1 Introduction

Optical fibers play an important role for sensing in harsh environments. They are resistant to electromagnetic interference, electrically passive and have multi-physical sensing capability [1]. They are flexible and compact and, therefore, they can be deployed near functional surfaces [2]. Applications comprise temperature, strain, humidity, and moisture measurement [3].

Laser metal deposition (LMD) is an additive manufacturing technology that allows welding onto existing metal parts [4]. It is a direct energy deposition technique. With an LMD system, two metallic pieces may also be welded together [5]. LMD is also used for improving functional surfaces by adding a metal layer [6]. Therefore, integrating optical fibers with LMD may make additional manufacturing steps for smart functional surfaces obsolete.

2 State of the art

Various approaches to embed electronics and sensors in metal parts have been proposed. Using LMD, Ostolaza et al. demonstrate embedding of mineral insulated copper cables into tool steel [7]. Petrat et al. [8] determine parameter combinations to embed nickel wires into chrome-nickel steel using LMD. Feldhausen et al. [9] encapsulate the ceramic components in steel. Werkle et al. [10] investigate effects of LMD process parameters on ceramic insulating shield. Optical fibers have been embedded with non-LMD technologies. Petrie et al. [11] embed copper-coated optical fibers in nickel layers on top of stainless steel substrate via ultra-sonic additive manufacturing.

Similarly, Hyer et al. [12] confine a functional fiber optic sensor in a stainless steel component using ultra-sonic additive manufacturing for measuring temperature and strain. Havermann et al. [13] use selective laser melting technology to embed optical fibers in stainless steel and demonstrate the ability of gratings to work as sensors for measuring temperature and strain in the harsh environment. Li et al. [14] propose a methodology to embed Fiber Bragg Grating (FBG) sensors in steel using laser-assisted shape deposition manufacturing. Maier et al. [15] employ powder bed-based additive layer manufacturing for embedding optical fiber sensors.

3 Objective

This work presents a novel method to embed fiber optical cables closely beneath metal surfaces of metal objects using laser metal deposition without affecting the light transmission properties of the fibers. For testing, sheet metal parts are employed.

4 Methodology

First, a metal sheet is laser-cut. Next, a rectangular groove is milled for placing the fiber optic cable in its center. The plastic-coated fiber to be embedded is sheathed with the ceramic sleeve for heat protection. Side walls for the optical fiber channel are welded onto the metal sheet. The fiber is then placed into the cavity between the two fabricated walls. A metal strip is placed on top of the fiber to protect it further. A single layer is printed with a scan speed of 0.6 m/min in order to minimize heat exposure. A purposeful delay of five minutes is introduced before printing a second layer. Roof wall is composed of three layers on the metal side by side (see Fig. 1).

Fig. 1
figure 1

Illustration of the experiment

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5 Experimental setup

The method is tested with a Trumpf TruLaser Cell 3000 and Trumpf Laser TruDisk 4001. Martensitic high chromium stainless steel powder is used for deposition. The experiment is conducted on S235JRC (1.0122) sheet metal using a single mode fiber G.657.A2, protected through the ceramic fibers’ sleeves (Item no. 080-0500, 4AS.044, Final Advanced Materials GmbH). The ceramic sleeve has a diameter of 1.6 mm and capable to withhold maximum continuous temperature of 1100 °C. A 2 mm nozzle that maintains 16 mm height from the substrate is applied. The roof walls and the side walls are deposited to encase the optical cable in metal completely. The process parameters for LMD are summarized in Table 1.

Table 1 Process parameters of the experiment
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After completing the encasing process through LMD (see Fig. 2), the functionality of the optical fiber is examined by testing the continuity of laser light being lit at one end and visible at the other end. Additionally, microcomputed tomographic (micro-CT) images of the optical fiber cable are analyzed to observe the effects of LMD on the fiber.

Fig. 2
figure 2

Fiber optic cable embedded in the metal sheet

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6 Result

The fiber optic cable maintains its fundamental light propagation functionality after being confined inside the metal. The metal sheet distorts by 15 degrees due to the continuous excessive heat in the center. The height of the welded surface from substrate is 3 mm and the width of the roof wall is 9 mm (see Fig. 1). The surface is rough and would need post-processing for the finish which has not been conducted for this test. LMD leaves visible burn marks on the ceramic sleeve. Figure 3a, b displays micro-CT images before and after the welding process. The two circles in the center are the fiber. Beyond a small empty area, the first PVC sleeve is visible. It is surrounded by protective fibers and not symmetrically aligned to the outermost PVC sleeve. Outside, the ceramic sleeve is visible. In Fig. 3b, metal particles as end points of visible lines of molten plastic can be observed. Despite all protective layers are affected by the welding heat, the optical fiber keeps its ability to propagate light. Visually inspecting the 3D data, no fiber ruptures are visible.

Fig. 3
figure 3

Microcomputed tomographic images of optical fiber cable a before and b after welding

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7 Discussion

Results show that with LMD, optic cables can be welded closely beneath sheet metal surfaces. The fiber is tightly enclosed inside the metal. It is protected from being directly exposed to the melt pool by inserting a metallic strip. This preserves the optical transmission capability of fibers. Nevertheless, the multiple surrounding sheaths keep the fiber intact. Since LMD parameter settings have not been optimize, results may be improved in future steps.

8 Conclusion and outlook

This work presents a method to embed fiber optic sensors beneath the functional surfaces of metal with LMD. It enables LMD to embed fiber optic-based sensors in the metal. However, further research is required to determine if the proposed welding process maintains the intactness and affects further optical characteristics of the fiber. In a next step, optimal LMD process parameters and long-term effects on the fibers should be investigated along with the sensitivity of the fibers to temperature and strain after LMD.