CN102680326B - An experimental device and method for thermal cracking test of aluminum alloy under the condition of actively applied load - Google Patents

Abstract

The invention relates to a device for testing hot crack of an aluminum alloy under the condition of an active applied load. The device mainly comprises a test sample mold unit, a connecting screw, a data testing unit, a frame-type connecting mechanism and a load application unit which are sequentially connected and positioned on the upper part of an equipment pedestal; and a data acquisition unitis connected with the data testing unit. Through the device for testing the hot crack of the aluminum alloy, information such as critical load and temperature change when a test sample is cracked canbe acquired, and data information is provided for constructing a relation between the solid phase rate of the aluminum alloy and the applied load; and test samples with different dendritic crystal growth directions can be acquired by constructing different cooling velocities of the aluminum alloy through different heat preservation materials, and a relation between the growth direction of dendritic crystal and the hot crack is sought. The testing device is simple in structure, reasonable in design and easy and convenient to operate, is applied to easy-cracking alloy and non-cracking alloy, further enriches the category of testing equipment used in the hot crack research process, and promotes research and development of the hot crack.

Description

An experimental device and method for thermal cracking test of aluminum alloy under the condition of actively applied load

technical field

The invention relates to an experimental device and method for testing hot cracking of an aluminum alloy under the condition of actively applying a load. The device can measure parameters such as critical load and temperature when the aluminum alloy is hot cracked, and belongs to the field of experimental research on metal materials and casting.

Background technique

In casting production, hot cracking is a common casting defect in castings, especially large castings, which has brought huge losses to industrial production. The occurrence of such casting defects is mainly related to the stress caused by the solid shrinkage of liquid alloy in the later stage of solidification. related to generation and development. Hot cracking of castings is a crack formed by castings at high temperature, and often occurs at the end of liquid alloy solidification, that is, in the solidus above the solidus line of the alloy. The occurrence of thermal cracks in castings is mainly due to the shrinkage stress of castings exceeding the bonding force between metal grains, and most of them occur along grain boundaries. From the observation of crack fractures, it can be seen that the metal at the cracks is often oxidized and loses its metallic luster. The cracks extend along the grain boundaries, the shape is zigzag, the surface is wider and the inner part is narrower, and some of them penetrate the entire end face of the casting.

A large number of studies have shown that the formation of hot cracks is a complex physical and chemical process, which includes heat conduction, fluids, and other factors that cause cracks. These factors include alloy composition, alloy solidification and thermodynamic properties, and process parameters of castings and molds. , mold material and some process control parameters and so on. At the beginning of the 20th century, foundry workers began to study the formation mechanism of thermal cracks, and proposed several different theories such as strength theory, liquid film theory, comprehensive theory, and intergranular bridging theory to explain the formation of thermal cracks; With the continuous development of the casting field, the numerical simulation technology of the temperature field and the flow field in the casting process has been fully developed, and the numerical simulation research of the stress field in the casting process has also been developed, which also indirectly promotes the numerical simulation of hot cracks in the casting process. Some research results have been achieved, and some theoretical models and criteria for hot crack prediction (such as Rappaz's RDG criterion) have been proposed, but there is still no general theoretical model or criterion that can predict hot cracks more accurately. Therefore, the construction of a hot crack prediction model suitable for actual conditions is also the focus of current research.

In recent years, with the continuous deepening of research on hot cracking, scientific and technological workers have designed and developed some testing equipment for qualitative or quantitative characterization of hot cracking. The representative one in China is the double test bar for sand molds designed and developed by Dalian University of Technology. Alloy thermal cracking-line shrinkage tester, cast alloy wire shrinkage-stress-hot cracking characteristic multi-function tester developed by Harbin University of Science and Technology, and metal-type ZSR alloy thermal cracking tendency tester developed by Shenyang University of Technology; foreign representative It is the N-Tec thermal cracking test mold of N-Tec Ltd. in Canada and the Instrumented ConstrainedRod Mold used for aluminum alloy thermal cracking testing by Worcester Institute of Technology in the United States. The basic principle of these devices is to limit and fix the two ends of the test rod or sample. During the solidification process of the liquid alloy, the test rod or sample will solidify and shrink, especially at the junction of the thickness of the test rod due to the difference in solidification speed. In addition to thermal stress, when the thermal stress exceeds the combined strength limit, cracks will occur. Some information and data during the formation of liquid alloy hot cracks can be obtained by using these devices, but the hot crack test equipment used in the current experiment process is basically designed using the principle of solidification and shrinkage of the liquid alloy itself. Research on hot cracking of easy-cracking alloys, but to study hot cracking of alloys with a small tendency to hot cracking, these equipment cannot meet the requirements.

