Mobile phones have become a communication tool that people, especially workers, can't live without. There are almost everyone, some people even have two or three or more, but their bodies are made of materials, not everyone. Clear two Chu. Among the world's three major mobile phone production groups (American Apple, South Korea's Samsung, China's Huawei), the use of aluminum, titanium, and magnesium. The titanium body is bright and strong, but the price is higher, the density is higher, 66.7% larger than aluminum; the density of magnesium is small, 36% lighter than aluminum, with a silvery hue, but the strength is better than aluminum. Low, corrosion resistance is far less than aluminum, and the price is more expensive than aluminum; therefore, aluminum is the best overall performance and cost-effective smartphone body material, and its recyclability and recyclability are better than titanium. And magnesium. The aluminum, magnesium and titanium that the author says here include various alloys. We know that the size of Apple's mobile phone - the screen size has been increased from iphone 5, giving the visual an unprecedented satisfaction, watching the mobile phone has become a wonderful enjoyment, has now been added to 140mm (5.5 inches). However, the fuselage material of the initial large-screen mobile phone has not been improved synchronously, and the thickness of the body is still thinning. As a result, the body of the iphone mobile phone is deformed by an external force, and even becomes curved, and sometimes it will be put down with carelessness. The baby's mobile phone is bent, and in some cases, the screen may be beaten and broken. This situation has not been fundamentally changed until the launch of iphone 6 plus. Apple is under increasing market pressure and users are increasingly complaining.
7075 aluminum alloy turned out to solve iphone 6S dilemma In order to solve the problem of low bending performance of the previous generation iphone 6 plus mobile phone body, the aluminum industry recommended that Apple use the ultra-strength aluminum alloy 7075 plate for aerospace industry to process the fuselage, brand new Both iphone 6S and iphone 6S plus are all made of 7075 alloy sheet produced by American Aluminum Corporation, and the expected good results have been achieved. The bending resistance is greatly improved. The situation of bending becomes dissipated when it falls, and the deformation does not occur. It is.
7075 aluminum alloy was developed by Alcoa for World War II aircraft. It was used to manufacture large bombers and fighters. It was finalized in 1944. It was 75S before July 1954 and registered with the American Aluminum Association in July 1954. Change to the unified four-digit grade 7075. While studying the 75S alloy in the United States, Japan and the Soviet Union were also studying such alloys. In 1945 and 1946, they successfully developed such aluminum alloys, which are Al-Zn-Mg-Cu alloys. Their strength is larger than that of Al-Cu-Mg hard aluminum, so it is called super-hard aluminum, until it is still the most deformed aluminum alloy with the highest strength performance, and it is one of the most used aerospace aluminum alloys. And 2024 type alloys are listed as two aerospace aluminum alloys, such as the front fuselage long stern of the C919 aircraft, the passenger observation window frame, the middle fuselage long raft, the keel beam edge, the keel beam web, the floor turning beam, the middle The rear fuselage long stern and the cargo door frame, the head shackles and the rims, the door frame, etc. are all made of different 7075 aluminum alloy materials. It can be said that without the support of the 7075-type aluminum alloy, such high values and more can not be created. Large passenger aircraft that are light, comfortable and fast. The aircraft can fly for the first time this year.
The composition of 7075 aluminum alloy has grown and expanded over the past 72 years. By January 2015, there were three common alloys (7075, 7175, 7475) registered in the American Aluminum Association (AA) 7075 aluminum alloy; 7275 alloy was drawn It is very alloyed because each has very little sales, and the 7375 alloy is eliminated. The composition of commonly used alloys is listed in Table 1. It can be seen from the data listed in the table that the composition of the alloy is developing in a direction of high purity, that is, the content of the main components (Zn, Cu, Mg, Cr) of the alloy remains substantially unchanged, while impurities The content of (Si, Fe, Ti, etc.) is getting lower and lower, so the conventional properties of the alloy do not change much, and the fatigue strength, fracture toughness and damage tolerance of the alloy are all decreased with the decrease of impurities. rise. These three alloys were developed by Alcoa. The 7175 and 7475 alloys have a slightly higher impurity content, so the raw material prices are slightly higher, and the smelting and purification processes are more complicated, so the price is higher. The prototype of the special mobile phone body can be used with the prototype alloy 7075, which can fully meet the requirements of the new smart phone replacement. Iphone 6 is made of 6xxx alloy. For iphone 6S and iphone 6S plus, 7075 alloy is the best partner. For the heat-strengthenable alloy, the main strengthening elements are Zn and Mg, and Cu also has a certain strengthening effect, but its main task is to improve the material's resistance to stress corrosion cracking.
