Brazing: Microstructural Characterization of a Motor Armature Join

Brazing is crucial in manufacturing reinforcement, as brazed joints must withstand significant loads and stresses resulting from their principal advantage of joining dissimilar materials with the slightest geometrical or microstructural modification. This results in better bonding performance and excellent structural rigidity and integrity. The automotive, aerospace, medical, and energy industries, among others, have increased their brazing applications because of such advantages. Brazing is a subcategory of welding processes where the materials are heated above the filler material´s melting point. A filler material has a characteristic liquidus temperature around 450 °C, which must be lower than the solidus temperature of the base materials. Such a difference allows the filler to melt and penetrate the base material, producing a strong joint upon solidification. Joining involves phenomena such as surface wetting, interdiffusion, and possibly some chemical reactions. Flow and spread of the filler occur mainly by capillarity and surface wetting, including significant diffusion of different chemical elements between both materials. The nature of the filler and base materials determines the atomic bonding between them, i.e. metallic, ionic, covalent, or mixed [1-4]. A heat input uniformly distributed and a controlled process time are the most critical brazing variables to avoid corrosion and oxidation of the joined component's surface, they also allow a close control of the final gran size [5]. Cu–P brazing alloys are broadly used in the automotive industry. A ternary alloy filler (Sil-Fos 15) with a chemical composition of 80 Cu – 15 Ag – 5 P (wt.%) and a melting range of 645-800 °C was employed in this research. This composition shows ductility because of the silver addition, whereas increased fluidity and fluxing/deoxidant characteristics are promoted by the presence of phosphorus. Moreover, P diminishes the alloy´s melting point due to the formation of the Cu3P eutectic (eutectic temperature of 714 °C). Finally, a larger amount of copper reduces the alloy´s melting temperature and promotes solid solution avoiding the formation of brittle phases [6]. Brazing studies usually report the microstructure of braze joints using sophisticated characterization equipment. This study returns to the elemental metallographic characterization to exhibit the microstructures of the 80 Cu – 15 Ag – 5 P alloy containing the Ag + Cu phases and the Cu3P eutectic. The microstructure was characterized by using a 970F Keyence optical microscope. The difference between the proportions of these phases and those present in the filler metal can be accounted for based on P loss due to self-fluxing during the brazing process. A metallographic study derived from the brazing welding of an electric motor armature is presented in this document. The joint fabrication involves a dual welding process, i.e. i) commutator (Cu)-filler (Sil-Fos 15)-conductor (Cu) and ii) conductor (Cu)-filler (Sil-Fos 15)-conductor Cu). Figure 1 clarifies the diagram of the armature sandwich welding joint assembly. The welding procedure is performed at 450 °C (842 °F), with a welding time of 320 milliseconds and an electrical current intensity of 5000 A. Additionally, a negative electrode force of 90 lbf and a positive electrode force of 50 lbf are applied, as detailed in Table 1. Process parameters for the armature welding-brazing process. Process parameters for the armature welding-brazing process. Table 2 shows and describes the reagents and times employed for the metallographic analysis of the samples. Etchants and conditions employed for etching the welded joints of copper- Sil-Fos 15 filler. Etchants and conditions employed for etching the welded joints of copper- Sil-Fos 15 filler. Figure 2 shows that a reduced space facilitates a greater accumulation of particles formed by the movement of copper atoms from the base metal towards the filler material. In a confined space between the base metal and the filler, there is less room for copper atoms to move, influencing the formation of a more homogeneous and stronger joint [7, 8]. Figure 3 revealed the formation of globular phases in both the conductor and the Sil-Fos 15 filler material. In the latter, the presence of a second phase resembling dendritic growth was also observed. This process plays a crucial role in the formation of a robust and enduring welding union. It is noteworthy that the formation of the globular phases occurs due to thermal energy and diffusion phenomena involved in the process, which enable the movement of atoms facilitating the effective integration of the base and filler materials. The gap between the base metal and the filler metal in this process influences the development of globular phases. A smaller gap results in the merging of these spherical phases, creating a unified and homogeneous phase. Conversely, a larger gap leads to a noticeable separation between these particles embedded within a matrix. Figure 4 displays a particle size analysis performed in the filler metal matrix, a mean particle size of 2.5 µm for the formed phase was found. Figure 5 displays the particle size distribution found for the particles within the metal base matrix. A bimodal particle size distribution was observed suggesting that the formation and growth of such particles occur at different rates. The metallography reveals a consistently fused and well-unified weld interface, demonstrating an even distribution of grains across the thermally affected zone. The absence of porosities, cracks, inclusions, or any other irregularities indicates a robust weld. Additionally, a gradual transition between the molten zone and the base metal was observed, showing no signs of segregation or sudden alterations in the structure. This study presented evidence of the formation of second phase particles in both the copper base alloy and the Sil-Fos 15 filler. The formation of a well-balanced microstructure in the latter, including an evenly distributed second phase, may be positive as such phase can act as a reinforcement, improving the mechanical properties of the welding union. In this investigation, metallography played a crucial role as a fundamental tool for examining the microstructures derived from the brazing-welding process employed to join an electric motor armature. This technique allowed for a precise assessment of eutectic phase formation, the uniformity of the joint, and the presence of potential discontinuities. Schematic of the armature sandwich assembly. The armature conductor includes a Sil-Fos 15 clip as a filler material. A. The micrograph shows that when the available space is reduced, there is an increased accumulation of second phase particle in the filler material. Image captured at 2000x magnification. B. Second phase particles observed in both the copper base material and the Sil-Fos 15 filler material, image captured at 2000x magnification. Size distribution of globular particles within the filler material matrix. Size distribution of globular particles within metal base matrix.