123 research outputs found
Thixoforming of EN AW-2014 alloy at high solid fraction
No full textEN AW-2014 extruded alloy slugs were thixoformed at 615 degrees C where the solid fraction is estimated to be 80%. The recrystallization process occurred during heating to the thixoforming temperature, between 550 degrees C and 600 degrees C, well above the solidus temperature owing to the pinning of grain boundaries by Al2Cu precipitates. The equiaxed polygonal grains thus obtained have become increasingly globular upon soaking. Si was enriched in the grain boundaries during soaking while the solid solution matrix was gradually depleted off Cu. The grain boundary composition has moved closer to that of the Al-Cu Si ternary eutectic with a lower melting point than the binary Al-Cu eutectic, facilitating grain boundary melting. The liquid phase has then penetrated between the grains, forming a more or less continous intergranular network. Microstructural features essential for forming in the semi-solid state were obtained after about 10 min at 615 degrees C. The subsequent forming process has occurred in the semi-solid state with no evidence of grain deformation. The thixoformed EN AW-2014 part was solutionized at 500 degrees C for 2 h and was subsequently quenched in water. Artificial ageing at 160 degrees C has produced hardness values as high as 160 HV after only 8h. It is concluded that the high strength wrought EN AW-2014 alloy feedstock processed by the RAP route respond to a thixoforming operation in a very favorable fashion. (C) 2011 Elsevier B.V. All rights reserved
High-temperature abrasive wear testing of potential tool materials for thixoforming of steels
No full textHigh temperature abrasive wear performance of Inconel 617, Stellite 6 alloys and X32CrMoV33 hot work tool steel was investigated. The wear resistance of the latter is degraded at 750 degrees C due to its inferior oxidation resistance. Extensive oxidation co-occuring with abrasive wear at 750 degrees C leads to substantial material loss due to the lack of a protective oxide scale, sufficiently ductile to sustain the abrasion without extensive spalling. The wear resistance of the Inconel 617 and Stellite 6 alloys, on the other hand, improves at 750 degrees C owing to protective oxides that sustain the abrasion without spalling. (C) 2010 Elsevier Ltd. All rights reserved
AlB3 master alloy to grain refine AlSi10Mg and AlSi12Cu aluminium foundry alloys
No full textThe potential of AlB3 master alloy in the grain refinement of AlSi10Mg and AlSi12Cu foundry alloys was investigated and compared with that of the AlTi5B1 master alloy, the standard grain refiner for most aluminium foundries. The latter refines the grain structures of both alloys. However, this performance is not nearly as good as that obtained in wrought aluminium alloys with the same grain refiner. The Ti-free AlSi10Mg and AlSi12Cu alloys, on the other hand, exhibit very small grains for the entire range of holding times when inoculated with AlB3. This implies a remarkable grain refining efficiency, typical of grain refined wrought aluminium alloys, as well as a strong resistance to fading of the grain refinement effect. Lack of Ti in the melt allows the entire B to form AlB2 particles, the perfect substrates, shortly before alpha-Al starts to crystallize. Aluminium castings can enjoy grains as small as those of the wrought alloys, well below 200 mu m, with an addition of 0.02 wt% B provided that their Ti content is controlled. (C) 2011 Elsevier B.V. All rights reserved
Investigation of Isothermal and Cyclic Oxidation of Plasma-Nitrided Hot-Work Tool Steel at Elevated Temperatures
No full textThe oxidation of a plasma-nitrided, hot-work tool steel at temperatures that cover a range of operations from post-plasma-nitriding oxidation to steel thixoforging processing was investigated. Thermal exposure at 500 degrees C led to the formation of a thin Fe-Cr spinel layer and an even thinner outermost layer of hematite. The former is the only oxide that grew on samples exposed to oxygen-lean conditions at 750 degrees C. A thick, multi-layered oxide scale formed on the surface when the plasma nitrided hot-work tool steel was held at 750 degrees C under atmospheric conditions. In this scale, the outermost hematite layer and the inner Fe-Cr spinel were separated by a magnetite layer. The oxide scale produced during thermal cycling at 750 degrees C was also multi-layered with an identical oxide scale configuration to that formed during isothermal exposure at 750 degrees C. The hematite layer, which retained its integrity during isothermal exposure at 750 degrees C, suffered small cracks that were instrumental in its fracture and spallation during thermal cycling. The distinct feature resulting from cyclic oxidation, however, was the wide gap that formed along the magnetite-spinel interface. Thermal expansion mismatch produced compressive stresses which in turn led to buckling of the magnetite layer and to its detachment; while, the spinel layer adhered to the tool steel substrate and survived throughout thermal cycling. Enrichment of nitrogen and the subsequent precipitation of N-2 gas were also believed to have contributed to the gap formation. Formation of such a gap poses a serious threat to the integrity of the oxide scale and was shown to be responsible for the spallation of the magnetite layer upon thermal cycling
Response to thermal cycling of duplex-coated hot work tool steels at elevated temperatures
No full textHot work tool steel samples were plasma nitrided and coated with a thin hard AlTiN film using the Cathodic Arc Physical Vapour Deposition process in the present work. The duplex-coated samples thus obtained were submitted to thermal cycling under conditions that mimic the steel thixoforming process. Compressive stresses generated at the surface due to a thermal expansion mismatch with the substrate lead to blistering of the AlTiN coating. These blisters produce wrinkles and develop into surface cracks with increasing number of cycles. AlTiN coating starts to spall off at those locations encircled by these cracks, leaving the substrate exposed to very hostile conditions. The nitrided substrate is very prone to oxidation and scaling is inevitable. Likewise, the nitrided surface fails to provide the required substrate support for the AlTiN coating as the hardening introduced at the surface by plasma nitriding is almost completely erased after only 350 thermal cycles. Having lasted for a maximum of 750 thermal cycles, the duplex-coated hot work tool steel fails to offer any improvement in the thermal fatigue performance of hot work tool steels at elevated temperatures. (C) 2011 Elsevier B.V. All rights reserved
