Dry forming of aluminum alloys – Wear mechanisms and influencing factors

Tribological contacts in sheet metal forming are accompanied by several wear phenomena. One of which is the transfer of material from the softer sheet material to the harder tool surface, namely adhesive wear. Forming of aluminum alloys makes high demands on forming processes. Aluminum alloys show a strong tendency of adhesion on common tool materials. Adhesions on tools reduce the surface quality, the dimensional accuracy of the parts and the process stability. In order to avoid adhesive wear during forming, nowadays a high amount of lubricant is applied to the aluminum sheets. Though economically and ecologically attractive, dry forming processes with aluminum sheets seem not to be possible. In order to develop advantageous tribological systems a comprehensive understanding of the acting mechanisms is necessary. This paper discusses the influence of the alloy composition and the influence of oxide layers on the adhesive wear in aluminum forming.     

High-strength AlN ceramics by low-temperature sintering with CaZrO3–Y2O3 co-additives

Aluminum nitride (AlN) ceramics with the concurrent addition of CaZrO₃ and Y₂O₃ were sintered at 1450-1700 °C. The degree of densification, microstructure, flexural strength, and thermal conductivity of the resulting ceramics were evaluated with respect to their composition and sintering temperature. Specimens prepared using both additives could be sintered to almost full density at relatively low temperature (3 h at 1550 °C under nitrogen at ambient pressure); grain growth was suppressed by grain-boundary pinning, and high flexural strength over 630 MPa could be obtained. With two-step sintering process, the morphology of second phase was changed from interconnected structure to isolated structure; this two-step process limited grain growth and increased thermal conductivity. The highest thermal conductivity (156 Wm⁻¹ K⁻¹) was achieved by two-step sintering, and the ceramic showed moderate flexural strength (560 MPa).     

对再生镁碳砖制备过程中碳化铝的研究

针对在镁碳砖或再生镁碳砖制备过程中被公认为有害物质的碳化铝进行了研究。在所研究的热处理温度范围内,确定了碳化铝的形成机理。在实验中研究了热处理温度和石墨加入量对碳化铝(Al4C3)形成的影响。     

Preparation and reaction kinetics of polypropylene‐graft‐cardanol by reactive extrusion and its compatibilization on polypropylene/polystyrene

Polypropylene‐graft‐cardanol (CAPP) was prepared by reactive extrusion with polypropylene (PP) and natural renewable cardanol, which improved the inherent defects of PP such as its chemical inertness and hydrophobicity. Moreover, the cardanol grafted onto PP resolved the degradation of PP during reactive extrusion and use. The effects of reactive extrusion on the change of the molecular structure of PP, the change in the free‐radical concentration during processing, and the compatibilization of CAPP on the PP/polystyrene (PS) composite materials were examined in this study. The constants of the grafting reaction rate at the beginning of reactive extrusion were also deduced. The results show that cardanol was grafted onto PP, and the p–π conjugate system in cardanol was observed to stabilize free radicals. The grafting reaction rate (R_g) at the initial stage of the grafting reaction process was calculated through the equation R_g = k_g[M·][Cardanol], where k_g is the constant of the apparent grafting reaction rate and [M·] is the concentration of free radicals in the reaction system. k_g first increased with the growth of temperature and then began to decrease when the temperature exceeded the critical temperature of 200°C. The mechanical properties showed almost no change after the samples were aged for 72 h. This was due to CAPP, which changed PP/PS to a ductile material from a brittle one. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 39911.     

Structure and properties of polypropylene/high‐density polyethylene blends by solid equal channel angular extrusion

The solid equal channel angular extrusion (ECAE) process on polypropylene (PP)/high‐density polyethylene (HDPE) blends was carried out. Scanning electron microscopy (SEM) was used to observe the sample structures. Results showed that ECAE process could make PP/HDPE blends to produce orientation structure. Impact performance of ECAE‐PP/HDPE samples after ECAE process improved remarkably, especially for ECAE‐PP/HDPE (90/10)‐O whose impact strength reached 91.91 kJ/m², 18.1 times higher than that of pure PP and 11.2 times higher than that of PP/HDPE (90/10). The mechanism of enhancing between HDPE and PP was discussed. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 39759.     

Perfluorosulfonated ionomers membranes: Melt‐processing and characterization for ion exchange applications

Perflurosulfonated ionomers membranes with different ionic‐exchange capacity were successfully fabricated via melt‐extruding and casting of their –SO₂F precursors. A systematical investigation of the thermal stability, crystallinity, and rheological properties of the precursors was performed to secure their optimized processing conditions. The tensile properties of acid‐form membranes are found to increase with base‐hydrolysis time, where a tensile strength of 38.2 MPa is readily obtained after 24 h's base‐hydrolysis. The content of –SO₂F or –SO₃H containing side‐chains plays an important role in the thermal stability, rheological, and mechanical properties of the precursor or the acid‐form membranes. The strong ionic interactions, attributed to the –SO₃H groups, lead to decreased crystallinity and tensile strength for different IEC membranes. The acid‐form membranes exhibit good proton conductivity and low methanol crossover in comparison with reference Nafion membrane. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 39944.     

