Important Discovery in Aluminum Alloy Laser Coaxial Fusion Additive Manufacturing

Aluminum alloy has unique advantages such as lightweight, high strength, and excellent corrosion resistance, and is highly favored in the aerospace manufacturing field. Laser Coaxial Fusion Additive Manufacturing (LCWAM) adopts beam shaping technology, which uses wire as the deposition material to melt and stack layer by layer. Compared to traditional side axis wire feeding technology, laser coaxial fuse additive manufacturing has advantages such as high deposition rate, high forming flexibility, and consistent front and rear motion forming quality. In addition, coaxial wire feeding has a special optical fiber action form of "light wrapping", which significantly changes the heat transfer form during the additive process. Additionally, aluminum alloys have strong reflectivity and thermal conductivity to lasers. The growth morphology and evolution mechanism of aluminum alloy grains under its special optical fiber action form are not yet clear, which seriously limits the promotion and application of aluminum alloy laser coaxial fuse additive manufacturing.

3D printing technology reference: Professor Zhan Xiaohong's team from Nanjing University of Aeronautics and Astronautics, in collaboration with the Ruike Laser team, recently published an article titled "Mechanism of Column to equiaxed to cylindrical grain transition during wire based directed energy disposal 205 C aluminum alloy utilization a coaxial head: Numerical simulation and experiment" in the Top Journal of Materials Processing Technology in the field of material forming. The article showcases the mechanisms of aluminum alloy laser coaxial fuse additive manufacturing and polycrystalline morphology evolution. Research work. The first author of the paper is Dr. Gao Zhuanni, and the corresponding author is Professor Zhan Xiaohong from Nanjing University of Aeronautics and Astronautics.

Research Strategy
In the process of aluminum alloy laser coaxial fuse additive manufacturing, researchers have discovered a special grain distribution pattern, where equiaxed fine grains are distributed at the bottom of the deposition layer, while layered grains exist at the top of the deposition layer. In addition, researchers have elucidated the formation mechanism of this special grain morphology by conducting macro micro cross scale "thermal microstructure" coupling simulations. The results indicate that the special structure of the light wrapped wire induces a high temperature gradient distribution and relatively mild liquid convection in the middle of the sedimentary layer, as well as a low-temperature liquid boundary layer at the edge of the molten pool, resulting in four types of polycrystalline morphology transformations: layered crystal, equiaxed crystal, equiaxed fine crystal, and columnar crystal.

Research findings
Grain morphology distribution
When the heat input (107.1J/mm) and substrate preheating temperature are low (160 ° C), equiaxed fine grain areas appear at the bottom of the deposition layer. When the heat input is 166.7 J/mm and the substrate preheating temperature is 160 ° C~230 ° C, the microstructure of the deposition layer is mainly composed of top layered crystals, middle equiaxed crystals, and bottom columnar crystals. At low heat input (107.1J/mm), high heat input (194.4J/mm), and high substrate preheating temperature (300 ° C), no layered crystals appeared at the top of the deposition layer. At a heat input of 194.4J/mm, the interior of the sedimentary layer exhibits typical additive microstructure characteristics composed of bottom columnar crystals and top equiaxed crystals.

Figure 1 Microscopic characteristics of sedimentary layer cross-section under different heat inputs

Figure 2 Microscopic characteristics on the cross-section of the deposition layer under different substrate preheating temperatures

Figure 3 Formation mechanism of equiaxed fine crystals and layered crystals

Precipitation phase and element distribution
At a heat input of 107.1J/mm, a small amount of Al2Cu second phase precipitates in the equiaxed fine-grained region, with a volume fraction of 13.75%. At a heat input of 166.7J/mm, the content of precipitated phases in the layered crystal zone is relatively low, with a volume fraction of 6.52%. The sedimentary layer has more second phase precipitates near the grain boundaries from the top downwards, and the volume fraction of precipitated phases at the bottom of the sedimentary layer is 9.66%.

Figure 4 SEM images of sedimentary layers under different heat inputs

Microhardness
The microhardness of the sedimentary layer shows a trend of first decreasing and then increasing from top to bottom, with the highest microhardness at the bottom (88.6 HV1) and the lowest microhardness at the middle and lower parts (72.8 HV1). The microhardness on both sides of the sedimentary layer is relatively low, while the microhardness in the middle is relatively high (79.9HV1). As the heat input increases, the microhardness of the deposited layer gradually increases, reaching its maximum value (85.9 HV1) at 166.7 J/mm heat input, and then decreases as the heat input continues to increase. As the preheating temperature of the substrate increases, the microhardness of the deposited layer shows a trend of first increasing and then decreasing. When the preheating temperature is 230 ° C, the microhardness reaches its maximum value.

