Hybrid manufacturing, which, e.g., combines additive manufacturing with conventional machining processes, can be a way of overcoming limitations currently encountered in additive manufacturing. Cryogenic milling might be a viable option for hard-tocut materials, as it leaves a contamination-free surface and can increase surface quality and tool life compared to conventional cooling concepts. In this study, the influence of cryogenic milling with carbon dioxide on titanium Ti-6Al-4V specimens manufactured with laser metal deposition (LMD) was investigated regarding tool wear and surface integrity in comparison to dry machining and machining with cooling lubricants. Moreover, additional layers of material were deposited on top of conventionally and cryogenically machined surfaces by means of LMD. The interface zone was then examined for defects. The milling process was closely monitored by means of thermal and high-speed imaging. Optical and tactile surface analysis provided evidence that lower roughness values and improved surface qualities could be obtained with cryogenic machining in comparison to dry machining. Moreover, significantly less tool wear was observed when a cryogenic cooling medium was applied. Although the utilization of conventional cooling lubricants resulted in satisfying surface qualities, substantial residual contamination on the milled surface was detected by means of fluorescence analysis. These contaminants are suspected to cause defects when the next layer of material is deposited. This is supported by the fact that pores were found in the weld bead applied on top of the milled specimens by means of LMD. Conversely, cryogenic machining resulted in very clean surfaces due to the residue-free evaporation of the coolant. Hence, a good metallurgical bonding between the weld bead and the milled substrate could be achieved. The results indicate the great potential of cryogenic milling in hybrid manufacturing, especially in terms of intermediate machining, as it provides residue-free surfaces for subsequent material deposition without an additional cleaning step and can significantly prolongate tool life.
Laser additive manufacturing is state‐of‐the‐art and regarded as a key technology in the future of production engineering [1]. Originating from a computer model, a new freedom of design can be achieved by simply adding layer by layer. Beyond the new perspectives and extended possibilities of conventional processes there are fields of application which already have been opened up. One of those is the highly innovative but demanding field of processing with heat resisting alloys (see Fig. 1). This field of engineering has traditionally been a door opener for hightech technologies such as aerospace or energy [2, 3].
The Advanced Telescope for High-ENergy Astrophysics (ATHENA) will observe “the hot and energetic universe,” which was determined as one of the most urgent scientific topics for a major future space mission by The European Space Agency (ESA). One of its three main components is the optical bench, a monolithic titanium structure that accommodates 678 mirror modules and keeps them accurately aligned. The immense but slender structure in the range of 2.5–3 m diameter at a height of 300 mm proves a challenge to manufacturing. A hybrid robot cell is developed using additive buildup via laser welding, combined with high-performance machining and the state of the art process and metrology monitoring and control. The present work focuses on the shielding of the laser induced melt pool, a key concern when processing titanium. The sensitive metal with unusual low heat conductivity requires a large area of high purity atmosphere to prevent embrittlement. However, the large hybrid system prohibits the use of a sealed enclosure, and therefore, a local shielding system is developed for the challenging case of the ATHENA optical bench’s hollow-chamber design. Since the present thin wall design poses a worst-case scenario in terms of heat dissipation and shielding flow for the shielding system, its effectiveness here can be applied to most other geometries enabling the flexibility for lot size one. The key features of the novel approach are the prevention of turbulence while keeping operation economical despite the large shielding area. The first is achieved by means of an integrated honeycomb screen and the latter by employing a layered flow with a higher velocity outer curtain and an air deflecting coflow. This system was numerically optimized, tested, and effectiveness proven by means of visual inspection, microstructural analysis, and measurement of material properties.
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