Failure analyses of cemented tungsten carbide extrusion die

Abstract

In this study, the failure of a cemented tungsten carbide extrusion die (WC–Co) which was designed for the production of 55 000 components but failed during production of third component has been investigated. The die is used for cold extrusion of auto parts components as sizing die. Sizes achieved by forging process are further refined to required fine tolerance. The component size is required to be controlled within ±20 μm. The investigations revealed that the failure is due to abusive machining of cemented tungsten carbide. Detail studies under different electrical discharge machining conditions have been made to investigate the formation of cracks and their morphology. The major cause of the failure is established as abusive electrical discharge machining conditions with higher pulse current, which lead to micro- to macrocracks at the machining stage. These cracks further enlarged under load conditions when extrusion was done. Based on this study experiments were performed under different machining conditions on the WC–Co material. Optimum parameters were established for machining and accordingly new die was designed and fabricated which is performing very well.     

Plasma channel formation for triggering arc welding

Abstract

A method of starting arc welding using a plasma channel formed between electrodes in a tungsten inert gas arc welding system was demonstrated. The plasma channel was generated by gas breakdown in the laser beam path. In a previous study by the present authors, the arc welding could be started using a laser produced plume. Results in the present study indicated that the laser energy required to start the process using the plasma channel was lower than that using the plume.     

Effect of electrical discharge machining process on crack susceptibility of nickel based heat resistant alloy

Abstract

The objective of this study was to investigate the effects of electrical discharge machining (EDM) process conditions on the crack susceptibility of a nickel-based superalloy. Because sequential processes are inevitable in the manufacturing industry, the effects of EDM and post-EDM processes on the crack propagation behaviour of the components need to be clearly understood. The altered material zone (AMZ) of EDM processed Hastelloy X revealed different morphology before and after solution heat treatment. Depending on the EDM dielectric fluid and the post-EDM process such as solution heat treatment, it was found that cracks existing in the recast layer could propagate into substrate when a 20% strain tensile force was applied at room temperature. In the case of the kerosene electrical discharge (ED) machined specimens, it was observed that carburisation and sharp crack propagation along the grain boundary occurred after the heat treatment. However, the deionised water ED machined specimens after heat treatment underwent oxidation and showed no crack propagation behaviour. A metallurgical evaluation of the microstructure of the AMZ was also conducted.     

Calendar of events

The influence of the gas type, air or nitrogen, and the treatment time in an RF glow discharge treatment on the shrink resistance properties of knitted wool fabric and wetting properties of keratin fibres were studied. Wetting properties were determined by means of contact angle measurements on single keratin fibres. This method allows measuring accurately the influence of the plasma gas type and treatment time on fibre hydrophilicity, and its modification with the time elapsed after plasma treatment. The modification of the surface properties should be taken into account, especially, when a biopolymer after-treatment is applied to achieve wool shrink resistance. Surface chemical changes were studied by means of XPS. Topographical changes in the wool fibre surface were observed by scanning electron microscopy (SEM) and surface damage was evaluated by means of the Herbig sac formation. Both air and nitrogen plasma treatments impart shrink resistance to wool fabric and hydrophilic properties to the keratin fibres. Even short exposure times are found to be enough to decrease drastically the advancing water contact angle and, therefore, to increase the shrink resistance effect. Slight differences were observed between the air and nitrogen plasma treatments. The time elapsed after the plasma treatment promotes an increase of the advancing contact angle and a decrease of chitosan adsorption. The plasma treatments studied here modify chemically the epicuticle but it is not removed.     

Role of Hydrogen Peroxide in Selective OH Group Functionalization of Polypropylene Surfaces using Underwater Capillary Discharge

Plasma chemical methods are well suited for introducing functional groups to the surfaces of chemically inert polymers such as polyolefins. However, a broad variety of functional groups are often formed. Unfortunately, for further chemical processing such as grafting of molecules for advanced applications a highly dense monotype functionalized polyolefin surface is needed. Therefore, the main task was to develop a selective surface functionalization process, which formed preferably only a single type of functional groups at the surface in high concentration. Amongst the novel plasma methods, the underwater plasma process (UWP) is one of most attractive options to solve the problem of monotype functionalization. Such plasma is an efficient source of ions, electrons, UV-radiation, high-frequency shock waves, radicals such as hydroxyl radical, and reactive neutral molecules such as hydrogen peroxide. In contrast to established gas phase glow discharge processes, the water phase limits the particle and radiation energies and thus the energy input into the polymer. By virtue of the liquid water environment, which moderates highly energetic plasma species, extensive oxidation, degradation, cross-linking and radical formation on the polymer are more limited as compared to gas plasma exposure. The variety of plasma produced species in the water phase is also much smaller because of the limited reaction possibilities of the plasma with water. The possibility to admix a broad variety of chemical additives makes underwater plasma even more attractive. Hydrogen peroxide and the catalyst (Fe-ZSM5) should influence and increase the equilibrium concentration of OH radicals in the underwater plasma process. It was found that these radicals played a very important role in OH functionalization of polyolefin surfaces. Hydrogen peroxide was identified to be the most prominent precursor for OH group formation in the UWP. The catalyst would affect the steady state of OH radical formation and its reaction with the substrate surface and thus accelerates the functionalization process.     