Contents of the invention

Aiming at the deficiencies in the existing technology and experimental devices, the present invention abandons the idea of using liquid alloys to freely shrink, and proceeds from the idea of actively applying load to the sample, and designs a method for performing aluminum alloy related heat under the condition of actively applying load. Crack test experimental device, the experimental device is simple in structure, reasonable in design, easy to operate.

The present invention is realized through the following technical solutions:

An experimental device for thermal cracking testing of aluminum alloys under the condition of actively applied loads, mainly including a sample mold unit, a connecting screw, a data testing unit, a frame-shaped connection mechanism, and a load application unit connected in sequence; the sample mold unit, connecting screw , the data testing unit, the frame-type connecting mechanism, and the load applying unit are located on the upper part of the equipment base; the data collecting unit is also connected with the data testing unit.

The sample mold unit includes a metal mold; the metal mold includes two metal half molds; a cavity is arranged inside the metal mold; a temperature measuring port is arranged on the upper part of the metal mold.

Insulation materials are installed in the cavity; the insulation materials are riser insulation materials or graphite materials.

The data testing unit includes a load testing mechanism and a temperature testing mechanism; the load testing mechanism is an "S" type sensor, and the "S" type sensor is connected to the sample mold unit through a connecting screw; the temperature testing mechanism is "K" Type nickel-chromium-nickel-silicon thermocouple, "K" type nickel-chromium-nickel-silicon thermocouple is directly inserted into the temperature measuring port on the upper part of the metal mold.

The data acquisition unit adopts a paperless recorder; the data sampling period of the paperless recorder is 0.1S.

The load applying unit includes a trapezoidal screw, a bearing seat, a shaft coupling and a speed-regulating motor connected in sequence; the trapezoidal screw is connected with an "S" type sensor through a frame-shaped connection mechanism. A full-thread screw is installed at the end of the connecting screw; a graphite plug is arranged at the connection between the connecting screw and the sample mold unit.

The output shaft of the speed-regulating motor, the frame-shaped connecting mechanism, the "S"-shaped sensor, the connecting screw, and the center line of the metal mold are all located on the same horizontal line, and the coaxiality during assembly is ≤0.15mm.

A method for testing aluminum alloy hot cracking by an aluminum alloy hot cracking test experimental device under the condition of actively applying a load, comprising the following steps:

(1) Before the experiment, check whether the paperless recorder, speed regulating motor, "S" type sensor, "K" type nickel-chromium-nickel-silicon thermocouple and coupling are working normally;

(2) Preheat the metal mold to keep the temperature of the metal mold at 200~250°C;

(3) Adjust the connecting screw to the initial position, and tighten the full-thread screw at the end of the connecting screw;

(4) Installed; insulation material, graphite plug and signal input data line of paperless recorder, and then ready for alloy pouring;

(5) After the alloy pouring is completed, according to the alloy type and alloy thermal cracking tendency, select the appropriate alloy solidification time and motor speed, and load the load until the sample is completely cracked or the sample is completely solidified. Stop the speed regulating motor and The work of the paperless recorder, the data recorded by the paperless recorder is transferred to the PC for processing, and finally the metal mold is opened to take out the sample.

The functions of the graphite plug are: one is to block the molten liquid metal, and the other is to reduce the friction between the screw rod and the mould.

The end of the connecting screw is equipped with a full-thread screw, which can make the metal mold and the connecting screw tightly combined, avoiding sliding and affecting the measurement of the load.

