Advanced Materials Research Vols. 29-30

Abstract: A feedstock of Ti(Al,O)-Al2O3 composite powders was produced by high energy milling of a mixture of Al and TiO2 powders followed by a thermal reaction process. The feedstock was then thermally sprayed using a high velocity oxygen-fuel (HVOF) technique on H13 steel substrates to produce Ti(Al,O)-Al2O3 composite coatings. The performance of the coatings was assessed in terms of thermal fatigue behaviour and reaction with molten aluminium (soldering). The composite powders and coatings were characterised using scanning electron microscopy (SEM) and X-ray diffractometry (XRD). This paper reports the experimental observations and discusses characteristics and potential applications of the composite coatings.

Abstract: Gamma TiAl based alloys are important materials with potential applications in aerospace and automotive applications due to their high specific strength and creep resistance. The major barrier for their applications is their limited ductility at room temperature and limited hot workability. One way of overcoming this barrier is to reduce the grain sizes to ultrafine grained (<500μm) or nanostructured (<100nm) level. In our present study, we attempt to produce bulk ultrafine grained Ti- 47Al-2Cr (at%) alloy using a combination of high energy mechanical milling of elemental powders to produce a very fine structured Ti/Al/Cr composite powder and consolidation of the powder using hot isostatic pressing (HIPping). It was confirmed that high energy ball milling using a planetary ball mill led to the formation of extremely fine Ti and Al layered composite structure. The thermal behaviour of the powder was studied using differential thermal analysis, and it was shown that the reactions between the Ti and Al phases in the fine structured composite powder occur at fairly low temperatures, below the melting point of the Al phase (660oC). The macrostructure and phase structure of the HIPped samples were also examined using optical and scanning electron microscopy and X-ray diffractometry (XRD). This paper is to report and discuss the results of this investigation.

Abstract: Cu-(2.5 or 5.0vol.%)Al2O3 nanocomposite balls and granules and Cu-(2.5vol.% or 5.0vol.%)Pb alloy powder were prepared by high energy mechanical milling (HEMM) of mixtures of Cu and either Al2O3 or Pb powders. It was observed that with the increase of the content of Al2O3 nanoparticles from 2.5vol.% to 5vol.% in the powder mixture, the product of HEMM changed from hollow balls into granules and the average grain size and microhardness changed from approximately 130nm and 185HV to 100nm and 224HV, respectively. On the other hand, HEMM of Cu–(2.5 or 5.0vol.%) Pb powder mixtures under the same milling conditions failed to consolidate the powder in-situ. Instead, it led to formation of nanostructured fine powders with an average grain size of less than 50nm. Energy dispersive X-ray mapping showed homogenous distribution of Pb in the powder particles in Cu–5vol.%Pb alloy powder produced after 12 hours of milling. With the increase of the Pb content from 2.5 to 5.0 vol.%, the average microhardness of the Cu-Pb alloy powder particles increases from 270 to 285 HV. The mechanisms of the effects are briefly discussed.

Abstract: Titanium and titanium alloys are the materials of choice for many industrial applications because of their attractive combination of low density, good mechanical properties, and high corrosion resistance, and titanium is the fourth most abundant metal in the earth crust (0.86 % by weight) behind aluminum, iron, and magnesium. However, titanium and titanium alloys are not widely. The reason for this is the high cost of titanium and titanium alloys! The cost gap for titanium and titanium alloys widens when they are used for fabricating components and structures. Consequently, much effort has been expended to reduce the cost of titanium and titanium alloys. In conjunction with the University of Waikato, Titanox Development Limited-New Zealand has been successful in creating a modified novel process to produce TiAl based alloy powders with different particle sizes and compositions at low cost. The process offers several benefits to the titanium industry the most significant one of which is that it displays the potential to significantly reduce the commercial production costs of Ti-Al based alloys. This paper describes the Titanox Development Limited technology in brief, and shows how it can economically produce titanium alloy powders for different industrial applications and making titanium alloys affordable. The process has been disclosed in a PCT (Patent Corporation Treaty) application which was approved in 2004 [1], and the related patent applications either have been approved or are being filed in different countries.

Abstract: M3:2 high speed steels with and without carbon addition were prepared by using powder metallurgy at sintering temperature between 1210 and 1280 °C. Densification, microstructure and mechanical properties of M3:2 high speed steels were investigated. Experimental results show that with 0.4wt% carbon addition, full density high speed steels were obtained at temperatures in the range 1240-1260 °C which is 40 °C lower than that of the undoped counterparts, leading to a sintering window expanded by 10 to 20 °C. By the addition of 0.4wt% carbon, the sintered steels show attractive combinations of bend strength and hardness over those of M3:2 steels without carbon addition. The results reveal that the addition of carbon will not only lower the sintering temperature and oxygen content, but also improve the mechanical properties of the sintered steels.

Abstract: Thermal spray deposition has been widely used as a coating process for applying thin protective layers to the need-to-protect materials, or substrates. Recent technological developments in thermal spray processing, particularly cold gas spraying (CGS) and supersonic plasma spraying (S-PS), have enabled some emerging applications for making structural components. This paper reports on the results of our recent attempts to obtain thick TiAl coatings using three coating techniques: atmospheric plasma spraying (APS), S-PS and CGS. We successfully achieved a 3 mm thick coating using both APS and S-PS techniques, but failed in cold spray. A significant phase change was observed of the powder particles experienced during both APS and S-PS processes. Nevertheless, a considerable quantity of titanium oxides was observed in the APS coating.

Abstract: Various manufacturing parameters involved in the ‘buoyancy method’ were inter-related. An equation based on unit cell models for a relation between volume expansion ratio (VER) of bulk microspheres in aqueous starch and microsphere size was derived. A good agreement between the equation and experimental data was found. The inter-microsphere distance (MID) concept was introduced and it was demonstrated that the MID can be calculated numerically for microspheres with known statistical data.

Abstract: The mixing required for post-mould gelatinisation of syntactic foams using starch as binder was studied. It was found that starch particles tend to adhere to hollow microspheres during mixing, forming agglomerates. A transition in the buoyancy of microsphere-starch phases was found at a particular volume fraction of starch. The transitional point was close to a volume fraction of starch at which a calculated relative density for a system consisting of multiple starch particles per microsphere was unity. Starch-microsphere inter-distance appears to be an important parameter affecting starch content in an agglomeration.

Abstract: IR chemical imaging has been used to characterise the dispersion of condensed tannin additives in a biodegradable plastic, poly(butylene succinate). By mapping key FTIR absorptions, acetylated tannin was found to remain in discrete aggregates, though the outer of these particles contained a mixed phase with PBS. In particles >100 μm, PBS does not appear to ingress the tannin core, only wetting the outer region. Analysis of a second tannin derivative indicated that there was a more uniform dispersion of the additive in the PBS matrix.

Abstract: Novel hyperbranched phenol-formaldehyde (HBPF) resin 1 has been prepared for the first time. Thorough characterizations (FT-IR, NMR, HPLC, MALDI-TOF MS and elemental analyses and rheological flow behavior) were performed to ascertain the structure of HBPF 1. The condensationcuring event of HBPF with diglycidylether of bisphenol-A (DGEBA) has been studied by differential scanning calorimetry (DSC) technique.