Research on the microstructure and properties of Al-Mg-Si alloy with different amounts of B element

The microstructure and properties of Al-Mg-Si alloy with different contents of B element were studied through microstructure, mechanical properties, and conductivity. The results showed that after adding the B element, the precipitates tended to precipitate along grain boundaries. As the amount of the B element increased, the grain coursing was severe, and the tensile strength was slightly improved. However, the impact on hardness was not significant, and the yield strength ratio showed a downward trend. When the addition amount of B element was 0.02wt.%, the Al-Mg-Si alloy had the best conductivity. Therefore, by comprehensive consideration of the conductivity and mechanical properties, the optimal addition amount of B was 0.02 wt.%.


Introduction
Al-Mg-Si alloy has the advantages of high conductivity, good corrosion resistance, and low density, which makes it an ideal material for overhead transmission lines.The materials of overhead conductors must meet the dual requirements of conductivity and mechanical properties.There are many strengthening methods for aluminum-based materials, including alloying, aging treatment, and plastic deformation, etc., but those methods will reduce the conductivity of aluminum materials to varying degrees.For example, the solid solution treatment causes solute atoms to enter the lattice, resulting in lattice distortion and enhanced electron scattering effect.Aging treatment causes the matrix to precipitate a second phase, blocking the directional movement of electrons.Plastic deformation could refine the grain size and increase quantity, improving the material strength, but hindering electron transmission [1,2] .As a result, the current research focuses on how to choose a strengthening method to maximize strength while maintaining conductivity [3,4] .
In this article, the Al-Mg-Si alloy with different amounts of B element was designed, which was expected to improve both electrical and mechanical performance simultaneously and obtain the optimal B element addition ratio.

Experimental details
The alloy with added B elements was referred to as ALB alloy, while the alloy without added alloying elements was referred to as Al alloy.The ALB alloy with different contents of element B was designed.The sample A-0 did not add the element B for comparative analysis.Based on the design chemical composition, ALB alloy ingots were prepared by using the resistance melting method.A resistance melting furnace was used for melting work.The starting materials were simple substances, including pure Al, pure Mg, pure Si, boron, and auxiliary materials.In the first step, the Al and Si elements were heated and melted.In the next step, pure Mg and boron were heated and melted.The melt was stirred every 3 minutes (a total of 15 minutes).After melting, the protective gas was added to the furnace chamber and further stirred to achieve complete alloying.After slagging, it was insulated for 40 minutes.After that, the melt was cast into cylindrical ingots.Each ingot was subjected to solution treatment at 530°C for 1.5 h, followed by water cooling, and finally aged at 170°C for 5 h.
The tensile tests were carried out using a tensile testing machine.The hardness tests were conducted by a hardness tester.The electrical performance tests were carried out by using a digital eddy current conductivity meter.
The direct reading spectrometer was used to detect the chemical composition, and the result is shown in Table 1.The microstructure of the alloys was observed by optical microscopy (BX53M, Olympus).Scanning electron microscopy was equipped with energy-dispersive X-ray spectroscopy.

chemical compositions
The content of the alloy elements was a little different from the design.However, this difference had no significant impact on mechanical and electrical properties.At the same time, the Fe element was detected in the ingot, and Fe was a major impurity element.Because the melting and casting tools used in industrial production were all made of steel or iron, they were inevitable to introduce the Fe element into the molten aluminum [5] .Although Fe was passively introduced, it was not a harmful element.Based on the phase diagram of the Al-Fe binary alloy, the solid solubility of the iron element in the α-Al matrix was very low, which was only 0.05% at the eutectic temperature.The content of element Fe would affect the grain size of the alloy when performing solid solution treatment on the alloy.Within a certain range, the grain size of AL alloy decreased with the increase of Fe content during solution treatment, due to the Fe-rich phase particles playing a promoting role in the nucleation process of the recrystallization.In addition, element Fe would form an Al-Fe-Si phase with the excess Si in the aluminum matrix.The above precipitates were usually coarse, which could strengthen the aluminum matrix, significantly improve the strength and hardness of the aluminum alloy and reduce the plasticity of the aluminum alloy.The presence of the element Fe did not have a significant impact on the conductivity of aluminum alloys.As the content increased, the conductivity tended to a state of stability.The conductivity of aluminum alloys could be improved by adjusting the content of Fe, Si and their ratio (Fe/Si) appropriately.

