Effect of fabrication process on the microstructure and dynamic compressive properties of SiCp/Al composites fabricated by spark plasma sintering

The effect of fabrication process on the microstructure and dynamic properties of SiC_p/Al composites was studied in this paper. Pure Al matrix composites reinforced with 20 vol.% SiC particles were fabricated by spark plasma sintering, and the pre-blended powders were prepared by two different processes. One was to mix the powders in conical flask by using a mechanical stirrer, and the other was the mechanical alloying process by using a planetary ball mill. The sintering temperature was also explored. The conventional split Hopkinson pressure bar was used to test the dynamic properties of these composites. The results show that the sintering temperature significantly affects the consolidation of the composites. The composites, which have not been fully densified, have very loose microstructure and poor mechanical properties. Mechanical alloying process can improve the microstructure and mechanical properties of the composites. These composites are rate dependent, their strengths increase with increasing strain rates.     

Preparation and properties of continuous glass fiber reinforced anionic polyamide-6 thermoplastic composites

Continuous glass fiber (GF) reinforced anionic polyamide-6 (APA6) composites were prepared via in situ ring-opening polymerization of caprolactam monomers. The effects of catalyst content, polymerization temperature and time on the viscosity average molar mass (M_v) and degree of crystallinity (X_c) were investigated in detail. The final mechanical properties of GF/APA6 composites were also studied. The results indicated that both high molecular weight and the high degree of crystallinity of resin matrix lead to the high mechanical properties of composites. Furthermore, the mechanical test results showed that the composites of plain woven fabric had tensile strength of 434 MPa and flexural strength of 407 MPa. The morphologies of tensile fracture surfaces of the composites specimens were observed through Scanning Electron Microscope (SEM). The SEM analysis showed that many disorganized nano-fiber crystals appear in the tensile fracture surfaces, which improve the mechanical properties of the matrix resin. The mechanical properties of the composites with different post-heat treatments were further investigated. The mechanical properties of the composites are significantly reduced after quenching treatment, but hardly improved after annealing.     

Cooling and crystallisation behaviour during vacuum-consolidation of commingled thermoplastic composites

This paper describes the experimental and modelling results of a study on the vacuum consolidation of commingled thermoplastic composites. Samples of Twintex^® glass/polypropylene (GFPP) fabric were melted, consolidated and cooled to room temperature by different cooling routes. The temperature field and the occurrence of a plateau in the through-thickness cooling profiles were modelled by coupling a non-isothermal crystallisation kinetics model with the energy equation, solved numerically using temperature-independent thermal properties for the resin. The model correctly predicted the onset of crystallisation and the degree of undercooling over a range of cooling rates. Predictions for the crystallisation rate, half-time, and temperature range agreed well with the literature data available.     

Thermal aging effects on mechanical and tribological performance of PEEK and short fiber reinforced PEEK composites

The effects of thermal aging on the properties of unfilled and random oriented short fiber reinforced PEEK and its composites have been studied. After the isothermal aging process, there is a remarkable decrease in degree of crystallinity but more organized crystallize structure achieved. As a result of transcrystalline layer formation, there was a considerable increase in the flexural modulus of materials. Thermal aging affects the impact properties of filled and unfilled PEEK dramatically. F_max, E_max and E · F_max results of both filled and unfilled aged PEEK and its composites are dramatically decreased. Thermal aging makes materials more brittle and there was a significant decrease in toughness. % Crystallinity is not the unique parameters to determine polymer’s performance. The orientation of crystals is another important parameter in microstructure and plays important role in mechanical and tribological properties of PEEK and its composites. There is a close relationship between thermal aging and microstructure. But there is not a linear relationship between microstructure and tribological properties. Microstructural changes after thermal aging serves developed mechanical properties. Increased mechanical properties results in improved tribological properties.     

