With rapid development of 3C industry, demand for aluminum alloy materials in the global market continues to increase, requirements for appearance quality and corrosion resistance of aluminum alloy products are also getting higher and higher. At present, aluminum alloy structural parts and decorative parts in 3C field on the market, such as mobile phone casings, notebook computer panels and other products, mainly adopt traditional semi-continuous casting → ingot homogenization → extrusion/rolling → machining, and then anodized. Cycle is long and cost is high, which obviously restricts further application of aluminum alloy materials in this field. Therefore, there is an urgent need to develop low-cost short-flow production processes. Die casting has characteristics of high production efficiency, near-net forming, high yield, and low production cost. It is widely used in mass industrial production of complex thin-walled precision castings of aluminum alloys for 3C. In order to obtain different color and surface quality, it is usually necessary to anodize it. Main factors affecting effect of anodic oxidation are: alloying elements, grain size, type of second phase, shape and size, and anodic oxidation process. Si content in traditional die-casting aluminum alloys is usually greater than 9% for good casting properties, and Fe content is greater than 7% for good mold release properties. Al-Si series cast aluminum alloys such as ADC12, which are currently widely used, have good die-casting performance, but Si element is not easily oxidized by anodization, impurity Si will turn gray on the surface of oxide film, even form defects such as black spots or black lines, which will adversely affect appearance quality; Cu element can improve strength and thermal stability of alloy, but it will make oxide film appear red and increase defects of oxide film; Fe element will cause black spots and other defects on the surface of alloy after anodic oxidation.
With rapid growth of demand for anodized aluminum alloys for 3C products, DM series of die-casting aluminum alloys developed by Mitsubishi for surface anodized treatment process has entered mass production, but there are still problems such as mold sticking, thermal cracking and insufficient strength. At present, there is no anodic oxidation die-casting alloy product suitable for mass production in China.
Al-Mn alloys have been widely used in many fields because of their good plasticity, corrosion resistance, electrical and thermal conductivity. In this project, through metal mold casting, spiral flow test mold is used, flow properties of Al-Mn alloys are characterized by comparing fluidity sample lengths of Al-Mn alloys and ADC12 alloys with different compositions; phase composition of alloy was determined by X-ray diffraction (XRD) phase analysis; morphology and distribution characteristics of each phase of alloy were determined by metallographic observation (OM), scanning electron microscope (SEM) and energy spectrum analysis (EDS); mechanical properties and anodic oxidation properties of alloy were tested by tensile test and anodic oxidation test, finally an Al-Mn series cast aluminum alloy suitable for die-casting with optimal composition and better comprehensive performance was obtained.
Graphical results
Four groups of Al-Mn alloys with different Mn content were prepared by metal mold gravity casting test, and their chemical compositions are shown in Table 1. The total mass of each group of test alloys is 30kg, 99.75% (mass fraction) of industrial pure aluminum and Al-10Mn master alloy are heated and melted in a crucible resistance furnace at a melting temperature of 760-770℃. A ϕ90mm*10mm sample was taken for component analysis. After alloy composition is qualified, use a single-rotor degasser to feed high-purity Ar gas to degas for 15 minutes, processed melts go through processes such as slag removal and standing still, finally alloy melt is poured into fully preheated iron fluidity test mold and copper wedge mold respectively. Pouring temperature is 720-740℃, and mold temperature is 150-180℃. Schematic diagram of wedge-shaped mold and actual ingot are shown in the figure 1.

Table 1 Chemical composition of Al-Mn alloy (%)

 

Fig.1 Mold and samples for performance testing of Al-Mn alloy

 

Fig.2 Schematic diagram of alloy fluidity test mold
A sample with a height of 10 mm was cut from middle of wedge-shaped ingot for microstructure observation, and sampling position is shown in Figure 3. Metallographic sample preparation process is as follows: sawing→mounting→rough grinding→fine grinding→polishing. In order to further observe grain distribution of ingot, anodic coating observation was carried out on sample. Coating solution was a mixed solution of sulfuric acid and phosphoric acid, and corrosion time was 120s. Microstructure of alloy was observed by JSM-6480 scanning electron microscope and Olympus JX51 metallographic microscope, composition of second phase of sample was analyzed by energy dispersive spectroscopy (EDS).

 

Fig.3 Al-Mn alloy microstructure and mechanical properties test

 

Fig.4 Fluidity tests of Al-Mn alloys with different Mn contents

 

Fig.5 Metallographic structure of Al-Mn alloys with different Mn contents

 

Fig.6 Average grain size of Al-Mn alloys with different Mn contents

 

Fig.7 SEM photos of Al-Mn alloys with different Mn contents

Table 2 EDS analysis of Al-Mn alloys with different Mn contents (%)

 

Fig.8 XRD analysis pattern of Al-Mn alloy with 2.0% Mn content

 

Fig.9 Mechanical properties of Al-Mn alloys with different Mn contents
Anodizing of aluminum alloy is one of the most common surface treatment methods, and anodizing process is actually surface corrosion process of alloy products. Microstructure of aluminum alloy determines quality of its anodic oxidation performance, uniform and fine grain structure is conducive to obtaining a higher quality oxide film layer. In addition, existence of second phase will have an important impact on its anodic oxidation effect, mainly because there is a difference in potential difference between second phase and substrate, coarse second phase is prone to localized corrosion during anodic oxidation process, reducing quality of oxide film.

 

Fig.10 Microstructure of Al-1.7Mn-0.1Fe Alloy Wedge-cast Ingot Cross-section

 

Fig.11 Grain structure in each region of wedge-shaped ingot cross section

 

Fig.12 Color difference of Al-Mn alloys with different Mn content and second phase area fraction

Table 3 Thickness of Al-Mn alloy anodized film with different Mn content
In conclusion
(1) As Mn content increases, fluidity of Al-Mn alloy first increases and then decreases. When Mn content is 2.0%, fluidity is the best, which can reach 97% of ADC12 alloy under same level of conditions, meeting requirements of industrial die-casting production .
(2) Second phase in Al-Mn alloys is dominated by Al6Mn and Al-Mn-Fe phases, and second phase is obviously coarse when Mn content is ≥ 2.0%.
(3) With increase of Mn content, tensile strength and yield strength of Al-Mn alloy first increased and then decreased, elongation decreased obviously. When Mn content was 1.7%, comprehensive mechanical properties of alloy were the best. Tensile strength and elongation reached 53MPa, 122MPa and 31% respectively.
(4) Color difference values of Al-Mn alloys with four kinds of Mn content are all lower than 0.5, and anodic oxidation film thickness is greater than 10 μm. When Mn content is 1.7%, anodic oxidation performance of alloy is the best.