Klaus Bauckhage

Piezoelectric Droplet Generator for the Calibration of Particle-Sizing Instruments

Using a piezoceramic tube and a continuous glass capillary, droplets in a diameter range between 10 μm and 100 μm can be generated. This corresponds to a volume of up to 0.6 pl. The velocity of the generated droplets depends on droplet size but is constant for each diameter. The liquid can be dosed as single droplet, as an accumulation of droplets or as a chain of droplets in a frequency range between 1 Hz up to 3 kHz. The lifetime of the droplets depends on droplet size and the chemical and physical properties of the dispersed liquid. In addition, for water droplets the humidity of the air near the droplet trajectory influences the lifetime of the droplets.

A mathematical model for cooling and rapid solidification of molten metal droplets

Abstract During the spray forming process, a continuous molten metal stream is atomized by impinging high speed inert gas jets. In the generated spray cone, the resulting metal droplets are rapidly cooled by the huge temperature difference to the surrounding gas phase and thereby partly solidify. After a certain flight and residence time inside the spray cone, the droplets impinge on the substrate and form the product (deposit). The material properties of this product depend on several process parameters and especially on the thermal state of the deposited droplets at impingement. Smaller droplets cool very fast and may impinge onto the product in a completely solidified state as solid metal powder particles. Larger droplets contain a higher amount of thermal energy and impact during the state of phase change or even still completely liquid. Therefore, describing the thermal history of metal droplets during flight in the spray cone is of great importance. In this contribution, a mathematical model is introduced to describe the cooling and solidification of individual metal droplets in the spray cone during the droplet–gas interaction in flight. By introducing this model into a standard two phase flow simulation model for the spray cone description, it is possible to calculate the transient droplet temperature and solid fraction contents of individual particles depending on overall process parameters and flight path.

Jet break up of liquid metal in twin fluid atomisation

For melt disintegration in spray forming and metal powder production, the liquid metal typically is atomised by means of twin fluid atomisation e.g. with inert gases. The first stage of the atomisation process is covered by the initialisation and development of surface perturbations on the liquid jet surface that subsequently grow and finally lead to the break up of the melt jet. The primary disintegration process affects all further stages of the atomisation process and hence influences the resulting spray characteristics. In this contribution, the influence of different material properties on the primary disintegration process of a metal melt in a free fall atomiser configuration is investigated. Surface instabilities of liquid metal jets up to the primary break up are analysed experimentally as well as analytically. Results of a linear instability analysis are discussed and compared to evaluations of high-speed video images. In comparison of both results the periodicity and main mechanism of the initialisation process is analysed.

In-situ particle temperature, velocity and size measurements in the spray forming process

The structure and material properties of spray formed products depend directly on the thermal state of particles before they impact the substrate or on the already deposited layer. Monitoring particle temperature, velocity and size can thus provide a unique tool for optimizing the material properties as well as controlling spraying conditions during deposition. In this paper, an optical sensing device based on the principle of high-speed pyrometry, developed for on-line monitoring of particle temperature, velocity and diameter of in-flight particles during thermal spraying conditions (e.g. plasma guns), is for the first time applied and examined in the spray forming process. Thermal radiation emitted by the particles is collected by a sensing head attached to the spray cone and transmitted through optical fibers to a detection cabinet located away from the dusty environment. Tests were carried out with different materials, spray pressures and measurement positions to exhibit the efficiency of the measurement system in the spray forming process.

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.

Piezoelectric Single Nozzle Droplet Generator for Production of Monodisperse Droplets of Variable Diameter

The further development of a droplet generator which functions without the use of pressure (drop-on-demand system) for the production of monodisperse droplets of variable diameter opens up new fields of application.

Numerical investigation of alternative process conditions for influencing the thermal history of spray deposited billets

Abstract In spray forming, during the spray deposition process and the subsequent cooling period a time dependent temperature field develops within the product and the solidification of the remaining liquid fraction takes place. In this paper, the time dependent thermal conditions and solidification behaviour in spray formed billets are investigated. A transient numerical simulation is carried out and compared with experimental results. A description of the relevant model and the developed program will be given and an investigation of the influence of different process parameters on the time dependent temperature field and the solidification history within billets are shown. The numerical model is based on a single-phase formulation of the energy equation. A non-orthogonal coordinate system is used for grid generation within the time dependent growing shape of the billet. Temperature measurements are carried out within the substrate and also in the lower part of the billet in the spray forming process. The material discussed throughout this contribution is CuSn6 (2.1020).

Sizing of inhomogeneous particles by a differential laser Doppler anemometer

A particle moving across the interference pattern of two intersecting laser beams scatters the incident light with a temporal variation that enables the calculation of particle velocity. This idea was realized in laser Doppler anemometry (LDA). By recording the light scattering pattern with a fast line scan sensor, the spatial modulation of the signal is also detected. This yields, in addition to particle velocity, information about the size and morphology of the particle. This is the working principle of a new measuring device presented in this paper, the differential laser Doppler anemometer (DLDA). The theoretical background, principle of data evaluation and first experimental results of water and emulsion droplets and of glass beads are described.

Micron and sub-micron aerosol sizing with a standard phase-Doppler anemometer

Abstract In this paper the extended application range of the standard phase-Doppler anemometer (PDA) to particle diameters below the commonly accepted lower limit of about 5–10 μm is described. The problems that accompany measurements on smaller particles are small scattering cross-sections and ambiguous relations between the measured phase difference ΔΦ and the particle diameter d . A simple, but flexible optical set-up for the transmitting part of a standard PDA system, and a numerical algorithm, which allows careful data evaluation from ambiguous ΔΦ ( d ) relations, help to manage these problems. Measurements on aqueous suspensions of micron and sub-micron sized dielectric and metalic monospheres, as well as on polydisperse aerosols, are presented and evaluated by means of the proposed algorithm.

Detection based on rainbow refractometry of droplet sphericity in liquid–liquid systems

The shape of droplets in liquid-liquid systems influences their mass and momentum transfer processes. The deviation from sphericity of rising droplets in liquid-liquid systems was investigated for different droplet sizes. Rainbow refractometry permits one to test, in this case, whether the use of laser-optical particle sizing will be correct or faulty. Since the assumption of spherical particle geometry is a general basis of laser-optical particle-sizing techniques such as rainbow refractometry or phase Doppler anemometry, deviation from the spherical shape results in a measuring error. A sphericity check based on rainbow refractometry is introduced.