Investigation of erosive wear behavior and physical properties of SGF and/or calcite reinforced ABS/PA6 composites

The aim of this study was to investigate the erosive wear behavior of glass fiber, CaCO₃ particle and glass fiber/CaCO₃ hybrid reinforced ABS/PA6 blend based composites. The samples were prepared by using melt mixing and injection molding techniques. The mechanical, thermal, morphological properties and erosive wear behavior were investigated in terms of reinforcing agent type and composition. It was observed that the tensile strength and modulus values of hybrid composites gave a value between tensile strength and modulus values of only fiber reinforced composites and only particle reinforced composites. From DSC analysis it was revealed that T_g and T_m of composites were not significantly affected by reinforcement; however, degree of crystallinity was found to be sensitive to reinforcement type and composition. The impingement angle was found to have a significant effect on the erosive wear behavior. The results indicated that composite materials exhibited maximum erosion rate at impact angle of 30° conforming their ductile erosion behavior. In order to investigate wear mechanisms, eroded surface analysis was done by scanning electron microscopy. Surface analysis showed that repeated impact of hard silica sand particles caused a local removal of the matrix from the fiber surface and led to form craters on the surface of the composite material.     

Mode I fracture resistance of glass fiber mat-reinforced polypropylene composites at various degree of consolidation

The energy release rate at steady-state fracture of glass fiber mat-reinforced polypropylene (GMT-PP) composites of different degree of consolidation was determined by various methods considering the mechanical energy (G_ss), acoustic emission energy (G_AE) and heat (G_h) release rates. All these energy release rates showed the same tendency, namely increase with increasing surface weight (consolidation degree) of the GMT-PP sheets. To determine the initiation fracture toughness (K_IC) the resistance curve concept was adopted. K_IC increased whereas the related strain energy release rate (G_IC) did not change with the consolidation degree.     

Tensile and thermomechanical properties of short carbon fiber reinforced polyamide 6 composites

In this study, carbon fiber (CF) reinforced polyamide 6 (PA6) composites were prepared by using melt mixing method. Effects of fiber length and content, on the mechanical, thermal and morphological properties of CF reinforced PA6 composites were investigated. Fiber length distributions of composites were also determined by using an image analyzing program. It was seen that the maximum number of fibers were observed in the range of 0–50 μm. Mechanical test results showed that, increasing CF content increased the tensile strength, modulus and hardness values but decreased strain at break values of composites. DSC results showed that T_g and T_m values of composites were not changed significantly with increasing CF content and length. However, heat of fusion and the relative degree of crystallinity values of composites decreased with ascending CF content. DMA results revealed that storage modulus and loss modulus values of composites increased with increasing CF content.     

Influence of cooling rate on interlaminar fracture properties of unidirectional commingled CF/PEEK composites

The influence of cooling rates on the mechanical property profile (transverse flexure properties and modes-I and -II interlaminar fracture toughness) has been investigated for unidirectional commingled CF/PEEK composites. A laboratory hot press with a steel mould was used to process the composites at 400°C for 60 min, at an applied pressure of 1 MPa. Cooling rates from fast (quenching in oil) to slow (hot press cooling) were achieved at ambient pressure. The results indicate that different matrix morphology was found at different cooling conditions, although deconsolidation occurred in the CF/PEEK composites during cooling. When the cooling rate was shifted from slow to fast, consolidation quality of the CF/PEEK composites was improved. The resulting effect of the consolidation quality and cooling rates on the mechanical property profile of commingled CF/PEEK composites is presented. It was found that the effect of the cooling rate on the mechanical property profile of the commingled CF/PEEK composites could not be isolated from the consolidation quality.     

Cooling rate influences in carbon fibre/PEEK composites. Part 1. Crystallinity and interface adhesion

The effect of cooling rate on the fibre–matrix interface adhesion for a carbon fibre/semicrystalline polyetheretherketone (PEEK) composite was characterised based on the fibre fragmentation, fibre pullout and short beam shear tests. The interface adhesion was correlated to the degree of crystallinity and the crystalline morphology, as well as the bulk mechanical properties of neat PEEK resin, all of which were in turn controlled by cooling rate. It was shown that the interface bond strength decreased with increasing cooling rate; the tensile strength and elastic modulus of PEEK resin decreased, while the ductility increased with increasing cooling rate through its dominant effect on crystallinity and spherullite size. The improvement of crystalline perfection and flattened lamella chains with high crystallinity at the interphase region were mainly responsible for the strong interface bond in composites processed at a low cooling rate. The interphase failure was characterised by brittle debonding in slow-cooled composites, whereas the amorphous PEEK-rich interphase introduced in fast cooled specimens failed in a ductile manner with extensive plastic yielding.     

