Laser-based powder bed fusion of Ti-6Al-4V structures with different surface-area-to-volume ratios in oxygen-reduced and oxygen-free environment

Titanium alloys, such as Ti-6Al-4V, are particularly susceptible to oxidation, which is why their processing in the laser-based powder bed fusion process is carried out conventionally in a protective gas atmosphere. However, this atmosphere still contains critical residual oxygen levels, which are to be eliminated as part of a new approach. This approach envisages doping the argon protective gas atmosphere with small amounts of the highly reactive gas silane (ratio < 1:1000). The residual oxygen content is particularly critical in filigree and thin-walled structures that have a high surface-area-to-volume ratio and are a typical field of application for this additive manufacturing process. Therefore, this work focuses on the manufacturing of Ti-6Al-4V structures with different surface-area-to-volume ratios in conventional argon (< 200 ppm residual oxygen) and argon-silane atmospheres (< 10-14 ppm residual oxygen) on an innovative laboratory machine. After processing, the specimens are analyzed for surface topography, microstructure, and Vickers hardness. In addition, energy-dispersive X-ray spectroscopy and X-ray diffraction measurements are carried out to further investigate the chemical composition and present phases in the as-built specimens. The influence of the different atmospheres and their residual oxygen content, the surface-to-volume ratio, and possible interactions between them are discussed

Investigations on the effect of post treatment utilizing ultrasonic standing waves on the hardness of laser beam welds in stainless steel

Laser beam welding is precise, quick and highly automatable. Nevertheless, disadvantageous hardness profiles can result and promote cracking. By an ultrasonic post treatment, crystal defects, internal stress and grain structure can be altered to achieve uniform hardness. In the investigations round bars with 30 mm diameter made from stainless steel grade 1.4301 are welded by laser in a rotational process. Ultrasonic excitation is applied utilizing a longitudinal mode of the system. The weld pool is positioned in the node or the antinode of the amplitude distribution. The excitation amplitude varies at 0/2/4 µm and the treatment durations at 0/5/10 min. The welds are evaluated by metallographic cross sections and hardness measurements. The results indicate the effects of acoustic residual softening and hardening. With standard deviations of about 2 %, the weld hardness is decreased by 3 % with nodal excitation and increased by 4 % with antinodal excitation. The difference between weld and base material hardness is not reduced since the base material is hardened at all ultrasonic parameters used

Influence of the ultrasonic vibration amplitude on the melt pool dynamics and the weld shape of laser beam welded EN AW-6082 utilizing a new excitation system for laser beam welding

Laser beam welding is a commonly used technology for joining similar and dissimilar materials. In order to improve the mechanical properties of the weld, the introduction of ultrasonic vibration into the weld zone has been proposed [5]. The ultrasonic system consists of an electronic control, a power supply, a piezoelectric converter and a sonotrode, which introduces the vibration into the weld zone. Its proper design is of great importance for the process performance. Furthermore, the effects of ultrasound in a melt pool need to be understood to evaluate and optimize the process parameters. In addition, it is important to find out the limits of ultrasonic excitation with respect to a maximum vibration amplitude. Therefore, firstly different methods of ultrasonic excitation are investigated and compared with respect to their performance. A system which is based on using longitudinal vibrations turns out to be the best alternative. Secondly, the system design is described in detail to understand the boundary conditions of the excitation and finally, simulations about the influence of ultrasonic vibrations are done by using a simplified model. The system is used to perform experiments, which aim at detecting the maximum vibration amplitude doing bead on plate welds of EN AW-6082 aluminum alloy. The experiments reveal a significant change of the weld shape with increasing ultrasonic amplitude, which matches the simulative findings. If the amplitudes are small, there is a marginal effect on the weld shape. If the amplitudes are high, melt is ejected and the weld shape is disturbed. In the present case, amplitudes over 4 µm were found to disturb the weld shape. © 2021, The Author(s)

Micro- vs. macro-phase separation in binary blends of poly(styrene)-poly(isoprene) and poly(isoprene)-poly(ethylene oxide) diblock copolymers

In this paper we present an experimentally determined phase diagram of binary blends of the diblock copolymers poly(styrene)-poly(isoprene) and poly(isoprene)-poly(ethylene oxide). At high temperatures, the blends form an isotropic mixture. Upon lowering the temperature, the blend macro-phase separates before micro-phase separation occurs. The observed phase diagram is compared to theoretical predictions based on experimental parameters. In the low-temperature phase the crystallisation of the poly(ethylene oxide) block influences the spacing of the ordered phase

NOVEL WELDING STRATEGY IN HIGH DEPOSITION RATE LASER-ASSISTED DOUBLE-WIRE WELDING PROCESS WITH NONTRANSFERRED ARC

Laser-assisted double wire welding with nontransferred arc melts material using an arc between two conveyed wires. Driven by gravity the molten metal drops onto the substrate. A laser beam is oscillated on the melt pool to bond the weld beads to the substrate without undercuts. Claddings at high deposition rates (11.6 kg/h) were performed with 316L on mild steel. The first welding strategy (AAA) is to weld adjacent beads (A) with a varied track spacing of 7 to 9 mm. The second strategy (ABA) consists of beads (A) welded at a distance of 14 to 18 mm from each other, so that a third bead (B) can be deposited in the space between. Claddings with the determined track spacing for AAA of 9 mm and ABA 18 mm were created in order to compare the resulting surface properties. The ABA cladding did not achieve a more uniform surface and less waviness than the AAA cladding.Mechanical Engineerin

