Investigation of High-Depostition-Rate Additive Manufacturing of Ti-6Al-4V via Laser Material Deposition

Rebar Hama-Saleh. Fraunhofer Institute for Laser Technology – ILT, Steinbach Straße 15, Aachen D-52074, Germany. Corresponding author : rebar.hama-saleh@ilt.fraunhofer.de Kerim Yildirim. Manufacturing Processes and Systems, Department of Mechanical Engineering, KU Leuven, Celestijnenlaan 300 box 2420, 3001 Leuven, Belgium. Susanne Hemes. ACCESS e.V., Intzestraße 5, Aachen D-52072, Germany. Andreas Weisheit. Fraunhofer Institute for Laser Technology – ILT, Steinbach Straße 15, Aachen D-52074, Germany. Constantin Leon Häfner. Fraunhofer Institute for Laser Technology – ILT, Steinbach Straße 15, Aachen D-52074, Germany.


Introduction oduction
Nowadays, Ti-6Al-4V is the most commonly used titanium alloy. Indeed, over half of the titanium alloy market is based on Ti-6Al-4V [1]. It belongs to the group of (α+β) alloys exhibiting high strength at medium density and excellent corrosion resistance, and is used in various sectors such as aerospace, automotive, medicine, chemical industry and power generation [2,3]. Such lightweight and strong alloys are especially suitable for highly loaded structures where weight reduction is a way to improve efficiency [4,5]. Additive manufacturing (AM) of titanium alloys has gained interest in the aerospace industry because of its potential for new design options as well as repair and hybrid manufacturing. AM can be used to manufacture parts such as turbine blades, blade integrated disks and combustors [6]. The typical deposition rate of Ti-6Al-4V using LMD is, however, below 1 kg/h and is commonly used for repair purposes, as it is not cost-effective for entire component build-up. Laser material deposition (LMD), also known as direct energy deposition (DED) or laser cladding, is an additive manufacturing method in which complex geometries can be formed by the multilayer arrangement of subsequent layers [1,2]. In this process, a laser beam is passed over the surface of the component, which is selectively melted. As seen in Fig. 1a, metal powder is injected into this molten pool with the aid of a carrier gas, typically a noble gas such as argon, which produces a molten metallurgical compound [3,4].
The addition of a noble gas as a shielding gas along the powder stream prevents the degradation of the powder and the molten bath. While it is obvious that near-net-shape technologies, such as AM, could lead to a more resource-efficient production than traditional production [4,7,8] [9], production costs in AM grow quickly with part size increases [10]. A way to overcome this is to increase the deposition rate on the scale of several kg/h.
In the present work, high-deposition-rate LMD (HDR-LMD) was equipped with a high-power diode laser and a zoomoptic for a large laser beam spot to improve the systematic ability of massive injected powder particles to melt. A powder nozzle was designed to meet the powder stream distribution requirements. By using the energy density index, ESAFORM 2021. MS13 (Additive Manufacturing), 10.25518/esaform21.486 486/1 the process window for the HDR-LMD process (deposition rate > 300 cm³/h) [11] was quantitatively determined. In addition, several systematic experiments were carried out to improve the bonding and porosity reduction of deposited HDR-LMD Ti-6Al-4V material, including process parameter dependence on track geometry and powder performance. This paper deals with understanding how the process conditions influence microstructure and texture evolution.  Fig. 1a, metal powder fed by an inert gas (typically argon) is introduced into the focused laser beam that generates a melt pool on the surface. The carrier gas and an additional gas stream through the beam path create an inert gas atmosphere around the melt pool [4,12]. When the beam moves on, the melt quickly cools down and solidifies [13]. While the inert gas fed through the powder feed nozzle shields the melt pool, the rest of the part is exposed to the surrounding atmosphere and the surface is oxidized. To avoid this oxidation, a global inert gas atmosphere was created by using an inert gas chamber filled with argon (Fig. 1b). The processing head is inside this chamber. As beam source, a 12 kW diode laser LDF 12000-100 from Laserline GmbH was used. with laser beam sour with laser beam source, po ce, pow wder f der feeder eeder, optic, po , optic, pow wder f der feed nozzle and inert g eed nozzle and inert gas chamber [14]. as chamber [14].
The Ti-6Al-4V powder material used for the experiments was supplied by TLS Technik GmbH. The measured chemical composition is listed in Table 1. The powder material was produced in the electron induction-melting gas atomization process (EIGA) and has a spherical particle shape and a particle size distribution between 45 and 90 µm. The Ti-6Al-4V substrate used in the LMD process was supplied by ATI Specialty Materials. The dimensions were 250 mm x 100 mm x 10 mm. The chemical composition is listed in Table 1. Prior to usage, the substrate plates were sandblasted to clean the surface and improve the absorption of the laser radiation.
T and feed rate were produced and analyzed in a preliminary study, in order to determine the minimum laser power for creating defect-free tracks. Based on the obtained results, the powder feed rate and laser beam diameter were increased in the main study to achieve a high-deposition-rate. In addition, the laser power was varied. The laser power and the powder feed rate were scaled accordingly. The scanning speed was kept constant. The laser power was varied for the laser beam diameter of 3 mm in a range of approximately 1450 W -1850 W and the powder feed rate ranged between approx. 6.61 g/min and 10.2 g/min. A complete list of the processing parameters is given in Table 2.
T Table 2. LMD pr able 2. LMD process par ocess paramet ameters ers 3.2 3.2 Influen Influence of laser po ce of laser pow wer on tr er on track geometry ack geometry The aim was to achieve an aspect ratio (height vs width of a track: aspect ratio Ar) between 0.25 und 0. 16 The aspect ratio drops from 0.35 to 0.16 when the laser power is raised from 3200 W to 4200 W. As can be seen in the graphs, there is a determined trend with the given parameters: between aspect ratio, track height and track width with the increasing laser power. When the laser power increases, track height remains steady.

