Deposition of Ti–6Al–4V using laser and wire, part I: Microstructural properties of single beads
Highlights
► Microstructural characteristics of Ti–6Al–4V single beads are investigated ► The Ti–6Al–4V single beads can be divided into five different zones ► Microstructural zones are similar for beads produced with or without wire
Introduction
In the present paper, the microstructural characteristics of Ti–6Al–4V single welding beads and their dependence on the process parameters are addressed. Single beads represent the fundamental unit for producing multi-layered components. Building up components layer by layer is mostly referred to as rapid prototyping, rapid manufacturing, or additive layer manufacturing (abbrev. ALM). Manufacturing near-net-shape components in a layer-by-layer fashion offers a great potential of time and cost savings in comparison to conventional manufacturing technologies. Especially aerospace components that are machined from costly wrought material at a low fly-to-buy ratio represent interesting applications for additive manufacturing. Wire-feed deposition is discussed as a promising technology in this area. To build up components controllably and predictably by numerous beads, and to understand the microstructural and mechanical properties, prior investigations on single beads are helpful which is verified in [1]. Furthermore, single beads provide data for the validation of finite element (FE) simulations. In [2], [3], single beads are used for simulation of an arc beam based wire-feed ALM process. In [4], [5], [6] single beads are used for simulation of a laser beam based powder-feed ALM process. Furthermore, [7] reports that single beads provide data for closed-loop process control to attain consistency of fabricated parts.
Regarding additive layer manufacturing, wire-feed processes have received less attention than powder-bed or powder-feed (also: blown-powder) processes according to [8] and according to worldwide studies reported in [9]. In 1980, early wire-feed ALM activities called Layerglaze™ were reported in [10]. While wire-feed systems have been used for welding purposes, there have been only a few studies on their potential to create 3D objects according to [11].
The increasing market demand for titanium serial production parts has promoted wire-feed processes in recent years, as repeatability, material properties, material usage, possible part size, and building speed have also become issues. In the literature, it is reported that using wire instead of powder feedstock could lead to a higher material quality of the deposited Ti–6Al–4V, specifically high density. This was figured out in an arc beam based process in [12] and in a laser beam based process in [13]. Additionally, [14], [15] reported of reduced contamination by using wire instead of powder feedstock.
Particular research on single beads for additive manufacturing using a laser beam was published in [16] from the University of Nottingham, in [17], [18] from the Fraunhofer Institute for Production Technology and in [19] from the University of Manchester. Research on single beads using an arc beam was published in [20] from the University of Sheffield & AMRC, using an electron beam in [21] from the NASA Langley Research Center, and using cold metal transfer in [22] from the University of Cranfield.
Section snippets
Wire-feed process
A laboratory wire-feed setup was developed and used for basic research in wire-feed ALM. It essentially comprised a Trumpf HLD 3504 Nd:YAG rod laser (diode pumped) with a maximum power of 3.5 kW, a Weldaix wire-feeder and a Kuka KR 100 HA (high accuracy) 6-axis robot. In an open box, which is permanently flooded by argon from its base (design of the box based on [23]), Ti–6Al–4V welding wire with extra low interstitials (ELI grade) is deposited onto a Ti–6Al–4V substrate. An overview and
Morphology and microstructure
Single beads are composed of different microstructural zones, depending on thermal history underwent. The schematic drawings in Fig. 5 were developed based on the thermal modelings of Ti–6Al–4V deposition in [3], [32], the microstructural and thermal modelings in [33], [34], the welding modeling in [35], the pseudo-binary phase diagram (Fig. 4), and microscopy observations. These illustrate the relationship between the peak temperatures underwent and the resulting microstructural zone at
Morphology and microstructure
Grain growth in (laser) weldments, in which a substrate of the same kind is present, is generally dominated by heterogeneous nucleation [53]. The grain structure near the fusion boundary is dominated by epitaxial growth [35]: solidifying crystals at the interface build upon the crystals or grains of the unmelted, solid parent base metal, take up the substrate's crystal or grain structure and orientation, and grow competitively according to [53]. During solidification, grains tend to grow in the
Summary and conclusions
Understanding the microstructure of single beads for characterization of multi-layered components might be as meaningful as understanding the properties of elementary cells for characterization of metals. Therefore, the microstructure of single beads is investigated in this paper. Relationships between the laser beam power, welding speed, wire-feed speed factor and particular microstructural features are determined and discussed. The experiments reveal fundamental microstructural and process
Acknowledgment
The activities at EADS Innovation Works were especially supported by Frank Palm, Achim Schoberth, and Dr. Claudio Dalle Donne.
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