Zn and Mg: main strengthening elements, can form n (MgZn2) and T phase (Al2Mg2Zn3), they have considerable solid solubility in Al, and change drastically with temperature rise and fall, the maximum solid solubility of MgZn2 in aluminum It is 28%, and it is reduced to 4%~5% at room temperature. It has strong aging strengthening effect. The increase of Zn and Mg content can greatly increase the strength and hardness, but it will make plasticity, stress corrosion resistance and fracture toughness. decline. Cu: When the Zn/Mg ratio is >2.2 and the Cu content is greater than the Mg content, Cu forms a strengthening phase S with Al and Mg to increase the strength of the alloy, and when the Zn/Mg ratio is <2.2, the S phase is hardly formed. Cu reduces the difference between the grain boundary and the intragranular potential, and can also change the structure of the precipitated phase and refine the precipitated phase of the grain boundary, which can inhibit the tendency of cracking along the crystal, thereby improving the resistance to stress corrosion cracking of the alloy. However, when the Cu content is >3%, the ability of the alloy to resist stress corrosion decreases. Cu increases the degree of supersaturation of the alloy, accelerates the artificial aging process of the alloy at 100 ° C ~ 200 ° C, expands the stability range of the GP zone, and improves the tensile strength, plasticity and fatigue strength.
Mn and Cr: Adding a small amount of Mn and Cr has a significant effect on the microstructure and properties of the 7075 alloy. They can produce a diffused compound during the uniform annealing of the ingot, prevent dislocations and grain boundary migration, thereby increasing the recrystallization temperature, effectively Preventing grain growth and refining the grain, ensuring that the material is in a non-recrystallized or partially recrystallized state after hot working and heat treatment, and improving the strength while making the material have better resistance to stress corrosion. In terms of improving the stress corrosion resistance, the effect of Cr is better than that of Mn. When the alloy is added with 0.45% Cr, the stress corrosion cracking life of the alloy is several tens to hundreds times longer than that of the equivalent Mn alloy.
Zr: The recent development trend is to replace Mn and Cr with Zr, because Zr can greatly improve the recrystallization temperature of the alloy. Whether it is hot-deformed or cold-worked material is non-recrystallized after heat treatment, Zr can also improve the quenching of the alloy. Permeability, weldability, fracture toughness, resistance to stress corrosion cracking, etc., are very effective microalloying elements in Al-Zn-Mg-Cu alloys.
Ti: refining alloy as-cast grains and increasing alloy recrystallization temperature
Fe and Si: In all alloys free of Fe and/or Si, they are unavoidable harmful impurities, from raw materials and tools and equipment used in the melting and casting process. These two impurities are mainly present in the form of hard and brittle compounds Al3Fe and free Si, and they can form coarse compounds such as (FeMn)Al6, (FeMn)Si2Al5, Al(FeMnCr) with Mn and Cr. Al3Fe has fine grain effect, but it is unfavorable for corrosion resistance. As the amount of insoluble phase increases, their volume percentage also increases. These insoluble phases are broken and elongated during processing deformation, and banded structures appear along the The direction of deformation is linear, consisting of short strips that are not connected to each other. Since the impurity particles are distributed in the interior of the grain and on the grain boundary, pores are generated at the boundary of a part of the particle-substrate during plastic deformation, and micro-cracks are formed, which becomes the source of macro-cracks, and it also promotes the premature development of the crack. At the same time, it also has a great influence on the growth rate of fatigue cracks, and has a certain ability to reduce local plastic deformation when it is destroyed. Since the Fe and Si compounds are difficult to dissolve at room temperature and function as a notch, they are likely to be crack sources and break the material, which is very disadvantageous for material elongation and fracture toughness. Therefore, when designing the new 7xxx series alloy, the content of Fe and Si is strictly controlled. In addition to the pure raw materials, corresponding measures should be taken during the smelting and casting process to prevent them from being mixed into the alloy melt.
