Volker Uhlenwinkel

Effect of thermomechanical treatment on spray formed Cu – Ni – Si alloy

Abstract The heat treatment response of a spray formed Cu – 2.4Ni – 0.6Si (wt-%) alloy has been investigated. The spray formed alloy was given various thermomechanical treatments prior to isothermal aging. These treatments included solutionising and/or cold rolling with different reductions in original thickness. The variation in hardness and electrical conductivity of the alloys was measured as a function of the aging time. The results indicated the highest peak hardness value of ~250 kg mm-2 for the alloy aged after solution treatment and cold rolling to 40% reduction in thickness, compared with the maximum hardness of 220 kg mm-2 for specimens aged directly in the as spray formed condition. However, the electrical conductivity after aging was observed to be a maximum of 65%IACS (International Annealed Copper Standard) in specimens cold rolled to 80% reduction in thickness before aging. The aging response was observed to accelerate with the degree of cold working. Optical, scanning electron and transmission electron microscopy were used for microstructural characterisation of the materials. Precipitation of the second phase was observed to dominate in deformation bands. The alloy showed evidence of discontinuous precipitation, particularly when the alloys were cold rolled before aging. The onset of discontinuous precipitation led to a drastic deterioration in hardness of the alloys. The precipitation behaviour of the alloy is discussed in the light of microstructural characteristics associated with various processing conditions of the alloy.

Synthesis of single phase i-AlCuFe bulk quasicrystal by spray forming

Abstract In the present study, an icosahedral single phase bulk quasicrystalline material based on Al62Cu25.5Fe12.5 has been synthesised by a spray forming route. Microstructural characterization showed an average grain size of 10 μm. The oversprayed fine powder showed the presence of β- and λ-phases, whereas, the deposit consited of the fully single phase bulk quasicrystalline material with compositional homogeneity. The hardness and fracture toughness measurements were carried out at different indentation loads of 50–500 g. The hardness values varied in the range 10.4–8.1 GPa and fracture toughness was seen to decrease with increasing load. The varionan of hardness with load, which is known as indentation size effect (ISE), has been established clearly. Fracture toughness value was constant in the load range from 200 to 500 g at 1.2 MPa m1/2. The cracking pattern after indentation at higher load has been observed to be intergranular as well as transgranular. The evolution of the single phase bulk quasicrystalline material has been discussed in light of the unique combination of atomization and deposition process elements in spray forming technique.

Strain-induced structural transformation of single-phase Al–Cu–Fe icosahedral quasicrystal during mechanical milling

A single-phase stable icosahedral quasicrystalline sample of high quality with the composition Al62.5Cu25Fe12.5 was produced by the spray forming technique. The material was further investigated by mechanical milling under an argon atmosphere to avoid oxidation during milling. At the initial stages of milling (within 5 h) a significant broadening of the diffraction peaks was observed, indicating a reduction of crystallite size and the introduction of lattice strain, which can be linked to phason strain of the quasilattice. Line broadening was noticed to increase with increasing milling time and in the material milled for longer time only a few broad diffraction peaks, which can be identified as a nanoscale bcc phase (i.e. disordered B2 phase, a ∼ 2.9 Å), were visible. At this stage the diffraction signals belonging to the quasicrystals were no longer observable, indicating a complete transformation of the quasicrystals into the bcc phase. Finally, the bcc phase formed during milling transformed back to the quasicrystalline phase during subsequent annealing treatment. The microhardness measured on the milled powders was found to decrease with increasing milling time, most likely as a consequence of the increased volume fraction of the ductile bcc phase. Attempts are made to rationalize the structural transformation.

Convective heat transfer from a billet due to an oblique impinging circular jet within the spray forming process

Abstract An experimental investigation was performed to study the heat transfer process from the surface of a billet due to an oblique impinging circular gas jet. The gas stream was generated by a circular array of 24 single jets within an atomizer nozzle. The application of this heat transfer condition can be found in the spray forming process during the cooling of the sprayed billet. At several positions on the billet surface the local heat transfer coefficients were measured. The measurement technique was based on the cooling of a surface mounted sensor having a sufficiently small Biot number. A comparison between published data and a test case had been carried out successfully. The distance between the atomizer and the billet, the gas mass flow rate, and the rotation angle of the billet were varied. Velocity measurements were executed to characterise the gas flow in the confined jet in the absence of the billet, while numerical flow field simulations were used to calculate the flow around the billet. The heat transfer coefficient distribution showed a significant peak at the stagnation point and decreased with increasing atomizer to billet distance and also with decreasing gas mass flow. The heat transfer coefficients averaged over the circumference of the billet were correlated empirically.

