Application of the hottest PQA new polymer quencha

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Application of PQA new polymer quenchant in aluminum alloy Abstract: This paper introduces the cooling characteristics of PQA polymer quenchant, the mechanical properties of aluminum alloy after quenching, quenching deformation and medium management

Keywords: polymer quenchant, aluminum alloy quenching, mechanical properties, medium management

Application of aluminum alloy of new polymer quenchabt PQA

Wang Yuling, Wang Shaojing, Fan yongqiang

(Nanjing zhoulian New Product and Technique Institute, Nanjing Aeronautical and Austronautical University Box435,Postcode 210016)

Abstract: This peper described the cooling characteristics of the PQA polymer quenchant, properties of aluminium alloy after quenching. Quenching distoreion and the managing of the PQA quenchant.

1. basic characteristics

1.1 the essence of PQA quenchant

pqa quenchant is a copolymer that takes two epoxy compounds as monomer raw materials, uses a certain initiator and catalyst, and reacts under controlled temperature and pressure. It is a polymer with anionic linear structure. It is a kind of PAG quenching medium, and its appearance is milky white to light yellow viscous liquid. It can be infinitely dissolved with water, that is, by adding any amount of PQA quenchant stock solution into water, a uniform aqueous solution can be obtained

When PQA quenchant is prepared into aqueous solutions with different concentrations, it will have different cooling rates, so as to adapt to the cooling rates required for quenching parts of different metal materials, shapes and thicknesses

1.2 reverse solubility and re solubility

pqa quenchant is a homogeneous and transparent aqueous solution. When its heating temperature reaches a certain value, the quenching medium immediately becomes turbid (milky white). The temperature at which this phenomenon occurs is called cloud point

quenching medium (above the cloud point) Asahi Huacheng (China) Investment Co., Ltd. held a project signing ceremony with Changshu new materials industrial park. When the temperature continues to increase and reaches a certain value, the polymer in the turbid quenching medium will precipitate and sink in clusters (because the proportion of the polymer is greater than that of water). The temperature at which the cluster polymer begins to precipitate in the quenching medium is called the reverse melting point

the polymer precipitated from the quenching medium due to the temperature rising above the reverse melting point will dissolve again as long as the temperature of the quenching medium drops below the reverse melting point, so that the quenching medium becomes uniform again. The phenomenon that the precipitated polymer re dissolves due to the decrease of temperature is called back solubility

the cloud point determines the actual allowable maximum temperature rise of this kind of water-based quenching medium in the quenching process, that is, the service temperature of the quenching medium should be lower than the cloud point. Reverse dissolution and back dissolution determine the cooling mechanism of PQA quenching medium when it cools red hot metal, which is the key for this kind of quenching medium to have ideal cooling characteristics, significantly reduce quenching deformation and prevent quenching cracking

1.3 cooling characteristics

1.3.1 reasons for PQA quenchant to reduce quenching deformation and prevent quenching cracking

when water is used as quenching medium, its biggest disadvantage is that the steam film is serious. Until near the low-temperature martensitic transformation temperature, the steam film breaks and enters the bubble boiling stage, resulting in the maximum cooling rate, which is easy to cause quenching deformation and quenching cracking

pqa water-soluble quenching medium is completely different. Due to the inverse solubility of this quenching medium, when a certain concentration of uniform quenching medium cools the red hot metal, the quenching medium near the metal surface will soon reach the inverse melting point, and cluster polymer precipitation will occur. When the mass polymer moves to the metal surface, it is adhered, thus forming a polymer isolation film on the metal surface. The greater the concentration in the quenching medium, the greater the thickness of the isolation film. Due to the poor thermal conductivity and heat insulation of polymer quenching medium, the quenching medium has high concentration, thick isolation film and low cooling speed. Therefore, by controlling the concentration, you can control the small and large cooling rate required for quenching, which can not only avoid the parts from hardening, but also prevent quenching cracking or excessive quenching deformation

due to different surface finishes and geometric shapes of quenched workpieces, quenching media such as water and oil form steam (gas) film on the surface of workpieces first and then break. Therefore, uneven cooling will inevitably lead to workpiece deformation. The most intense bubbly boiling process of PQA quenching medium cooling only occurs on the outer surface of the isolation film, not on the surface of the workpiece. Therefore, the uneven cooling caused by different surface finish and geometry of the workpiece is eliminated, and the quenching deformation caused by uneven thermal stress of the workpiece is eliminated

the isolation film formed on the surface of the workpiece during quenching thickens with the extension of the cooling time. The phenomenon of back dissolution of the isolation film does not occur until the temperature below the reverse melting point. Therefore, the cooling rate of PQA quenching medium at low temperature is not strong, which can avoid excessive organizational stress during martensitic transformation, so as to prevent the risk of quenching cracks

1.3.2 cooling rate

adopt a pure silver probe with a handle with a diameter of 16 and a length of 48mm, and use the extrusion method to connect the hot end of the thermocouple to the geometric center of the probe. The heating temperature of the probe is 800 ± 2 ℃, the thermoelectric potential signal when the probe is cooled in the medium is collected by the microcomputer, and the cooling curve and cooling characteristic curve (relationship between temperature and cooling speed) are drawn by the xw-4675 digital plotter controlled by the microcomputer. Figure 1 shows the characteristic curves of different types of Quenchants and PQA quenchants with different concentrations. It can be seen from the figure that the relationship between temperature and cooling rate of PQA quenchants with different concentrations has the following three characteristics

Figure 1 PQA cooling characteristic curve

1.3.3 three characteristics of cooling rate

△ when the concentration of quenching medium is greater than 15%, the cooling rate decreases with the increase of concentration

because the greater the concentration of quenching medium, the thicker the polymer diaphragm formed on the surface of pure silver probe, the more obvious the heat insulation effect, and the most intense bubble boiling of cooling only occurs on the outer surface of the isolation film, and the quenching medium does not directly contact the metal surface, Therefore, the cooling rate of quenching medium is low