The microstructure of ALB alloy
Fig. 1 was the metallographic microstructure of the ALB alloys.The grain size of AL alloy was 30 μm to 50 μm and the precipitated phases existed both inside the grain and at the grain boundaries.After 0.02% B was added to the AL alloy, there was no significant change in the grain size and a decrease in precipitation phases within the grains.As the content of B increased, the grain size of the alloy increased, the amount of the precipitates at grain boundaries increased, and the grain boundaries coarsened.
(g) AL-0.13B(h) AL-0.15BFig. 1 Metallographic microstructure of the ALB alloys Fig. 2 was the back scattered-electron images of the Al-Mg-Si and Al-Mg-Si-0.02Balloys.The microstructure was composed of a matrix (dark area) and a small amount of precipitated phases (bright area).The precipitated phases presented both within and at grain boundaries, and the precipitated phases within the grain were spherical or short rod-shaped, with a size of 1 μm to 10 μm.The precipitates at grain boundaries exhibited a long strip-like morphology.The microstructure was composed of a matrix (dark area) and a small amount of precipitates (bright area).Compared with Al-Mg-Si alloy, the number of precipitates increased which mainly precipitated along grain boundaries, resulting in coarsening of grain boundaries.The precipitated phases also presented both within and at grain boundaries, and the precipitated phases within the grain were spherical or short rod-shaped with a size of 1 μm to10 μm.The precipitates at the grain boundaries exhibited a long strip-like morphology.  2 shows the energy spectrum analysis results of the feature areas in Fig. 2. It can be seen that the types of the precipitated phase were the same before and after adding element B. The precipitated phase containing Mg existed within the grain in the form of a round ball or short rod.The precipitation phase of element Fe was in the form of long strips or rods, existing within the grains or at grain boundaries.The element Si was detected in the precipitated phases both within and at the grain boundaries.The element Si had the largest electronegativity, so it was easier to form a precipitated phase with other elements.
Therefore, it could be seen that after adding element B, the type of the precipitates was unchanged, but the aggregation state of precipitates changed, and the precipitates were more inclined to precipitate along grain boundaries.As the B atoms were relatively light and had a small amount of addition, no precipitate containing element B was detected.3 shows the electrical conductivity curve, and Fig. 4 shows the tensile strength and Vickers hardness test curves.When the addition amount of element B was 0.02 wt.%, the AL alloy had the best conductivity.As the content of element B increased, the conductivity of the alloy showed a slight downward trend.It was due to the precipitates segregated toward grain boundaries, the gains became coarsened after the addition of element B, which could enhance the scattering and absorption of the free electrons in the matrix.It harmed the conductivity of the alloy.After adding element B, the tensile strength of the alloy slightly increased compared to the alloy without element B. The addition of B element had a slight effect on the hardness of the alloy and even reduced the hardness.Fig. 5 shows the yield ratio curves of ALB alloys.The yield ratio of the alloy without element B was the highest.As the content of element B increased, the yield ratio showed a downward trend.It was due to the significant increase in tensile strength, while the yield strength only slightly increased after adding element B, which resulted in a decrease in the yield strength ratio.The higher the yield ratio was, the better the processing performance of the material was.Therefore, the AL alloy ingot required a higher yield ratio for subsequent extrusion and drawing work.Considering the conductivity and mechanical properties of the ALB alloy, when the addition of B was 0.02%, the alloy exhibited good comprehensive properties.Fig. 5 Yield ratio curves of ALB alloys 4. Conclusions 1) After adding element B, the precipitates tended to precipitate along grain boundaries.As the amount of B element increased, the grain coursing was severe.
2) When the amount of B element was 0.02wt.%, the Al-Mg-Si alloy had the best conductivity.
3) It was recommended to add 0.02wt.%B to the Al-Mg-Si alloy as the alloy exhibited good comprehensive properties.
AL-0.02B Fig. 2 BSD images of the ALB alloys Table

Table 1
Chemical compositions of the ALB alloys (wt.%)