Laser Aided Additive Manufacturing of Amorphous Coatings | Laboratory of Laser Aided Additive and Subtractive Manufacturing

Energy savings has become a very crucial issue that has triggered an extensive research interest throughout the world in developing new harvesting methods as well as to minimize the losses occurred during transmission of the generated electricity. A significant part of the power generation is made by burning fossil fuels that increase the CO2 emissions causing several environmental issues. Hence the efficient usage of energy is not only economical but also environmental friendly. The losses incurred during transmission of the generated power can be greatly minimized by improving the efficiency of the transformers. Even a subtle increase in the efficiency can save billions of dollars that signifies the thrust involved in this research area.

The transformer efficiency can be greatly improved by using suitable soft magnetic material with low coercivity and high saturation magnetization. Amorphous and nano-crystalline materials possess excellent soft magnetic properties compared to currently used grain oriented Fe-Si steels and there has been a great interest to further improve the properties of these materials. Attempts have been made to improve the magnetic properties by different combinations of alloying additions like P, Cu, Zr, and Nb. However, addition of alloying elements not only increases the cost but also reduces the saturation magnetization. It is difficult to obtain a nano crystalline microstructure without these alloying additions due to low nucleation density as well as rapid coarsening of the precipitates that deteriorates the magnetic property. This can be overcome by increasing the nucleation density and restricting the growth of the nano-crystals. LME is a promising route in obtaining very high heating and quenching rates by appropriately choosing the laser parameters. The high heating rate results in increase in nucleation density and fast quenching reduces the growth rate of the crystallites. Thus a suitable microstructure can be obtained by using LME without any alloying additions. This in turn results in improvement of magnetic properties and hence improves the efficiency of the transformers.

It has been reported that laser irradiation of amorphous Fe-Si-B results in selective crystallization of the amorphous matrix resulting in the formation of homogeneous crystalline phase at the edge of the laser irradiated region (Fig. 1a) and partial devitrification (Fig. 1b) takes place at the center of the irradiated region. This is very well explained on the basis of the change in free volume due to thermal stresses experienced at the edge and center of the laser track (Fig. 1c). The generation of different thermal stresses developed along the laser track is explained on the basis of different thermal histories experienced due to Gaussian distribution of laser beam intensity. Under the set of laser processing parameters employed in the current efforts, the resulting homogenous nano-crystalline phase at the edge of the laser track has a crystallite size of (~30 nm) that is within the regime where ferromagnetic exchange interactions dominate impeding the magnetization following the easy axis of the structural unit. These results in a significant decrease in coercivity for laser processed samples compared to conventional annealing (Fig. 1d) as the magnetic anisotropy averages over several structural units and also reduce in magnitude. Thus laser processing can be utilized to achieve precise microstructural changes at the submicron to nano scale levels in the magnetic materials to improve the efficiency of the transformers and savings in energy that consequently likely to reduce the environmental pollution.

**Figure 1:** TEM Bright field image (a) near edge (b) center of the track (c) Schematic of the laser track, (d) Comparison of B-H curves for laser processed and conventional annealed samples

This understanding is further extended to multi-pass laser processing and the magnetic properties of the laser annealed samples are compared with conventionally annealed samples. It is observed that laser annealing resulted in higher nucleation rate compared to furnace annealing (Fig. 2). The high nucleation rate is attributed to annealing at higher temperatures coupled with extremely high cooling rates. Furthermore, partitioning of Si into the α-Fe(Si) phase is altered in laser processed samples due to higher nucleation rate (Fig. 2). The amount of Si partitioning affects the magnetic moment of α-Fe(Si) phase and higher Si content results in lower saturation magnetization. Thus an increase in saturation magnetization in laser processed samples is due to lower Si partitioning. Although an increase in saturation magnetization is observed, higher coercivity values of the laser processed samples can be attributed to higher grain size. Further experiments were performed to reduce the grain size and to obtain a good combination of lower coercivity and higher saturation.

**Figure 2:** Microstructure evolution and composition analysis of multi-pass laser processed sample.

In order to reduce the coercivity, laser processing was performed on a magnetic substrate to increase the heat conduction as well as to alter the diffusion kinetics by the influence of magnetic field. A significant reduction in grain size can be realized for the samples processed on a magnetic substrate that resulted in near zero coercivity values (Fig. 3). Furthermore, a crystallographic texture is observed at the edge of the laser track where as a random texture if observed at the center (Fig. 6). This is attributed to higher annealing temperatures at the center of the laser track compared to the edge region. The higher temperatures resulted in annealing in the paramagnetic regime at the center where as lower temperatures resulted in annealing in ferromagnetic regime at the edge of the laser track resulting in a crystallographic texture.

**Figure 3.** Microstructure evolution in sample processed on a magnetic substrate.

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