Influence of carbon nanotube-polymeric compatibilizer masterbatches on morphological, thermal, mechanical, and tribological properties of polyethylene

Study was made of the effect of multiwall carbon nanotubes (MWCNTs) and polymeric compatibilizer on thermal, mechanical, and tribological properties of high density polyethylene (HDPE). The composites were prepared by melt mixing in two steps. Carbon nanotubes (CNTs) were melt mixed with maleic anhydride grafted polyethylene (PEgMA) as polymeric compatibilizer to produce a PEgMA-CNT masterbatch containing 20 wt% of CNTs. The masterbatch was then added to HDPE to prepare HDPE nanocomposites with CNT content of 2 or 6 wt%. The unmodified and modified (hydroxyl or amine groups) CNTs had similar effects on the properties of HDPE-PEgMA indicating that only non-covalent interactions were achieved between CNTs and matrix. According to SEM studies, single nanotubes and CNT agglomerates (size up to 1 μm) were present in all nanocomposites regardless of content or modification of CNTs. Addition of CNTs to HDPE-PEgMA increased decomposition temperature, but only slight changes were observed in crystallization temperature, crystallinity, melting temperature, and coefficient of linear thermal expansion (CLTE). Young’s modulus and tensile strength of matrix clearly increased, while elongation at break decreased. Measured values of Young’s moduli of HDPE-PEgMA-CNT composites were between the values of Young’s moduli for longitudinal (E₁₁) and transverse (E₂₂) direction predicted by Mori-Tanaka and Halpin-Tsai composite theories. Addition of CNTs to HDPE-PEgMA did not change the tribological properties of the matrix. Because of its higher crystallinity, PEgMA possessed significantly different properties from HDPE matrix: better mechanical properties, lower friction and wear, and lower CLTE in normal direction. Interestingly, the mechanical and tribological properties and CLTEs of HDPE-PEgMA-CNT composites lie between those of PEgMA and HDPE.     

Effect of surface modification of nanosilica on crystallization,thermal and mechanical properties of poly(vinylidene fluoride)

Composites were prepared by solution blending ploy(vinylidene fluoride) (PVDF) and nanosilica which modified by different organic modifiers. Infrared analysis showed that the crystalline structure of PVDF was changed by the addition of RNS-A (silica with amino terminated group), while similar crystalline structure as pure PVDF was observed for composites with DNS-0 and DNS-2, unmodified silica and alkyl terminated group silica, respectively. With differential scanning calorimeter (DSC) and dynamic mechanic thermal analysis (DMTA) techniques, crystalline structure, thermal, and mechanical properties of the composite films were examined. As the DSC results showed, addition of SiO₂ would lead to the increased cooling crystallization temperature (T _c), implying that SiO₂ nanoparticles could act as nucleating agents, however the degree of crystallinity of PVDF was not elevated significantly. In the complementary modulated DSC curves, multi-melting peaks associated with non-reversing portion were observed and were explained from the viewpoint of melting-recrystallization in the DSC heating scan. In addition, dynamic mechanical properties as well as the thermal stability of the composites are also influenced by SiO₂. As manifested by the corresponding DMTA and thermogravimetric analysis (TGA) results, a strong interaction should exist between PVDF and SiO₂ nanoparticles.     