Figure 5 Microhardness of Deposited Layer under Different Process Parameters

Temperature field calculation
As the heat input increases, the length of the double ear shaped isothermal surface corresponding to 386.4 ° C gradually decreases, the width gradually increases, and gradually approaches both sides of the substrate. As the preheating temperature of the substrate increases, the size of the 357.8 ° C isothermal surface, which is a semi ellipsoidal shape with a short front and a long back, gradually increases. When the preheating temperature reaches 230 ° C, the shape of the isothermal surface changes to a double ear shape. As the substrate preheating temperature and heat input increase, the peak temperature and solidification rate (R) of the deposition layer increase, while the temperature gradient (G) and cooling rate (G × R) decrease. The increase in heat input leads to a stronger ability of the sedimentary layer to absorb heat, which is the reason for the increase in peak temperature.

Figure 6 Temperature field and isothermal surface distribution in the middle of the deposition process under different process parameters

Figure 7: The peak temperature and solidification parameters of the deposition layer as a function of heat input and substrate preheating temperature

Microscopic organization simulation
The solidification process of the molten pool starts from the melting boundary and advances towards the center of the molten pool as the temperature decreases. As the molten pool continues to cool, the length of columnar grains continues to increase. When the undercooling in the middle of the molten pool increases to a certain extent, equiaxed grains randomly nucleate. As the solidification process progresses, equiaxed crystals continue to grow and push towards the top of the melt until the entire solidification process is completed. When the heat input is low (107.1J/mm), a low-temperature liquid boundary layer appears at the edge of the molten pool during the initial solidification stage. The high melting point metal compound Al3Zr, as a non-uniform nucleation point, participates in the grain nucleation and growth process within the liquid boundary layer, ultimately forming equiaxed fine grain areas at the boundaries of the melt pool. During the subsequent solidification process of the molten pool, the nucleation and growth of the entire deposition layer are completed through CET transformation. In addition, there is an evolution process from columnar crystals to equiaxed crystals and then to layered crystals inside the melt pool. The initial growth stages of columnar and equiaxed crystals exhibit synchronous growth and competition with each other, exhibiting CET transformation characteristics. As the solidification process progresses, the high temperature gradient at the top of the sedimentary layer and relatively mild liquid convection provide an ideal environment for the nucleation and growth of layered crystals. Therefore, during the nucleation and growth process of equiaxed grains in the middle of the sedimentary layer, layered crystals begin to nucleate and grow above the equiaxed grains at the top of the sedimentary layer, and grow parallel to the top of the sedimentary layer in the<110>direction.

At the same solidification time, as the heat input and substrate preheating temperature increase, the cooling rate of the melt decreases, the length of columnar crystals gradually decreases, the growth rate of equiaxed crystals slows down, and the proportion of solid phase area gradually decreases. As the temperature gradient and cooling rate of the melt increase, the primary dendrite spacing (DPr) gradually decreases. During the solidification process, larger dendritic arms grow by consuming smaller dendritic arms, as smaller dendritic arms have a larger specific surface area. The lower the cooling rate during the solidification process, the more sufficient the time for grain coarsening, and the larger the spacing between secondary dendrite arms.

Figure 8 Evolution of grain growth during aluminum alloy laser coaxial fuse additive manufacturing process

Figure 9 Changes in the proportion of equiaxed crystal solid area and columnar crystal length with solidification time under different heat inputs and substrate preheating temperatures

Figure 10 Growth morphology and dendrite spacing of 5-core columnar crystals under different temperature gradients and cooling rates

research conclusion
(1) When the heat input is 166.7J/mm and the substrate preheating temperature is 160 ° C~230 ° C, the deposition layer is mainly composed of columnar crystals at the bottom of the melt pool, equiaxed crystals in the middle and upper parts, and layered crystals at the top. Layered crystals mainly appear at the forefront of columnar or equiaxed crystals, showing polycrystalline growth parallel to the local heat flow direction. At a heat input of 107.1J/mm and a substrate preheating temperature of 160 ° C, equiaxed fine grain areas appeared at the bottom of the deposition layer.

(2) At a heat input of 166.7J/mm, the maximum microhardness value of the deposited layer (85.9HV1) was obtained. The average hardness value of layered crystals located at the top of the sedimentary layer is higher than that of equiaxed crystals, while the average hardness value of equiaxed fine crystals located at the bottom of the sedimentary layer is slightly higher than that of columnar crystals.

(3) The peak temperature and solidification rate of the sedimentary layer increase with the increase of heat input and substrate preheating temperature, while the temperature gradient and cooling rate are the opposite. As the temperature gradient and cooling rate decrease, the primary dendrite arms gradually thicken, and the spacing between primary and secondary dendrites increases. The simulation results indicate that there is a competitive relationship between columnar crystals, equiaxed crystals, and layered crystals.

Source: Yangtze River Delta Laser Alliance