△ when the concentration of quenching medium is below 15%, the cooling rate increases with the increase of concentration

because the heat insulation effect of isolation film is not important when the concentration of quenching medium is low. On the contrary, the infiltration effect of a few polymer molecules on the metal surface can reduce the thickness of steam film of simple water or even cannot form, so the cooling rate is improved

△ the cooling rate is low in the low temperature stage

because the isolation film formed on the surface of the metal (workpiece) can only be dissolved back into the quenching medium when the metal surface temperature is below 80 ℃ of the reverse melting point. Therefore, the insulation effect of the isolation membrane can be maintained until low temperature. In addition, polymer macromolecules move slowly at low temperature, with poor fluidity and weak convective heat conduction effect. Due to these two reasons, the cooling rate of the low-temperature cooling stage of the metal workpiece, that is, the phase transformation stage, is not large, so it is beneficial to reduce the structural stress and prevent quenching cracking. This beneficial effect is more obvious for higher concentration of quenching medium solution. As the aluminum alloy requires rapid cooling between 400 ℃ - 250 ℃ during solid solution treatment, the PQA quenching medium solution with higher concentration can meet this cooling characteristic requirement

2. Application of aluminum alloy quenching

2.1 concentration selection

2.1.1 when the thickness of aluminum alloy sheet metal parts is 0.8 ~ 3mm, 20 ~ 30% PQA is selected. The lower limit is selected for the thicker one. Use the upper limit for those who are thin

2.1 there are 4 openings on the valve body. 2 20 ~ 25 PQA is selected for aluminum alloy castings, and the lower limit is taken for parts and their thickness, and the upper limit is taken for parts and their thickness

2.2 mechanical properties after solution and aging

ly12 the mechanical properties of plates with different thickness after conventional heating, quenching in PQA quenching medium with different concentrations and natural aging are shown in Table 1. It can be seen from table 1 that the test data are above the specified value of metallurgical standard. Table 1 Properties of LY12 plates with different thickness after quenching in different quenching media and natural aging

yield strength, LY12 plates with 1mm and 2mm thickness, tensile test σ 0.2, slightly higher than that of water quenching

the specimen of fatigue test is shown in Figure 2

Figure 2 repeated bending fatigue specimen

due to limited conditions, the test is comparative. The samples are heated in a nitrate furnace (500 ± ℃), quenched in water and 30% PQA quenching medium respectively, and tested on a repeated bending fatigue tester after natural aging, with a frequency of 1240 times/minute. Under symmetrical cyclic stress σ- Under the condition of 9kg/mm2, the test of 1mm thick plate shows that the fracture cycle n of lyl2 plate quenched in 30% PQA quenching medium solution is close to that quenched with water

the number of broken samples (percentage in the total number of samples) is 75% for water quenching and 43% for 30%pqa quenching. The average life of fracture is 0.9x106 times for water quenched and 1.06 times for 30% PQA quenchant × 106 times. Therefore, it can be explained that the fatigue data of conventional water quenching and PQA quenching medium quenching are basically at the same level. However, from the fatigue test, the quenching effect of 30% PQA quenching medium has a better trend

to sum up, using PQA aqueous solution as the quenching medium, the mechanical properties of aluminum alloy plates are no worse than that of water quenching

3. Deformation effect

the deformation statistics of various sheet metal parts quenched in PQA quenching medium show that compared with water quenching, the quenching deformation can be reduced by 50 ~ 90%. The comparison of quenching deformation of some sheet metal parts is shown in Table 2

table 2

4. Intergranular corrosion

lyl2 plate is quenched in PQA quenching medium of the concentration when the thickness is less than 3mm. After natural aging, there is no intergranular corrosion

after lyl2 aluminum plate is quenched in PQA quenching medium and aged, the sample is polished and polished on the cut cross section (without the influence of aluminum coating), and corroded at room temperature (26 ~ 28 ℃) in 100 ml of water plus 2 ml of hydrochloric acid and 3 g of sodium chloride solution. Observe whether grain boundary corrosion occurs under the microscope. The results showed that there was no intergranular corrosion after 24, 48 and 72 hours of corrosion of 3mm thick LY12 plate (without aluminum coating on the surface) cooled by aqueous solution below 30% PQA quenching medium. Low concentration PQA quenched medium in the second half of the 19th century will not produce intergranular corrosion

5. Quenching medium preparation

5.1 quenching tank

△ quenching tank can be of steel structure and cement structure. However, the steel structure shall not be painted at will. Some paints are not miscible with PQA, resulting in the pollution of quenching medium

△ capacity of quenching tank, 10 kg PQA quenching medium solution is recommended for each kg of quenching metal. In case of mass production or continuous quenching, it should be prepared according to the capacity of 15 kilograms of quenching medium solution per kilogram of metal

5.2 preparation of quenching medium solution

△ prepare quenching medium solution according to volume concentration. That is, first put the required volume of water into the quenching tank, and then pour the required volume of PQA quenching medium stock solution into the water. Because the specific gravity of PQA quenching medium is greater than that of water, the PQA quenching medium stock solution just poured will sink at the bottom, so it is required to stir during preparation until the uniform solution is to. Under normal use conditions, PQA quenching medium solution does not crystallize, does not precipitate, and has the function of automatic mixing, so there is no need to stir to rapidly expand the capacity of battery diaphragm

△ prepare quenching medium solution according to height. That is, when the quenching tank is of regular cubic and square cylindrical shape, the height can be used to represent the concentration. For example, when the height of quenching solution is 1 meter, put water at the height of 0.8 meters, and then

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