Laser metal deposition of steel components using machining waste as build material
Gas atomised metal powders are generally used in laser cladding. However, costly new powders prompt the use of machining swarf as an alternative build material. This paper reports on this investigation and establishes its feasibility.
Microstructure study of direct laser fabricated Ti alloys using powder and wire
Abstract A compositionally graded material has been fabricated using direct laser fabrication (DFL). Two types of feedstock were fed simultaneously into the laser focal point, a burn resistant (BurTi) alloy Ti–25V–15Cr–2Al–0.2C powder and a Ti–6Al–4V wire. The local composition of the alloy was changed by altering the ratio of powder to wire by varying the feed rate of the powder whilst maintaining a fixed feed rate of wire-feed. For the range of compositions between about 20% and 100% BurTi only the beta phase was observed and the composition and lattice parameter varied monotonically. The grain size was found to be much finer in these functionally graded samples than in laser fabricated Ti64. Some samples were made using the wire-feed alone, where it was found that the microstructure is different from that found when using powder feed alone. The results are discussed in terms of the power requirements for laser fabrication of powder and wire samples.
Modelling the Marangoni convection in laser heat treatment
Epitaxial Laser Metal Forming (E-LMF) consists in impinging a jet of metallic powder onto a molten pool formed by controlled laser heating and thereby, generating epitaxially a single crystal deposit onto the damaged component. This new technique aims to be used for the repair and reshape single crystal gas turbine components. Because of the very localised melting pool, the high temperature gradients produced during the process must be carefully controlled in order to avoid both the columnar-to-equiaxed transition (CET) and the appearance of hot tears. To this end, heat flow modelling is required to establish the relationship between process parameters such as laser power, beam diameter and scanning speed, and the local solidification conditions. When modelling the heat transfer within the sample, it is necessary to include the liquid flow pattern generated by the surface tension driven convection known as the Marangoni effect. Indeed, the fluid flow in the liquid pool dictates the shape of the traces as shown by the measurements carried out at EPF-Lausanne in re-melting experiments. A three dimensional (3D) model is implemented in the finite element software calcosoft in order to model the development of the fluid convection within the liquid pool. It is shown that the velocities due to natural convection are of the order of 1 mm/sec whereas Marangoni convection produces velocities of the order of 1 m/sec. Moreover, at low scanning speeds, the liquid pool becomes larger than the beam diameter and the development of Marangoni eddies leads to a widening and deepening of the pool. The local solidification conditions such as the thermal gradient and the solidification speed can be extracted at both the solidus and liquidus temperatures to assess the risk of CET and hot cracking.