Dynamics of the wetting process on dielectric barrier discharge (DBD)-treated wood surfaces

Protection and preservation of wood properties in exterior environments can only be ensured if the surface is coated with a paint or varnish. In our experiments a dielectric barrier discharge (DBD) was used as a wood surface pretreatment for improvement of the subsequent deposition of thin paint layers from solutions onto these surfaces. As the adsorption, interfacial interactions and adhesion of paints are strongly dependent on surface wettability, the dynamics of the wetting process were analyzed. The results show that the water contact angle decreases after the DBD treatment, proving a more wettable surface. Additionally, the spreading of paint solution on the DBD-treated surface is more isotropic, showing a lower tendency to elongate along the wood fiber orientation.     

Ultra-Thin Polymer Layer Deposition by Aerosol–Dielectric Barrier Discharge (DBD) and Electrospray Ionization (ESI) at Atmospheric Pressure

Polyolefins are chemically inert and do not adhere well to metals, polymers or inorganics. To overcome this problem, polyolefin surfaces were modified thermally, plasmachemically, or by flame treatment with different oxygen-containing groups, however, unfortunately, such treatments were accompanied by undesired, adhesion lowering polymer degradation. To solve this dilemma, solutions of synthetic polymers and copolymers were prepared, sprayed into the barrier discharge or electrosprayed without discharge and deposited as thin adhesion-promoting layers. The deposited polymer layers from poly(vinylamine), poly(ethylene glycol)–poly(vinyl alcohol) copolymers and poly(acrylic acid) were endowed with monotype functional groups. Using the aerosol — dielectric barrier discharge only a fraction of functional groups survived the deposition process in contrast to the electrospray in which all functional groups were retained.     

Preparation of Cotton Materials Using Corona Discharge

Abstract

The present work concerns the study of preparation processes when this operation is preceded by a CORONA discharge made on dry raw fabrics. The influence in the whiteness degree, hydrophility, starch removal and uniformity of properties of the cellulosic material after preparation is studied.

Combined preparation processes are widely used in order to replace solutions where desizing, alkaline boiling and bleaching are individual operations, being a main interest to find the optimal conditions to achieve alkaline oxidative treatments with an uniform and more intense cleaning action.

CORONA discharge is able to strongly modify hydrophility of cotton fabrics in raw stage, which is determinant to increase liquids absorption and uniformity of processed fabrics. Results of long-bath, pad-roll, pad-batch and pad-steam bleaching processes in laboratory conditions are compared using or not wetting agent in the recipes and including CORONA discharge as a preparation operation.

The transfer of this technology to industry regarding the implementation in textile plant in a very near future is being worked, supported by results as the present ones.     

Corona discharge treatment of EVAs with different vinyl acetate contents

Four ethylene vinyl acetate (EVA) co-polymers with different vinyl acetate (VA) contents (9–20 wt%) were treated with corona discharge to improve their adhesion to polychloroprene (PCP) adhesive. The thermal properties of the EVAs decreased as their VA content increased, caused by a decrease in crystallinity. The elastic and viscous moduli of the EVAs decreased and the temperature and modulus at the cross-over between these moduli decreased with increasing VA content. Contact-angle measurements (water), infrared spectroscopy (ATR-IR), X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM) were used to analyse the surface modifications produced in the corona-discharge-treated EVAs. The corona discharge treatment produced improved wettability and created roughness and oxygen moieties on the EVA surfaces. The higher the VA content and the higher the corona energy, the more significant modifications were produced on the EVA surface. The VA content also affected the T-peel strength values of treated EVA/polychloroprene + isocyanate adhesive joints, as the values increased with increasing VA content. Mixed failure modes (interfacial + cohesive failure in the EVA) were obtained in the adhesive joints produced with corona discharge treated EVAs containing more than 9 wt% VA. The accelerated ageing of the joints did not affect the T-peel strength values, but the locus of failure in most cases became fully cohesive in the EVA, likely due to the higher extent of curing of the adhesive.     

Glow discharge plasma brazing of Fe–Ni alloy in vacuum

Abstract

A novel vacuum brazing technique, termed glow discharge plasma brazing, is investigated. During the brazing process, the heating temperature of base metals is proportional to the square of the operating barometric pressure and the operating voltage, and the temperature distribution of the base metals is easily regulated by means of suitable measures. The ion beam from the glow discharge anode can efficiently sputterclean the surface of the base metals and the filler metal, which improves the wetting and spreading properties of the filler metal. Unlike the traditional vacuum brazing process, a high quality braze of Fe–Ni alloy is achieved at lower vacuum (a pressure of 5– 30 Pa or higher) by using the glow discharge brazing method. The brazing technique has a promising application in industry.