The structure of 7075 aluminum alloy is composed of α(Al) solid solution and second phase particles; the second phase particles are: intermetallic compounds formed during melt solidification, such as Al7FeCr, Al3Fe, Mg2Si, etc., their size is large, 0.5μm~10μm, crushed during pressure processing, insoluble in solid solution, so heat treatment can not change it, because of their existence, reduce the fracture toughness of the alloy; the Cr-containing diffused particles Al12Mg2Cr and Mg3Al18Cr2 are homogenized, The pre-heating and solution treatment before pressure processing, from the solid solution, the size of 0.05μm~0.5μm, has obvious inhibition and inhibition on recrystallization and grain growth; aging strengthening particles, mainly GP area, It dissolves into the solid solution during solution treatment, and precipitates from the solid solution during aging. Its shape and size vary greatly with time and temperature, which is an important factor affecting the properties of the alloy. In the T6 state, the strengthening point is mainly ≤1nm. In the GP region, in the T76 state, the strengthening particle is mainly a η' transition phase of 5 nm to 6 nm, and in the T73 state, it is a transition phase of 8 nm to 12 nm and an equilibrium phase η of 20 nm to 80 nm.
Physical and chemical properties of 7075 aluminum alloy
7075 aluminum alloy is the most widely used 7xxx series heat-treppable reinforced ultra-high-strength deformed aluminum alloy, which can be processed into various semi-finished products. The heat treatment conditions are: T6, T73 and T76. T6 has the highest strength and low fracture toughness, is sensitive to stress corrosion cracking, and its toughness decreases with temperature, so it should not be applied at low temperature. T73 has the lowest strength but has high fracture toughness and excellent stress resistance. Corrosion cracking and anti-flaking corrosion performance; T76 material has good stress corrosion resistance than T73 material. I recommend the use of T6 material to manufacture the mobile phone body, because of its high strength, good cutting performance, easy processing, and simple heat treatment process.
The static strength of the alloy is higher than that of the 2024 alloy, and the fatigue performance is comparable. It has good room temperature forming properties in the annealed state (O) and solution treatment state (W), not only in the fields of aerospace vehicles, military equipment, etc. It has a wide range of applications and is also gaining increasing applications in consumer products, especially in the manufacture of electronic products. However, its operating temperature should not be higher than 125 °C. The deepening temperature of 7075 aluminum alloy is 477 ° C ~ 638 ° C. The thermal conductivity of 7075-T6 alloy at 25 ° C, 50 ° C, 100 ° C, 125 ° C, 150 ° C, 200 ° C is (124, 128, 147, 157, 170) W / (m · ° C), T73 material The thermal conductivity at 25 ° C is 156 W / (m · ° C), 25.8% higher than that of T6 material; the specific heat capacity of 7075-T6 materials at 25 ° C, 50 ° C, 100 ° C, 125 ° C, 150 ° C, 200 ° C is (796, 879, 921, 963, 994, 1005) J / (kg · ° C); 7075 alloy at (25 ~ 100) ° C, (25 ~ 125) ° C, (25 ~ 150) ° C, (25 ~ 200) The average coefficient of linear expansion at °C is (23.8, 23.9, 24.1, 24.4) × 10-6/°C; the room temperature resistivity of 7075-T6 alloy is P=57.4nΩ·m, and the conductivity is 17.7MS/m~20.6MS. /m, T73, T76 material is 22.0MS/m~24.4MS/m
7075 aluminum alloy has good oxidation resistance, except for stress corrosion, other corrosion resistance is equal to 2024.
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