Effect of Interlayer Composition on the Properties of Laser-Directed-Energy-Deposition-Based Additively Manufactured Copper-Stainless Steel Wall Structures

Laser additive manufacturing (LAM) is a significant subset of additive manufacturing (AM) technologies that plays a pivotal role in creating components with minimal material wastage, thereby contributing to sustainability efforts [1]. The sustainability aspect of LAM comes from the fabrication methodology, which uses the digital data of a part to selectively add the material as per the required geometry using layer-wise building methodology. The above methodology minimizes/eliminates the need for machining of the components, leading to minimal waste generation. LAM is generally categorized into two primary methods: laser powder bed fusion (LPBF) and laser-directed energy deposition (LDED). The key distinction between them lies in how they feed the raw material, which can be in the form of either powder or wire. In addition, LAM deploys a high-power laser beam as a heating source to melt the feedstock material. LPBF finds application in building highly complex parts using fine powders and lasers with beam diameters in micron scale. Out of the two LAM techniques, LDED is used for building multi-material components due to its uniqueness in a combination of material and shape design freedom [1,2]. In addition, conventional methods employed for achieving spatial variations in compositions or structures include power metallurgy, vapor deposition, centrifugal casting, and welding. These traditional techniques are associated with challenges such as slower production rates, lower bond strength, limited geometrical complexity, and restricted material versatility [1]. LDED accomplishes this multi-material capacity through two significant approaches. Firstly, it employs pre-mixed powder feeding—in which various powders, each representing a specific material—are simultaneously introduced into the melt-pool. This approach facilitates the creation of components with gradient properties, known as functionally graded materials (FGM), which exhibit varying material compositions at different locations within the component to meet specific performance requirements [3,4]. Secondly, by employing a multi-feeder system, LDED can produce multi-material components by utilizing multiple feeders, each delivering a different material. These feeders operate at predetermined rates, offering precise control over the material composition at various points within the component. This method can be particularly valuable in cases where materials need to be strategically positioned for optimal performance [5]. LDED using powders has wide applications due to its geometry control, precision, and easy process control. Numerous studies have been conducted to demonstrate the capability of LDED for building FGM or multi-material components [3,4,5,6,7,8,9].

The fabrication of multi-material components aids in joining materials with a significant difference in thermo-physical properties and materials that form intermetallics/brittle precipitates at the interface [10]. In addition, the multi-material components develop properties that are different from the individual materials aiding in tailored structures, properties, and performance [11]. Out of various combinations of multi-material, FGM or multi-material of copper (Cu)–stainless steel (SS) stands out as a significant and ongoing research area with diverse applications, including tooling, cryogenic systems, power generation, and heat transfer industries [12]. Cu-SS FGMs or multi-material structures are meticulously engineered to leverage the advantageous thermal conductivity of Cu in combination with the superior strength and corrosion resistance of SS within a single component. However, the production of Cu-SS FGMs or multi-material components presents substantial challenges owing to the pronounced disparities in thermo-physical properties—such as thermal expansion coefficient, melting point temperature, and thermal conductivity—between Cu and SS. Additionally, the limited solubility of Cu and Fe in each other even at higher temperatures increased the complexity of building Cu-SS structure using LDED. It may be noted that the solubility of Cu and Fe with each other is poor (maximum Cu solubility in Fe is about 10 wt. % even at higher temperatures (i.e., ~1550 K), whereas the solubility of Fe in Cu is less than 5 wt. % at 1550 K [13,14].These above-mentioned challenges often result in the formation of solidification cracks and porosity at the interface of LDED-built direct Cu-SS joints [6,15,16,17]. One of the techniques employed to address these issues stemming from abrupt interfaces is the creation of FGMs, which offer a more gradual transition in material properties.