Micr Microstructur ostructure (incl. EBSD e (incl. EBSD, thermal imaging) , thermal imaging)
The as-built microstructures in Ti-6Al-4V produced by AM can be quite complex, mainly due to solid state phase transformations of the solidified β-phase. Typical structures are Widmanstaetten basket-weave [18,19], acicular or martensitic microstructures [20][21][22] depending on the process conditions.. Fig. 3 shows the fine basket-weave morphology inside primary β-grains in a single layer of a sample produced by LMD.
The EBSD analysis in Fig. 4 shows that the primary β-grains have a columnar form. The growth direction of the grains is determined by the temperature gradient at the solid-liquid interface, which is strongly directed into the solid material of the substrate. Large size columnar grains, therefore, grow parallel to the direction of cooling. Depending on specific laser power and powder feed rate, the primary beta grain size varies from a few hundred microns to several millimeters. The α microstructure formed inside the prior β-grains varies depending on the cooling rate and the corresponding thermal cycles. Often, a phase of discontinuous grain boundary α is also observed at the boundaries of prior β-grains [23]. Rapid solidification is a consequence material added in a layer by layer fashion and consists of wide prior β-grains that grow epitaxially across several layers [24][25][26]. In addition, the cooling rate depends on several factors including part geometry, position in the part, process gas flow rate, laser power, laser scanning speed, and build-up strategy of the layers.
Grain growth appears during solidification from previously deposited AM layers and ultimately regulates the crystallographic nature of the AM structure via partial or complete remelting of the previously formed underlying layer [27]. Throughout solidification, the structure near the melt pool boundary is controlled by the base metal.
However, further from the melt boundary, the microstructure is controlled by competitive growth. Competitive growth appears among dendrites with various crystallographic orientations in the polycrystalline materials [28].  scanning speed 1500 scanning speed 1500 mm/min, the corr mm/min, the corresponding micr esponding microstructur ostructure of c) P1(bott e of c) P1(bottom) and d) P5 (t om) and d) P5 (top). op).
The microstructure of the cross-section of the LMD Ti-6Al-4V parts is caused when previously deposited material is reheated during the subsequent deposition process. This leads to microstructural coarsening. Fig.5 c and d show this clearly from the first layer of the sample to the thirteenth layer. The β-phase coarsening appears as the temperature increases during deposition, a result of the constant laser energy input during the process. Martensite is expected when the material cools down from temperatures above the β-transus temperature (Tβ =994°C) faster than 140°C/s and below the martensite starting temperature Ms=575°C [29]. Therefore, the β-transus temperature is indicated by a green dashed line in the plot (Fig. 5a).     Fig. 7. The lowest porosity results for large beam diameters (< 0.2 %). For the smaller beam diameters and the lower laser power, the porosity levels are slightly higher, probably because the melt pool does not exist as long as for larger beam diameters, leaving less time for pores to escape from the melt [30].

Har Hardness measur dness measurement ement
The effect of increasing powder feed rate at constant laser power is also obvious. The porosity increases since the amount of laser power is not sufficient to melt all particles completely, leaving bonding defects inside the volume.
However, this effect is very pronounced only at the lowest power values used in the experiments.

Conclusions Conclusions
The aim of this paper is to investigate the influence of high-deposition-rates on the microstructure, hardness and porosity of Ti-6Al-4V single tracks and bulk volumes produced by the laser material deposition process. Higher deposition rates than the typical ones used for repair processes (below 1 kg/h) were achieved by enlarging the laser beam diameter, increasing laser power and powder feed rate.
The knowledge gained in the course of this work shows that it is possible to generate defect-free volumes at high-deposition-rates up to 5 kg/h. The volumes have a density of 99.98 %.
In the course of this work, we found that the microstructure is influenced by the process conditions where beam diameter, laser power and thermal history of each layer play an important role. In all layers, large primary β-grains form across the layers and are aligned strongly along the main route of heat transfer into the solid volume, indicating that anisotropic mechanical properties may occur (still to be determined). During solid state transformation α (basket weave) or α martensite (needles) form depending on the local cooling rate.
The micro-hardness measured in the deposited area (approximately 330 ±10 HV) is slightly higher than the typical hardness of forged Ti-6Al-4V. This effect is related to the higher cooling rate in AM.