study, the extent of jute and viscose fibre breakage during the extrusion process on the fracture toughness and the fatigue properties was investigated. The composite materials were manufactured using direct long fibre thermoplastic (D-LFT) extrusion, followed by compression moulding. The fracture toughness (K_IC) and the fracture energy (G_IC) of the PP–J30 composites were significantly improved (133% and 514%, respectively) with the addition of 10 wt% viscose fibres, indicating hindered crack propagation. The addition of viscose fibres resulted in three times higher fatigue life compared with that of the unmodified jute composites. Further, with the addition of (2 wt%) MAPP, the PP–J30–V10 resulted in a higher average viscose fibre length of 8.1 mm, and the fracture toughness and fracture energy increased from 9.1 to 10.0 MPa m^1/2 and 28.9 to 31.2 kJ/m², respectively. Similarly, the fatigue life increased 51% compared with the PP–J30–V10, thus demonstrating the increased work energy due to hindrance of the propagation of cracks.     

Eutectic Al–Si–Cu–Fe–Mn alloys with enhanced mechanical properties at room and elevated temperature

In this paper, we report a novel kind of eutectic Al–Si–Cu–Fe–Mn alloy with ultimate tensile strength up to 336 MPa and 144.3 MPa at room temperature and 300 °C, respectively. This kind of alloy was prepared by metal mold casting followed by T6 treatment. The microstructure is composed of eutectic and primary Si, α-Fe, Al₂Cu and α-Al phases. Iron-rich phases, which were identified as BCC type of α-Fe (Al₁₅(Fe,Mn)₃Si₂), exist in blocky and dendrite forms. Tiny blocky Al₂Cu crystals disperse in α-Fe dendrites or at the grain boundaries of α-Al. During T6 treatment, Cu atoms aggregate from the super-saturation solid solution to form GP zones, θ″ or θ′. Further analysis found that the enhanced mechanical properties of the experimental alloy are mainly attributed to the formation of α-Fe and copper-rich phases.     

Enhancing mechanical properties of Mg–Sn alloys by combining addition of Ca and Zn

We developed new wrought Mg–2Sn–1Ca wt.% (TX21) and Mg–2Sn–1Ca–2Zn wt.% (TXZ212) alloys with high strength and ductility simultaneously, produced by conventional casting, homogenization and indirect extrusion. A partial dynamically recrystallized microstructure, with the micron-/nano-MgSnCa particles and G.P. zones dispersing, was obtained in TX21 alloy extruded at 260 °C (TX21-260). The TX21-260 alloy exhibited yield strength (YS) of 269 MPa, ultimate tensile strength (UTS) of 305 MPa, while those of the TX21 alloy extruded at 300 °C decreased to be 207 MPa and 230 MPa respectively. For TXZ212-260 alloy, on the other hand, MgSnCa, MgZnCa and MgZn₂ phases were observed, and the average grain size increased to be ∼5 μm. The YS and UTS of TXZ212-260 alloy evolved to be 218 MPa and 285 MPa, and the elongation (EL) reached as high as 23%. The high strengths of TX21-260 alloy were expected due to the high number density of nano-MgSnCa phases, G.P. zones and ultra-fine grain size (∼0.8 μm). The high EL of 23% in TXZ212-260 alloy was consistent with the high work-hardening rate, which was attributed to the larger grain size, more high angular grain boundaries, presence of more nano-particles and the weaker texture.     

Short cracks initiated in Al 6013-T6 with the focused ion beam (FIB)-technology

The fatigue crack growth behaviour of short corner cracks in the Aluminium alloy Al 6013-T6 was investigated. The aim was to determine the crack growth rates of small corner cracks at a stress ratio of R = 0.1, R = 0.7 and R = 0.8 and to find a possible way to predict these crack growth rates from fatigue crack growth curves determined for long cracks. Corner cracks were introduced into short crack specimens, similar to M(T) – specimens, at one side of a hole (Ø = 4.8 mm) by cyclic compression (R = 20). The precracks were smaller than 100 μm (notch + precrack). A completely new method was used to cut very small notches (10–50 μm) into the specimens with a focussed ion beam. The results of the fatigue crack growth tests with short corner cracks were compared with the long fatigue crack growth test data. The short cracks grew at ΔK-values below the threshold for long cracks at the same stress ratio. They also grew faster than long cracks at the same ΔK-values and the same stress ratios. A model was created on the basis of constant K_max-tests with long cracks that gives a good and conservative estimation of the short crack growth rates.     

Equivalent crack size model for pre-corrosion fatigue life prediction of aluminum alloy 7075-T6

Corrosion damage can significantly reduce the service life of aluminum alloy structures and endanger the structural integrity of aircraft. Here, aiming at center-hole sheet specimens of aluminum alloy 7075-T6, uniaxial fatigue tests and post-fracture analysis are performed to investigate the effect of corrosion pits on the pre-corrosion fatigue behavior. Then the best correlated parameters between corrosion pits and equivalent cracks are identified through Pearson correlation analysis. It is found that for single-crack initiations ar_ECS (equivalent crack depth 1 aspect ratio) vs. ar_cri (critical pit depth 1 aspect ratio) are best correlated with correlation coefficient of 0.9, while the best correlated parameters for multi-crack initiations are ar_ECS (equivalent crack depth 1 aspect ratio) vs. r_cri (aspect ratio) with correlation coefficient of 0.69. Equivalent crack size (ECS) models are correspondingly developed with these best correlated parameters for single- and multi-crack initiations, respectively. The pre-corrosion fatigue lives predicted with our models agree well with the experimental results and the maximum error factor is about 1.6.     

A newly developed calcia/titanium modified magnesia-based investment mold for titanium casting

Calcia (CaO) investment has the high stability with melted pure titanium (Ti) but the poor manipulation usually leads to low success rate of complete castings. Magnesia (MgO)-based investment can produce the high success rate of pure Ti casting though. However, the MgO-based investment should be accompanied with Ti powder to meet the accuracy of Ti castings. In this study, a pure titanium casting using Ti powder modified MgO-based investment and 0.1–0.5 wt.% CaO additive was investigated. Experimental results showed that pure Ti cast with a modified MgO-based dental investment with Ti/CaO resulted in a higher success rate of the complete casting and a decrease in the interfacial surface reaction at high casting temperatures as compared to unmodified MgO-based investments with Ti powder. This newly developed Ti modified MgO-based investment and 0.1–0.5 wt.% CaO additives had an acceptable marginal accuracy with a marginal gap of less than 50 μm; the 0.4 wt.% CaO additive gave the best accuracy. Based on a micro-chemical analysis, the addition of 0.1–0.5 wt.% CaO to the Ti modified MgO-based investment significantly reduced the reaction layers and casting defects in pure Ti casting, which had proven to enhance Ti–porcelain bonding; the investment modified with 0.5 wt.% CaO had the thinnest reaction layer.