Effect of uncoated calcium carbonate and stearic acid coated calcium carbonate on mechanical,thermal and structural properties of poly(butylene terephthalate) (PBT)/calcium carbonate composites

PBT/CaCO₃ composites were prepared in a single screw extruder with particle content varying from 0–30% by weight. The influence of surface treatment of the particles, with and without stearic acid (SA), on the mechanical, thermal and structural properties was studied. The experiments included tensile tests, impact tests, differential scanning calorimetry (DSC), scanning electron microscopy (SEM), X-ray diffraction (XRD) and Fourier transform infrared spectroscopy. The composite systems containing SA coated CaCO₃ were found to exhibit better mechanical properties as compared to composite systems containing uncoated CaCO₃, with the S3 system (20% of SA coated CaCO₃) exhibiting best combination of mechanical properties. Thermal study revealed that particle type and content had no influence on the melting temperature but the crystallization temperature, % crystallinity and thermal stability increased on increasing the CaCO₃ content in PBT matrix. Morphological observation indicated that in PBT composites containing SA coated CaCO₃, the coupling agent favours a better polymer filler interaction rendering inorganic polymer interface compatible, which is also evident from better mechanical and thermal properties.     

Effect of solidification conditions on microstructure,mechanical and wear properties of Mg–5Al–3Ca–0.12Sr magnesium alloy

In the present study, the microstructure, mechanical and wear properties of AXJ530 alloy under different solidification condition were investigated. AXJ530 alloys were cast in a multi-step permanent mould casting (PMC) with five different cooling rates, and also in high pressure die casting (HPDC). The effect of cooling rate was determined for the room temperature mechanical properties and the dry sliding wear resistance of the AXJ530 alloys. The results showed that grain size of AXJ530 alloy was refined and thinner lamellar eutectic phase formed at higher cooling rate. It was concluded that these changes led to the observed concurrent increases in ultimate tensile strength (σ_uts), yield strength (σ_0.2) and elongation (δ) of the AXJ530 alloy. The relationship between grain size and yield strength/hardness agreed with Hall–Patch behavior. The dry sliding wear rate of the PMC specimens decreased with increasing of cooling rate, but micro-porosity/inclusion in the HPDC specimen decreased its wear resistance properties. Abrasion was determined to be the dominant wear mechanism for the AXJ530 alloys.     

Consolidation of GF/PP commingled yarn composites

Consolidation quality and corresponding mechanical properties of GF/PP thermoplastic composites manufactured from a commingled yarn system have been investigated. A small compression mould with a laboratory hot press was used to apply the different processing variables (i.e. pressure, holding time and processing temperature). The consolidation quality of finished samples was characterized mainly through (a) microscopic studies of the material's microstructure, (b) density measurements, and (c) evaluations of mechanical properties using a small transverse flexure testing facility. A model for qualitatively describing the impregnation and consolidation processes in commingled yarn based thermoplastic composites was applied to predict variations of void content during consolidation and the time, temperature and pressure required to reach full consolidation. Based on a desired, minimum level of void content (X _v =2.0%), optimum processing windows for manufacturing of GF/PP commingled yarn composites are suggested.     

Influence of flow restriction on the microstructure and mechanical properties of long glass fiber-reinforced polyamide 6.6 composites for automotive applications

In this study, polyamide 6.6 with 40 wt% of long glass fibers was processed by injection molding into a mold with the ability to simulate controlled flow restriction. The mechanical properties of the molded test specimens were evaluated to verify the application of the composites as substitutes for metallic materials in automotive applications and the influence of flow restriction on these properties. Mathematical models were used to calculate the tensile strength of the composites in order to validate the experimental results. It was found that the presence of the controlled flow restriction affects fiber length as well fiber orientation, both of which influence the final mechanical properties of the composite. Moreover, a high degree of anisotropy in the mechanical properties was observed.     

Non-isothermal forming of glass fiber/polypropylene commingled yarn fabric composites

The commingled glass fiber/polypropylene (GF/PP) composites were fabricated using a double-belt press and the influence of temperature and velocity on the consolidation quality and mechanical performance of the composites were investigated. The contents and distribution of the voids in the composites were changed with varying the level of the processing parameters, and the composite tensile and flexural properties were dramatically decreased when the void contents were beyond 3.6%. Reducing the viscosity of the matrix or increasing the uniformity of fibers in the commingled yarns was important to improve the consolidation quality. But the production efficiency was not improved as hoped by increasing the velocity at higher temperatures because of the weakened interfacial bonding. The crystal form and crystallinity degree had no obvious changes under different processing conditions, but the composite mechanical performance was enhanced when the degree of order in the crystal structures was decreased, which might be caused by more effective stress transferring or less initial cracks.     

