Ultrasonic Treatment of Molten Aluminum: Principles, Applications And Prospects

Ultrasonic treatment of molten aluminum: principles, applications and prospects

1. Introduction

Aluminum and its alloys occupy an important position in machinery manufacturing, transportation, aerospace and other fields due to their excellent properties such as high specific strength, corrosion resistance, and easy recycling. The grain size and organizational morphology of aluminum alloys directly affect the final performance of the material. To obtain excellent performance, the key is to break the original large dendrites into small, uniform equiaxed grains. At the same time, the hydrogen dissolved in the aluminum melt is the main cause of defects such as pinholes and pores in castings. How to degas efficiently is also a long-term problem faced by the metallurgical industry.

Traditional treatment methods - such as rotational degassing, adding chemical grain refiners, etc. - although effective to a certain extent, have problems such as efficiency bottlenecks, changing alloy composition, and increasing the difficulty of recycling. The emergence of ultrasonic treatment (UST) technology provides a green and efficient solution for the refining and structure control of aluminum melt.

2. Technical principles

The core of ultrasonic treatment of aluminum melt lies in the synergistic effect of cavitation effect and acoustic flow effect.

(1) Cavitation effect. When high-power ultrasonic waves are introduced into the aluminum melt, periodic alternations of positive and negative pressure will occur in the liquid. In the negative pressure stage, the melt is "torn apart" to form tiny cavitation bubbles; in the positive pressure stage, these bubbles collapse sharply, instantly generating local high temperature, high pressure, and strong shock waves. This cavitation has multiple functions: on the one hand, the shock wave generated by cavitation can break the growing dendrites and rush them into the melt to become a new crystal core; on the other hand, cavitation activates tiny impurity particles (mainly Al₂O₃) in the melt, making them the basis for heterogeneous nucleation, thereby promoting nucleation and refining grains.

(2) Acoustic flow effect. Ultrasonic waves will produce nonlinear effects when propagating in the melt, forming large-scale acoustic flows and microacoustic flows. The powerful acoustic flow can significantly improve the uniformity of the melt temperature field, change the solidification conditions of the melt, and change the solidification mode of the aluminum melt from layer-by-layer solidification to volume solidification, effectively inhibiting the growth of columnar crystals and forming a uniform equiaxed crystal structure. The acoustic flow effect also promotes macroscopic stirring and mass transfer of the melt, making the temperature and chemical composition more uniform.

Under the dual effects of cavitation and acoustic flow, ultrasonic treatment achieves a "package" optimization of the aluminum melt—degassing, slag removal, grain refinement, and tissue homogenization are completed simultaneously.

3. Core Applications

3.1 Degassing (removal of hydrogen)

Hydrogen is the main harmful gas in aluminum melt. Most defects such as pinholes and looseness in castings originate from the precipitation of hydrogen during the solidification process. The mechanism of ultrasonic degassing is that the cavitation effect of ultrasonic waves produces a large number of bubble cores in the melt, and the dissolved hydrogen concentrates in these bubbles and continues to grow until it can be smoothly discharged from the melt.

Ultrasonic degassing has a remarkable effect. Research shows that ultrasonic degassing can reduce hydrogen content by more than 50%, thereby increasing the density and plasticity of castings. In the experiment of AlSi12Fe aluminum melt, after being treated with 1000W ultrasonic power for 1 minute, the density equivalent of 40kg aluminum melt reached 1.28%. Some studies have even shown that the elastic vibration of ultrasonic waves can completely degas the molten metal within a few minutes.

3.2 Grain refinement

Grain refinement is a key way to improve the mechanical properties of aluminum alloys. The mechanism of ultrasonic grain refinement includes the following aspects:

Broken dendrites: The shock waves and jets generated by cavitation can cut off and destroy growing dendrites and increase the number of crystal nuclei.

Promote nucleation: Cavitation activates impurity particles (mainly Al₂O₃) in the melt, making them become crystal cores and promoting nucleation.

Increase the degree of subcooling: Ultrasonic vibration reduces the effective subcooling of the molten metal and reduces the critical nucleation radius, thereby increasing the nucleation rate.

Experimental results show that the solidified structure of aluminum melt treated by ultrasonic treatment has been refined to varying degrees. After ultrasonic treatment is applied to the 7055 aluminum alloy melt, the grains are refined, the structure is homogenized, the tensile strength is significantly increased, and the plasticity is also greatly improved. In pure aluminum experiments, ultrasonic treatment achieved a grain refinement effect of up to 48%.

