In laser cladding thermal contraction of the initially liquid coating during cooling causes residual stresses and possibly cracks. Preweld or postweld heating using inductors can reduce the thermal strain difference between coating and substrate and thus reduce the resulting stress. The aim of this work is to better understand the influence of various thermometallurgical and mechanical phenomena on stress evolution and to optimize the induction-assisted laser cladding process to get crack-free coatings of hard materials at high feed rates. First, an analytical one-dimensional model is used to visualize the most important features of stress evolution for a Stellite coating on a steel substrate. For more accurate studies, laser cladding is simulated including the powder-beam interaction, the powder catchment by the melt pool, and the self-consistent calculation of temperature field and bead shape. A three-dimensional finite element model and the required equivalent heat sources are derived from the results and used for the transient thermomechanical analysis, taking into account phase transformations and the elastic-plastic material behavior with strain hardening. Results are presented for the influence of process parameters such as feed rate, heat input, and inductor size on the residual stresses at a single bead of Stellite coatings on steel.

The computer simulation technique is applied t o the electron diffraction contrast of inclusions. A new formulation of t h e Bloch wave theory of diffraction contrast in the twobeam case and a cubic interpolation scheme are used t o diminish the calculation time. Bright and dark field images of coherent ellipsoid-shaped particles in an elastically isotropic crystal have been calculated and printed using exact elastic distortion fields. Taking into account only the dipole part of the elastic fields has not great influence on the images. Contrast details are discussed on the basis of perturbation expansions for the beam intensities.Die Methode der Computersimulation des Beugungskontrasts wird auf Einschliisse angewendet. Um Rechenzeit zu sparen, werden eine neue Formulierung der Blochwellengleichungen fur den Beugungskontrast im Zweistrahlfall und ein kubisches Interpolationsverfahren eingefiihrt. Theoretische Hell-und Dunkelfeldbilder von kohkrenten ellipsoidformigen Einschliissen in einem elastisch isotropen Kristall werden angegeben, die mit Hilfe des exakt berechneten elastischen Distortionsfeldes in der Umgebung der Teilchen gewonnen wurden. Es zeigt sich, daB der Beugungskontrast im wesentlichen durch den Dipolanteil der elastischen Felder bestimmt wird. Kontrasteinzelheiten werden unter Verwendung storungstheoretischer Ausdriicke fur die Strahlintensitaten diskutiert.

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