聚芳醚酮树脂非等温结晶动力学及其复合材料湿热老化性能EI北大核心CSCD

聚芳醚酮(poly(aryl ether ketone),PAEK)热塑性复合材料因其突出的韧性、耐老化性能、耐疲劳性能,以及成型加工低成本、高效率、可回收等一系列优势,得以替代部分传统热固性复合材料,并已在国外航空等高端工业领域取得成功应用。PAEK是一种半结晶高聚物,复合材料不同的成型工艺条件导致树脂基体聚集态结构的差异,对其力学性能、耐老化性能等均存在显著的影响。对国产PAEK树脂的结晶动力学进行了研究,通过控制成型降温条件,采用模压工艺制备了树脂结晶度不同的连续碳纤维增强复合材料,并对其进行了湿热老化处理,评价了老化处理前后复合材料的弯曲性能。结果表明:采用较大的降温速率能够提高树脂的结晶速率,但会降低树脂的结晶度。在随炉冷却、空气冷却和水冷却三种不同的降温条件下,制备的复合材料结晶度分别约为36.3%,29.7%,26.5%,结晶度的显著差别导致复合材料的弯曲强度随结晶度的降低而减小。经湿热老化处理240 h后的弯曲强度保留率分别为88.8%,86.7%,86.6%,较高的结晶度能够较好地抵抗水对复合材料界面的破坏,提升复合材料的耐湿热老化性能。     

A new method for estimating the isothermal devitrification and crystallization of mold powder slags from non-isothermal DSC data

The rate of formation of crystalline phases from liquid and glassy mold powder slags is of foremost importance in the performance of molds used for continuous casting of steel. This study shows how the Induction Period (of Šimon and Kolman) and the Kissinger methods can be combined in a kinetic model to evaluate the isothermal rate of formation of crystalline phases from thermo-analytical data – onset temperature, T_i, peak maximum temperature, T_m, shape index, S, and conversion at peak maximum, x_m – collected at various linear heating and cooling rates. The diagram of the extent of isothermal transformation as a function of time calculated for a commercial mold powder, used for casting low carbon steels, shows good agreement with the degree of transformation observed in photomicrographs of glass disks devitrified isothermally, at several temperatures for different times. Additionally, qualitative and quantitative X-ray diffraction results obtained at room-temperature from glass powder samples treated isothermally and quenched also show good accord with the degree of transformation predicted with the kinetic model developed in this work.     

Microstructure and thermal shock resistance of Al2O3 fiber/ZrO2 and SiC fiber/ZrO2 composites fabricated by hot pressing

Al₂O₃ chopped fiber/ZrO₂ and SiC continuous fiber/ZrO₂ composites were fabricated by hot pressing at 1550°C and 15 MPa in vacuum. The mechanical properties of thermally shocked composites were measured at room temperature by four-point bending. The addition of Al₂O₃ fibers into ZrO₂ matrix degraded the fracture strength, but improved significantly the thermal shock resistance. In addition, the mechanical properties of SiC fiber/ZrO₂ composites were much lower than those of monolithic ZrO₂ because of the presence of microcracks on the surface. The SiC fiber/ZrO₂ composites showed an excellent thermal shock resistance.     

Microstructure and properties of SiCw/6061 aluminium alloy composites after compression at temperatures around solidus of matrix

Abstract

The effect of compressive deformation at temperatures around the solidus of the matrix on the microstructure and properties of SiC_w/6061 aluminium alloy composites was investigated. It was found that the temperature, strain rate, and amount of deformation affect whisker distribution and breakage, densification and uniformity of composites, and SiC_w/matrix alloy interfacial bonding. The microstructural evolution due to compression affects the properties of the composites, which is considered to be the most important aspect for evaluating high temperature plastic forming of the composites. The optimum parameters for compressive deformation were determined by analysing the microstructure and the properties of the composites.