A small volume, local shielding gas chamber with low gas consumption for Laser Wire Additive Manufacturing of bigger titanium parts

This paper shows how additive manufacturing of large size titanium parts can be achieved by means of a mobile shielding gas chamber, without the consumption of excessive amounts of shielding gas. While welding, the oversized cover of the chamber can be slid to the sides without opening it. The laser head is only partly inserted into the chamber through the cover. This enables a small sized chamber and allows a quick filling with argon. Since the chamber has a low leakage, only small amounts of argon (5 l/min) are needed to maintain a sufficient welding atmosphere with less than 300 ppm oxygen. For large sized parts, the chamber can be repositioned on the substrate. It has flexible parts which can be fit to the already welded structures that otherwise would prevent the chamber from being put flat on the substrate. The limited build space inside the chamber requires a new welding strategy, which is suggested.Mechanical Engineerin

PROCESS-INTEGRATED ALLOY ADJUSTMENT IN LASER DEPOSITION WELDING WITH TWO WIRES

For Direct Energy Deposition (DED) with wire as filler material, the material selection is mostly limited to commercially available welding wires. This limits the achievable material properties for cladding and Additive Manufacturing purposes. Using a coaxial deposition welding head, in which two different wires can be fed and controlled individually, the alloy composition can be adjusted in the common process zone in-situ. In this study, the two wire materials AISI 316L and ER 70S-6 are used in different mixing ratios to fabricate single weld seams. The different mixing ratios are achieved by varying the wire feed rates. The material content in the weld is varied between 0% and 100% in 20% steps. The weld seams are examined with regard to the distribution of alloying elements, hardness and microstructure. Homogeneous mixing of the two materials was achieved at all mixing ratios. At a content of 40% or more of ER 70S-6 in the weld seam, there was a drastic change in the microstructure and a significant increase in hardness. The microstructure changed from austenitic to ferritic-pearlitic, which was accompanied by an increase in hardness from 170 HV0.1 to 428 HV0.1.Mechanical Engineerin

Influence of Silane-doped Argon Processing Atmosphere on Powder Recycling and Part Properties in L-PBF of Ti-6Al-4V

In the additive manufacturing of metal powders, the residual oxygen in the processing atmosphere plays a crucial role, especially in highly reactive materials like titanium alloys. Besides oxidation of the built parts, it leads to oxygen pick-up into the unmolten powder. Since oxidized particles cannot be removed during recycling, the powder properties deteriorate after multiple uses. In this work, Ti-6Al-4V powder was processed under conventional argon atmosphere (residual oxygen content < 0.01 vol%) as well as silane-doped argon atmosphere (< 0.001 vol% silane in argon). The silane-doping leads to a residual oxygen content of < 10-20 vol%. The powder was sieved and used 5 times for each atmosphere. The powder properties morphology, chemical composition and flowability were analyzed for virgin as well as reused powder. Furthermore, the roughness and relative density of the built parts were evaluated. It was hypothesized that oxygen-free production improves recyclability and thus resource efficiency.Mechanical Engineerin

Towards experimental process parameter development for Ti-6Al-4V TPMS lattice structures with application to small scale dental implants using micrographs

Ti-6Al-4V is a widely used alloy in implant engineering and lattice structures can be applied to locally match the stiffness of the implant to the stiffness of bone. Triply periodic minimal surface (TPMS) structures are popular due to their curved surface, which supports a good manufacturability and osseointegration of the implant. Tests with different TPMS structures showed a strong interaction between design factors and manufacturing parameters resulting in the need for individual parameter development. However, to the best of our knowledge, the most work in the current literature focusses on mechanical and biological examinations of TPMS structures manufactured with standard parameters. As process parameters influence the structural properties, the optimum values for further analysis may not have been investigated (e.g., their influence on microstructure and mechanical properties). In this work, a design of experiments approach is used to develop process parameters. As computer tomography scans are resource intensive for large scale parameter development, a sparser approach using micrographs for porosity analysis is introduced. Small structures with unit cell size as small as 1.0 mm are fabricated on a laser powder bed fusion industrial machine. Our initial studies show that the design factor pore size is negligible in comparison to wall thickness when optimizing internal porosity.Mechanical Engineerin

Combination of Cladding Processes with Subsequent Hot Forming as a New Approach for the Production of Hybrid Components

A new process chain for the manufacturing of load-adapted hybrid components is presented. The "Tailored Forming” process chain consists of a deposition welding process, hot forming, machining and an optional heat treatment. This paper focuses on the combination of laser hot-wire cladding with subsequent hot forming to produce hybrid components. The applicability is investigated for different material combinations and component geometries, e.g. a shaft with a bearing seat or a bevel gear. Austenitic stainless steel AISI 316L and martensitic valve steel AISI HNV3 are used as cladding materials, mild steel AISI 1022M and case hardening steel AISI 5120 are used as base materials. The resulting component properties after laser hot-wire cladding and hot forming such as hardness, microstructure and residual stress state are presented. In the cladding and the heat-affected zone, the hot forming process causes a transformation from a welding microstructure to a fine-grained forming microstructure. Hot forming significantly affects the residual stress state in the cladding the resulting residual stress state depends on the material combination.Mechanical Engineerin