The literature highlights the complexity of fabricating Cu-SS joints (FGM and direct joining) through AM and welding, with the outcome influenced by the Cu-SS composition and processing parameters [15,16,18]. The investigation carried out by Noecker II et al. involved tungsten arc welding of tool steel–Cu composites, and it was observed that as the concentration of Cu decreased, there was an increased vulnerability to the crack formation during the solidification process [17]. This susceptibility varied from low to high as the Cu weight percentages changed from 100% to 50% in the Cu-SS systems [17]. In a similar manner, Articek et al. underscored the significance of having sufficient terminal Cu available during the solidification process. They observed higher tensile strength in Cu–tool steel components deposited using LDED with various compositions, in contrast to pure samples. Their findings led to the conclusion that issues related to porosity and crack formation primarily result from suboptimal process parameters [15]. Zhang et al. incorporated a layer of a nickel-based superalloy between them as a solution to address these challenges, allowing for the successful production of multi-material Cu–tool steel injection molds using LDED [16]. In another study, Osipovich et al. showcased the potential of wire-feed based AM, particularly through a twin-feeder system, for creating transition joints in Cu-SS materials [19]. Furthermore, in one of our recent research endeavors, we built bulk Cu-SS FGM with varying grading percentages, employing specific composition-dependent parameters [12]. This investigation revealed that when employing multi-track and multi-layer depositions, which involve repeated thermal cycles, liquation cracking tends to occur in regions with lower Cu compositions [12]. One effective approach to mitigate liquation cracking is to minimize the number of thermal cycles by constructing wall structures. Additionally, Cu-SS wall structures offer the potential to replace traditionally manufactured stainless steel fins, thereby improving cooling efficiency and reducing fin dimensions. This enhancement in cooling performance and size reduction of fins contributes to the development of radiators, chillers, condensers, and more. Furthermore, in terms of sustainability, LDED is one of the techniques that can be used for building thin-walled components with minimal wastage and reduced distortion as compared to subtractive manufacturing techniques [20].

LDED of FG Cu-SS wall structures is conducted using an in-house-developed LDED system, as shown in Figure 1(a,i) [12], and the flow chart shown in Figure 1(a,ii) reveals the experimental procedure. For the experiments, commercially available SS 304L powder and Cu powder are employed. SS 304L steel grade is used in the present investigation, considering the better weldability of SS304L along with its application in heat transfer equipment such as heat sinks, tubes-pipes, injection moulds, heat exchangers, dissimilar joints, etc., in power generation, the tooling industry, the aerospace sector, the medical industry, etc. [3,12]. Before the deposition process begins, the Cu and SS powders are meticulously blended with various compositions, denoted as Cu _X SS _100-X, with “X” taking on values of 20, 40, 50, 60, and 80. This blending is carried out thoroughly for a duration of 3 h at a rotational speed of 70 revolutions per minute utilizing a specialized powder-mixing machine.

Full factorial experiments are carried out by varying laser power (600–1800 W), scan speed (0.3–0.7 m/s) and powder feed rate (4–8 g/min) at three-level each for depositing tracks and wall structures of Cu-SS mix composition [12]. Furthermore, Cu-SS FGM wall structures are deposited as per Figure 1b at the identified process parameter (combination of laser power, scan speed, and powder feed rate) as mentioned in Table 1 and are used for depositing individual blended compositions of Cu-SS [12,21]. Subsequently, LDED-deposited wall structures are sectioned perpendicular to the scan-direction using wire EDM, mounted, and polished by following standard metallurgical guidelines. Chemical etching is conducted using a blended solution composed of 5 g of ferric chloride, 25 mL of hydrochloric acid, and 100 mL of ethanol. Optical microscopy, utilizing the LEICA DM 2700M model, is employed to conduct a qualitative examination of structural integrity and the detection of cracks. In parallel, scanning electron microscopy (SEM) equipped with energy-dispersive spectroscopy (EDS), featuring the Carl Zeiss Sigma model, is utilized for elemental mapping and fractographic analysis. Vickers micro-hardness measurement is carried out at a load of 100 gm for a dwell time of 10 s (make: Omnitech, model: MVH-S Auto). Single-cycle automated ball indentation (ABI) is performed to measure the energy storage capability of the material with respect to different Cu-SS compositions. ABI is performed using a spherical hardened tool steel ball indenter with a diameter of 1 mm (make: Biss). ABI test is carried out for a loading cycle of 50 N load and an unloading cycle up to 5 N load with a pre-load of 5 N. The area under the curve is estimated to measure the energy absorption capability of the material. To assess the strength of the Cu-SS joint, micro-tensile testing is conducted using a universal testing machine with a loading capacity of 5 kN (Manufacturer: SDAtlas, Model: H5kL) at a strain rate of 0.001 s⁻¹. Micro-tensile samples are sub-size specimens prepared by following the standard mechanical tensile testing methodology available in the literature, as shown in Figure 1c [12]. The tensile specimen is designed in such a way that all the interfaces and graded regions lie within the gauge length.