Abstract: Causes of thermal cracking of middle frame of die-cast ADC12 aluminum alloy mobile phones were analyzed using methods such as direct reading spectrometer, scanning electron microscope and metallographic inspection. Results show that hot cracking occurs at grain boundary, with a large number of pores and carbon-oxygen inclusions distributed on both sides, and there are more massive β (Al9Fe2Si2) brittle phases, which hinders feeding channel of alloy, increase brittleness of alloy, and worsen mechanical properties of material. At the same time, due to influence of thermodynamic factors during cooling process, internal structure is unevenly distributed, resulting in large thermal stress, which causes thermal cracks in the middle frame of mobile phone during die-casting process.
With development of 5G communications, traditional metal casings have a strong shielding effect on mobile phone signals. Current mainstream appearance material in the development of smartphones uses a double glass/ceramic + aluminum alloy mid-frame solution. Compared with deformed aluminum alloy mobile phone middle frames, die-cast aluminum alloy middle frames have advantages of excellent formability, simple process, and high production efficiency. ADC12 aluminum alloy die-casting parts are very suitable for mass production due to their high yield, good surface quality, high dimensional accuracy and low subsequent processing volume, and are widely used in automotive and electronic communications fields. As a core structural component, middle frame of a mobile phone plays an important supporting role in smartphones and has high requirements in terms of strength. In casting production, hot cracks may have serious consequences, especially mechanical properties will be seriously affected, resulting in product scrapping. Currently, research on ADC12 aluminum alloy mainly focuses on optimization of alloy composition, there are few reports on distribution of structural defects during die-casting process and impact of defects on mechanical properties of alloy. This article uses optical microscope, scanning electron microscope, direct reading spectrometer and other means to observe and analyze thermal cracking defects in the middle frame of mobile phones made of ADC12 aluminum alloy die-casting. It uses direct reading spectrometer and other methods to observe and analyze, and then proposes improvement measures to provide a reference for avoiding such defects in production process of mobile phone middle frames.

Main raw material used for ADC12 aluminum alloy in this test is recycled aluminum. Chemical composition of sample near fracture is analyzed using a direct reading spectrometer and compared with standard composition. Results are shown in Table 1. It can be seen that composition of hot cracked sample is within standard range.
Main chemical composition of ADC12 alloy

Clamping force of die-casting machine used is 300kN, material handle thickness is 15mm, mold temperature is 200℃, injection force is 330kN, punch diameter is 60mm, injection pressure is 116MPa, injection time is 3.5s, cooling time is 2.0s, and mold retention time is 8.0s. In this test, mold was preheated to 150℃ (actually measured temperature of mold surface) through a mold temperature machine, and then die-casting was performed. Action stroke position of mold handle during die-casting process: the first fast position is 100mm, the second fast position is 240mm, boost position is 280mm, and tracking position is 375mm. Macro photo of middle frame of produced mobile phone and its thermal crack defect is shown in Figure 1. It can be seen from figure that thermal crack occurs at hot joint of casting. This is because at the end of solidification of alloy, temperature of molten metal drops rapidly, which reduces flow rate of molten alloy, easily produces shrinkage porosity and shrinkage holes, and reduces feeding effect of molten metal on cracks.

 

Analysis and testing methods such as Phonex scanning electron microscope and metallographic microscope were used to analyze fracture morphology, metallographic structure and micro-region components of cracked specimen.

Figure 2 shows fracture morphology of thermal crack in the middle frame of mobile phone. From Figure 2a, it can be seen that fracture morphology of thermal crack is relatively rough, its edges are uneven and staggered, there are inclusions on both sides of crack and around it. It can be judged that hot cracking occurs at grain boundary. This is because when alloy solidifies to semi-solid stage, thermal stress at grain boundary gradually increases. At this time, strength limit of alloy is smaller, strength limit of semi-solid state of material is lower than thermal stress at grain boundary. That is, stress and strain generated by shrinkage in final stage of solidification exceed limit range that material can withstand, which is main cause of thermal cracking. Hot cracks generated at this time cannot heal in time, strength of alloy will decrease, and hot cracks will expand further. As can be seen from Figure 2b, there are brittle fracture areas and ductile fracture areas in fracture surface, with brittle fracture being main area. There are a large number of holes concentrated in some areas inside fracture cracks, which are initially determined to be pores. Due to large number of holes, which are widely distributed and relatively dense, they can be judged to be pores caused by recycling of waste materials such as air entrainment and exhaust plates during die casting. They seriously reduce mechanical properties of alloy. Moreover, there are a large number of inclusions distributed around pores. EDS micro-area composition analysis results of each point in Figure 2 are shown in Table 2. It can be seen from Table 2 that each point contains carbon, oxygen, nitrogen, aluminum, silicon and iron elements. It is inferred that it is mainly carbon, oxygen and nitrogen inclusions and harmful iron phase (β-AlFeSi), and it can be speculated that inclusions are derived from auxiliary materials in die-casting process, such as release agents, granular oil, etc., causing specimen to exhibit brittle fracture characteristics.