Response to thermal cycling of plasma nitrided hot work tool steel at elevated temperatures
No full textThermal fatigue performance of plasma nitrided hot work tool steel was investigated under conditions encountered by thixoforging dies in semi-solid processing of steels. Plasma [landing does not offer any improvement in the thermal fatigue performance of hot work tool steels at elevated temperatures, due essentially to poor resistance to oxidation and to temper softening. Fe(3)O(4) and Fe(2)O(3) scales produced on the nitrided surface fail to sustain the thermal stresses produced by thermal cycling. They spall off, generating fresh surfaces for further oxidation. This sequence leads to substantial material loss and impairs the integrity of the surface beyond a quality level that would be tolerated in steel thixoforming. The surface hardening provided by plasma nitriding is also completely erased. The tempered martensitic structure is replaced by fine. equiaxed ferritic grains implying a dynamic recrystallization process during thermal cycling. (C) 2010 Elsevier B.V All rights reserved
Melt treatment of Al–Si foundry alloys with B and Sr additions
No full textIndividual Sr and B additions to Al-Si alloy melts fail to provide either grain refinement or modification. Sr and B must be used together to achieve both features for high-quality sound castings. Sr and B solutes in addition to insoluble SrB6 particles are introduced to the melt when Sr and B are used together. SrB6 particles and B solutes, which form the AlB2 particles later during solidification, are believed to be responsible for the marked improvement in grain refinement in the co-addition practice. Sr atoms in liquid solution, on the other hand, are adsorbed on the surface of the Si plates leading to twinning once the melt cools to the eutectic solidification temperature. A series of experiments have shown that a very fine grained matrix and a fine fibrous eutectic are readily obtained when the AlSi7Mg0.3 alloy is inoculated with 100 ppm B and 150 ppm Sr
A calorimetric analysis of the response to heating of EN AW-2014 alloy formed in the liquid, solid and semi-solid states
No full textA calorimetric analysis was undertaken to understand the phase evolution during DC cast, extruded and thixo-forming of EN AW-2014 alloys. Extruded EN AW-2014 alloy responds to a heating cycle by the precipitation of the equilibrium theta-CuAl2 phase and its nonequilibrium precursors. This precipitation sequence is largely missing in the cast and thixoformed EN AW-2014 counterparts, suggesting that the precipitation capacity of these alloys is largely exhausted at very low temperatures. An additional endotherm is noted in the cast and thixoformed counterparts evidencing an additional soluble constituent, Cu2Mg8Si6Al5. Extruded EN AW-2014 alloy undergoes recrystallization between 500 degrees C and 550 degrees C while cast and thixoformed alloys undergo only grain growth. Cooling from solid-state forming is relatively faster and helps to retain much of Cu in solution offering a full precipitation sequence during a subsequent heating cycle ensuring substantial age hardening in the extruded alloy. A separate solution treatment is required to take advantage of the age hardening capacity of cast and thixoformed alloys
Processing of twin-roll cast thin AlFeSi strips for the manufacture of aluminium finstock
No full textThe thin strips produced with the new generation twin roll casters experience profoundly different temperature and strain gradients in the caster roll gap and thus require different down stream processing cycles. An attempt was made in the present work to identify processing cycles for the manufacture of finstock from thin AlFeSi strips. The grain structure in strips homogenized directly at the cast gauge is reorganized through growth processes while the grain structure in strips homogenized after cold rolling is reorganized via recrystallization. Finstock samples cold rolled without a homogenization treatment start to recrystallize at 300 degrees C in a very sluggish fashion whereas finstock samples submitted to a homogenization treatment undergo a faster recrystallization starting at 225 degrees C. Finstock with intermediate temper mechanical properties can be more easily achieved with a back-annealing process cycle without a homogenization treatment owing to a sluggish recrystallization. Soft temper finstock, on the other hand, requires a process cycle with a homogenization treatment
High temperature sliding wear behaviour of Inconel 617 and Stellite 6 alloys
No full textThe high temperature wear performance of Inconel 617 and Stellite 6 alloys was investigated and compared with that of the X32CrMoV33 hot work tool steel. The wear performance of the latter at 750 degrees C is judged to be very poor due basically to its inferior oxidation resistance. Extensive oxidation co-occurring with wear at 750 degrees C leads to substantial material loss basically due to the lack of an adhesive oxide scale, sufficiently ductile to sustain the wear action without extensive spalling. The wear resistance of the Inconel 617 and Stellite 6 alloys at 750 degrees C is relatively superior. The adhesive oxides growing slowly on Inconel 617 and Stellite 6 alloys sustain the wear action without spalling and are claimed to be responsible for the superior wear resistance of these alloys at 750 degrees C. (C) 2010 Elsevier B.V. All rights reserved
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