Solidification Sequence of Spray-Formed Steels

Solidification in spray-forming is still an open discussion in the atomization and deposition area. This paper proposes a solidification model based on the equilibrium solidification path of alloys. The main assumptions of the model are that the deposition zone temperature must be above the alloy’s solidus temperature and that the equilibrium liquid fraction at this temperature is reached, which involves partial remelting and/or redissolution of completely solidified droplets. When the deposition zone is cooled, solidification of the remaining liquid takes place under near equilibrium conditions. Scanning electron microscopy (SEM) and optical microscopy (OM) were used to analyze the microstructures of two different spray-formed steel grades: (1) boron modified supermartensitic stainless steel (SMSS) and (2) D2 tool steel. The microstructures were analyzed to determine the sequence of phase formation during solidification. In both cases, the solidification model proposed was validated.

Analysis of the spray forming process using backscattering phase-Doppler anemometry

Abstract A phase-Doppler anemometer (PDA) was extended for the analysis of metal melt gas-atomization. Because of the optical properties of liquid or partly solidified spherical particles with a high value of the imaginary part of the refractive index (which means that measurements can be executed, using the reflected light from the particles), the instrument was arranged in a 180° backscattering position, with a combined transmitting and receiving system. This compact assembly described in principle already by Bauckhage in 1988 and 1989 can easily be moved to realize different measuring locations in the spray cone, needing only one window. Operating from one side of the spray cone scanning PDA-measuring positions from inside the two phase flow could be carried out within milliseconds per measuring position, thus allowing the analysis even for short atomization duration. Due to high particle rates in the spray cone and a difficult optical behavior of the solidified metal particles (rough surface), a new signal processing had to be developed, which can be subdivided into a signal recording and a post processing. During the first part a signal is completely recorded for a definite duration without using any trigger criteria. This results in a kind of signal band, that contains bursts as well as noise. In the second part the bursts are removed from the signal band by using the signals power spectrum after a fast-Fourier transformation (FFT).

Grain size softening effect in Al62.5Cu25Fe12.5 nanoquasicrystals

Inverse Hall-Petch (IHP) behavior in nano-quasicrystalline Al62.5Cu25Fe12.5 is reported. Powders with varying grain sizes were produced by mechanical milling of spray-formed quasicrystals. The hardness of the milled powders increased with decreasing grain size down to about 40 nm and decreased with further refinement, demonstrating the IHP behavior. This critical grain size was found to be larger compared to other metallic nanocrystalline alloys. This IHP behaviour has been attributed to the structural complexity in quasicrystals and to thermally activated shearing events of atoms at the grain boundaries.

Cooling conditions for the generation of bulk metallic glasses by droplet deposition

Abstract The cooling rate during material processing until glass transition rate is the key parameter for the production of bulk metallic glasses. But in the past, little attention has been paid to advanced production techniques such as deposition of molten metal sprays or spray forming, which offer elevated cooling rates. In this work, cooling conditions during spray forming were investigated due to its utmost importance for producing amorphous structures. Spray forming is treated in this work as a three step cooling process consisting of droplet flight phase, splat phase and deposit phase. All cooling steps were simulated for different droplet sizes. The surface temperature of the deposit was found to play an important role in the production of metallic glasses via spray forming. The simulation model can be used to find suitable spray conditions for the generation of bulk metallic glasses.

Specific enthalpy measurement in molten metal spray

A caloric probe to measure average specific enthalpies of molten metal droplets in a spray cone of a free fall atomizer was developed. Molten tin was used in this caloric probe to collect the particles of the spray. The results of standard deviation of the consistency measurements were examined comprehensively. The experiments were made with low carbon steel, bearing steel AISI 52100 and copper. The process parameters, gas mass flow rate and melt mass flow rate were varied. The specific enthalpy within the spray cone clearly decreased with increasing radial distance from the center of the spray cone and with increasing axial distance to the nozzle. A correlation between median particle diameter and average specific enthalpy was clearly observed. Experimental results are compared with data from numerical simulations.

Stiff, light, strong and ductile: nano-structured High Modulus Steel

Structural material development for lightweight applications aims at improving the key parameters strength, stiffness and ductility at low density, but these properties are typically mutually exclusive. Here we present how we overcome this trade-off with a new class of nano-structured steel – TiB2 composites synthesised in-situ via bulk metallurgical spray-forming. Owing to the nano-sized dispersion of the TiB2 particles of extreme stiffness and low density – obtained by the in-situ formation with rapid solidification kinetics – the new material has the mechanical performance of advanced high strength steels, and a 25% higher stiffness/density ratio than any of the currently used high strength steels, aluminium, magnesium and titanium alloys. This renders this High Modulus Steel the first density-reduced, high stiffness, high strength and yet ductile material which can be produced on an industrial scale. Also ideally suited for 3D printing technology, this material addresses all key requirements for high performance and cost effective lightweight design.