 

EDS analysis results of each point in Figure 2

Figure 3 shows microstructure of alloy at hot crack position observed under an optical microscope. Metallographic structure is composed of primary α-Al and α-Al+ eutectic silicon phases. Light color is matrix structure and dark color is eutectic structure. It can be seen from figure that hot cracks are formed and developed on grain boundaries of alloy matrix phase. When cracks first begin to form, amount of intergranular separation and shrinkage is relatively small, and eutectic liquid is prone to shrinkage. It can be seen from Figure 3a that there are a large number of pores distributed at hot crack, which deteriorates mechanical properties of alloy, which is consistent with results observed in fracture morphology; grain size distribution of alloy in Figure 3b and c is uneven, coarse dendrites coexist with fine spherical crystals. Presence of a large number of coarse dendrites leads to a reduction in thermal cracking resistance of alloy, indicating that structure is affected by thermodynamics and other factors during cooling process, causing uneven microstructure and generating large thermal stress. Because thick dendrites cannot slip and reduce stress as easily as equiaxed crystals and spherical crystals, they are prone to thermal cracking. Figure 3b shows structure at tip of a hot crack. Some of cracks have been filled and compressed by eutectic liquid during formation process, some of them cannot be filled and compressed due to insufficient remaining eutectic liquid, so cracks are intermittent. After formation of hot cracks, due to narrow solidification interval of ADC12 alloy, when temperature of alloy liquid drops rapidly, solidification speed accelerates and proportion of solid phase increases rapidly, resulting in a significant decrease in mold filling ability of alloy liquid, so that formed hot cracks can no longer be filled and compressed. Moreover, amount of eutectic liquid is less than amount of feeding required for cracks, causing thermal crack to be unable to heal, so thermal crack further expands. Figure 3c shows structure after thermal crack has expanded. It can be seen that crack width increases significantly, with the widest point reaching 20 μm.

 

In order to further analyze microstructure morphology of hot crack, alloy sample was observed with a scanning electron microscope, and characteristic parts of hot crack were analyzed by energy spectrum. Results are shown in Figure 4 and Table 3 respectively. It can be seen that components of white massive and needle-like structures in microstructure contain aluminum, silicon, and iron elements, which are judged to be β (Al9Fe2Si2) brittle phases. A large number of larger-sized massive β-AlFeSi phases are on both sides of crack, and distribution is disorderly. It blocks feeding channels between dendrites, making it more difficult for eutectic liquid to feed, and at the same time increases pore defects. Because β-AlFeSi phase has a small gas-solid interface energy, pores are easy to nucleate along β-iron phase and form grow up. In addition, large harmful iron phase affects feeding of alloy and blocks passage of liquid feeding, thus further increasing tendency of alloy to crack.
EDS analysis results of each point in Figure 4

Table 3 EDS analysis results of each point in Figure 4

 

In view of thermal cracking defects that occurred during production process of ADC12 mobile phone middle frame, following improvement measures are proposed:
(1) Increase R angle at hot joint to reduce thermal stress there during solidification process;
(2) Reduce die-casting material head and strictly prohibit use of exhaust plate in furnace to prevent increase of alloy gas content and carbon, oxygen and nitrogen inclusions;
(3) Strictly control die-casting temperature to prevent aluminum liquid from absorbing air, while improving β-AlFeSi phase morphology and improving high-temperature mechanical properties of alloy;
(4) Increase mold preheating temperature to 200℃. Increasing mold preheating temperature can reduce cooling rate, slow down heat loss during solidification process of alloy, increase feeding capacity of alloy liquid, thereby reducing tendency of alloy to produce hot cracking;
(5) Add ≤0.1% Al-Ti-B refiner before alloy die-casting to improve alloy microstructure and make α-Al phase evenly distributed. At the same time, use low-temperature annealing at 250~290℃ for 10~16s, thereby reducing internal stress and improve its strength.
After controlling production process of die-cast ADC12 aluminum alloy mobile phone middle frame according to above measures, resulting casting did not suffer from hot cracking, as shown in Figure 5.

 

(1) When die-casting to produce ADC12 aluminum alloy mobile phone middle frames, hot cracking occurs at grain boundaries. This is because when alloy solidifies to semi-solid stage, strength limit of alloy is lower than thermal stress at grain boundaries, and remaining liquid phase in final stage of solidification is insufficiently fed.
(2) Through fracture morphology and metallographic observation, it was found that there were a large number of pores and carbon-oxygen inclusions distributed on both sides of hot crack. Scanning electron microscope test results showed that there are more massive β (Al9Fe2Si2) brittle phases on both sides of crack, which hindered feeding channel of alloy, increase brittleness of alloy, and worsen mechanical properties of material.
(3) By optimizing mold structure, controlling melting process and improving die-casting process, thermal stress during alloy solidification process is reduced and mechanical properties of material are improved, thus eliminating thermal crack defects of die-cast ADC12 aluminum alloy mobile phone middle frame.