3.3 Removal of inclusions

It is very difficult for tiny inclusions in the metal solution to float, and only their aggregation will make floating easier. Ultrasonic treatment can promote the accumulation and delamination of inclusions, thereby achieving the removal effect. In addition, cavitation and acoustic flow effects can also reduce component segregation and improve the uniformity of the casting structure.

4. Key process parameters

The effect of ultrasonic treatment is comprehensively affected by multiple process parameters.

Ultrasonic power. Power is the core parameter that determines cavitation intensity and treatment effect. The power of a single radiating head of industrial-grade equipment can reach 2500W. If the power is too low, sufficient cavitation effect cannot be produced, while if the power is too high, it may cause additional energy consumption and thermal effects. AlSi12Fe aluminum melt can achieve good degassing effect by processing it at 1000W power for 1 minute.

Processing temperature. Aluminum melt processing is usually carried out at temperatures around 750°C. Temperature affects the viscosity of the melt and the solubility of hydrogen, which in turn affects the degassing and refinement effects. Some studies have pointed out that as the processing time increases, the melt viscosity decreases and it becomes difficult for bubbles to escape, so the processing time needs to be reasonably controlled.

Processing time. Ultrasonic treatment time usually ranges from 1 minute to tens of minutes. It should be noted that the longer the time, the better the effect - too long ultrasonic time may lead to a decrease in ingot density and degassing rate.

Tool head immersion depth. The working depth of the ultrasonic tool rod affects the effective action area. Theoretical calculations show that the effective cavitation range in the melt is within the 30mm area below the end face, while the acoustic flow effect extends throughout the entire melt flowable area.

Ultrasonic frequency. Frequency affects the size and distribution of cavitation bubbles. Low-frequency ultrasound produces larger cavitation bubbles, which are suitable for applications such as degassing and grain refinement. The response of different aluminum alloy systems to frequency may be different and needs to be optimized for the specific alloy.

Effect and value

The comprehensive improvement effect of ultrasonic treatment on aluminum melt has been confirmed by a large number of studies:

Improved mechanical properties: After ultrasonic treatment of pure aluminum, the tensile strength can be increased by up to 17%, and the elongation is increased from 16% in the untreated state to 28%. The average tensile strength of pure aluminum ingots increases by 22.3% longitudinally and 20% transversely.

The structure is significantly refined: the solidified structure of the ingot treated by ultrasonic treatment is significantly refined, and the grain size can reach 77~405μm. The ingot without ultrasonic treatment formed a coarse and uneven microstructure.

Multifunctional integration: Ultrasonic treatment is a multifunctional melt processing technology that integrates degassing, slag removal, grain refinement, and tissue homogenization. Compared with traditional chemical grain refiners, ultrasonic treatment does not change the chemical composition of the alloy and does not increase the difficulty of recycling. It is a sustainable green processing method.

Ultrasonic treatment of metal aluminum melt is an efficient melt treatment technology that integrates degassing, slag removal, grain refinement, and tissue homogenization. Its core lies in the synergistic effect of ultrasonic cavitation effect and acoustic flow effect - the cavitation effect breaks dendrites, promotes nucleation, and expels gas, while the acoustic flow effect uniforms the temperature field and composition field, and changes the solidification method. A large number of studies have shown that ultrasonic treatment can significantly improve the mechanical properties and casting quality of aluminum alloys without changing the chemical composition of the alloy. It is a green and sustainable processing technology. With the deepening of technological understanding and the improvement of equipment level, ultrasonic aluminum melt processing technology will play an increasingly important role in aerospace, transportation, recycled aluminum and other fields.

Parameter	influence	reference
Power	Determine the accuracy and processing depth	The power ranges from 200W to over 2500W. For example, for 7050 aluminum alloy, the refinement effect is better at 200W, and the degassing efficiency is higher at 240W.
Frefuency	Affect the intensity of cavitation effect	Generally, the best results are achieved within the low-frequency range of 17-22 kHz, especially for the low-frequency and high-intensity ultrasonic waves in the 15-20 kHz range.
Process time	There exists an optimal value.	Too short a time will not achieve good results, while too long a time may lead to grain coarsening or a rebound in porosity. For example, the 7050 alloy works best when treated for 90 seconds.
Melt temperature	Affecting melt viscosity, hydrogen solubility and cavitation intensity	There is an optimal temperature range, and an appropriate medium temperature is conducive to improving the processing effect.
Melt volume	The power must be matched to the volume of the melt.	For large-scale molten materials, multiple probes or higher power may be required, or even non-contact ultrasonic technology may be employed to ensure the processing outcome.
Horn size / shape	Affecting the distribution of the ultrasonic field	Tools with smaller end areas (such as Φ10mm) have a better refinement effect.