Effects of a titanate coupling agent on the mechanical and thermo‐physical properties of talc‐reinforced polyethylene compounds

An experimental study was carried out to investigate the effects of a titanate coupling agent on the mechanical properties, moisture absorption, and thermal conductivity of talc‐filled high‐density polyethylene (HDPE). Talc (0–35 wt %) was used as reinforcement particulate filler in an HDPE matrix and samples were prepared in a micro‐compounder and an injection molding machine. Isopropyl tri(dioctyl)phosphate titanate (0.5 wt %) was used as coupling agent. Composites with and without coupling agent were evaluated for changes in mechanical and thermo‐physical properties, morphology, and void content. Addition of the titanate coupling agent most often resulted in an increase in stiffness and tensile strength. Furthermore, both the void content and the elongation at break of composites were reduced. Results also showed that the coupling agent had no effects on the thermal conductivity, thermal diffusivity, and specific heat capacity of the composites. In addition, it was observed that the coupling agent was more effective at low concentrations of filler. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40449.     

Effects of rare earth samarium oxide on the properties of polypropylene‐graft‐cardanol grafted by reactive extrusion

Rare earth elements can improve the performance of polymers because of their special 4f orbitals. The nucleation and stabilization of radical groups of rare earth particles can affect the structure of polypropylene (PP) and its properties. In this study, samarium oxide (Sm₂O₃) particles were used as a cocatalyst and nucleating agent in polypropylene‐graft‐cardanol (CAPP) grafted by reactive extrusion. The properties of polypropylene‐graft‐cardanol containing modified Sm₂O₃ with a titanate coupling agent (CAPPMS) were investigated by ultraviolet–visible spectrometry, polarizing microscopy, differential scanning calorimetry, scanning electron microscopy, universal testing, and capillary rheometry with the reference of CAPP containing unmodified Sm₂O₃ particles. The results show that the titanate coupling agent (TCA‐401) coated on the surface of the Sm₂O₃ particles improved the dispersion of the Sm₂O₃ particles and the adhesion between the Sm₂O₃ particles and CAPP matrix. The Sm₂O₃ particles promoted more cardanol to graft onto PP. Acting as nucleator for CAPP, the Sm₂O₃ particles increased the crystallization rate, increased the melting temperature, and decreased the spherulite size of CAPP. The modified Sm₂O₃ particles showed a greater effect on the mechanical and rheological properties than the unmodified Sm₂O₃ particles did. The tensile strength, impact strength and flexural strength of CAPPMS increased by 10 MPa, 0.64 kJ/m², and 6.5 MPa, respectively, compared to those of CAPP when we used 4.5 mol % modified Sm₂O₃ particles. The viscosity of CAPPMS increased to a certain extent in the presence of the modified Sm₂O₃. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 41012.     

New breathable films using a thermoplastic cross‐linked starch as a porogen

Breathable films, which find in variety of product applications, are conventionally made using mineral porogens such as calcium carbonate (CaCO₃). This article addresses a novel biodegradable and highly breathable film without inorganic porogens. Unexpectedly, a thermoplastic cross‐linked natural polymer (corn starch) was used successfully to create tortuous passages for film breathability. This concept was demonstrated using two types of thermoplastic cross‐linked corn starches as porogens and contrasted to control samples: native corn and chemically cross‐linked starches, respectively. The films discussed had increased breathability and mechanical properties relative to the control samples. The film morphology reveals that filler was irregular when thermoplastic starch or CaCO₃ was used. The difference in filler from chemically modified cross‐linked starch and thermoplastic cross‐linked starch was observable as well. It is believed that spherical particles provided by thermoplastic cross‐linked starch helps film debonding and porosity during the film stretch processes. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 41016.     

In situ biaxial stretching for the platelet formation of an ethylene–vinyl alcohol copolymer through multistage stretching extrusion and its effect on the gas‐barrier properties of polyethylene

The morphological evolution of ethylene–vinyl alcohol copolymer (EVOH) and its effect on the gas‐barrier properties of high‐density polyethylene (HDPE) were investigated. HDPE/EVOH blends were prepared through a multistage stretching extrusion, which combined an assembly of force‐assembling elements (FAEs) with an extruder. Scanning electron microscopy confirmed that with an increasing number of FAEs, the biaxial‐stretching field existing in each FAE transformed the dispersed EVOH phase into well‐defined platelets along the flowing plane. Dynamic rheological results further revealed that the formation of the platelets enlarged the interfaces between the dispersed barrier phase and the matrix; this not only led to the decline of the complex viscosity but also created more tortuous paths for the diffusion of gas molecules. Compared with that of the non‐FAE specimen blended with 25 wt % EVOH, the oxygen permeability coefficient decreased more than one order of magnitude when one FAE was applied. The structural model for permeability indicated that the enhanced barrier resulted from the increased tortuosity of the diffusion pathway, which was provided by the aligned high‐aspect‐ratio platelets. Compared with the previous biaxial‐stretching method, multistage stretching extrusion provided a simple and economical way to generate a laminar structure of the dispersed phase in the matrix phase without the application of an external stretching force. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40221.