The thermal insulation material can be selected from different materials capable of thermal insulation of the solidification of the alloy, and its main function is to construct different cooling rates of the liquid alloy, and the alloy can achieve different dendrite growth directions due to different solidification rates.

The loading generally begins to apply the load in the liquid-solid coexistence region of the alloy.

The data collected by the paperless recorder can be transferred to a PC through a storage device such as a U disk for corresponding data processing; the data sampling period of the paperless recorder is 0.1S, and the data collection is stopped when the temperature value drops to 300°C.

The outstanding features of the technical solution of the present invention are as follows:

1. The speed-regulating motor is used to actively apply the load, and the intensity and speed of the load are realized by adjusting the speed of the motor.

2. Use thermal insulation materials to control the solidification speed of the sample, and achieve different solidification speeds or cooling speeds of the samples by selecting different sizes and types of thermal insulation materials, and then achieve the purpose of controlling different dendrite growth directions of the liquid alloy.

3. The connecting screw is equipped with a full-thread screw. After the experiment is completed, the screw can be easily removed from the connecting screw for easy disassembly.

4. The data is collected using a paperless recorder to facilitate subsequent data processing.

The beneficial effect of the present invention is: starting from the idea of actively applying load to the sample, a complete set of experimental device for hot cracking test of aluminum alloy is designed, which can obtain the critical load and temperature change when the sample is cracked and other information, to provide data information for the construction of the relationship between the solid phase rate of aluminum alloy and the applied load; it is also possible to obtain samples with different dendrite growth directions by selecting different insulation materials to construct different cooling rates of aluminum alloys, so as to The relationship between dendrite growth direction and hot cracking was explored. The experimental device has a simple structure, reasonable design, and easy operation. It is suitable for alloys that are prone to cracking and alloys that are not prone to hot cracking. It further enriches the types of testing equipment used in the research process of hot cracking and promotes the research and development of hot cracking.

Description of drawings

Fig. 1 is a structural representation of the present invention;

Fig. 2 is a top view of the present invention;

Fig. 3 and Fig. 4 are the relationship graphs of temperature, applied load and time obtained by testing the A356 alloy under different conditions according to the present invention.

In the figure: 1-insulation material; 2-metal mold; 3-full wire screw; 4-graphite plug; 5-connecting screw; 6-sensor; 7-frame connection mechanism; 8-trapezoidal screw; Shaft device; 10-speed regulating motor; 11-equipment base; 12-bearing seat; 13-temperature measuring port.

Detailed ways

The present invention will be further described below in conjunction with the accompanying drawings and embodiments.

As shown in Figures 1 and 2, an experimental device for thermal cracking of aluminum alloys under the condition of actively applied loads mainly includes a sample mold unit connected in sequence, a connecting screw 5, a data testing unit, a frame-shaped connecting mechanism 7, and a load application unit. Unit; sample mold unit, connecting screw 5, data testing unit, frame-type connection mechanism 7, load applying unit located on the upper part of the equipment base 11, and the data acquisition unit is also connected with the data testing unit.

The sample mold unit includes a metal mold 2; the metal mold includes two metal half molds; the metal mold 2 is provided with a cavity; the metal mold 2 is provided with a temperature measuring port 13; the thermal insulation material 1 is installed in the cavity of the metal mold Inside, the insulation material 1 is a riser insulation material product or graphite material.

The data testing unit includes a load testing mechanism and a temperature testing mechanism; the load testing mechanism is an "S" type sensor 6, and the "S" type sensor 6 is connected to the sample mold unit through a connecting screw 5; the temperature testing mechanism is a "K" type nickel-chromium - Nickel-silicon thermocouple, "K" type nickel-chromium-nickel-silicon thermocouple is directly inserted into the temperature measuring port 13 on the upper part of the metal mold 2 .

The data acquisition unit adopts a paperless recorder; the data sampling period of the paperless recorder is 0.1S, and the data acquisition is stopped when the temperature value drops to 300°C.

The load applying unit includes a trapezoidal screw 8 , a bearing housing 12 , a coupling 9 and a speed-regulating motor 10 connected in sequence; the trapezoidal screw 8 is connected to the "S" type sensor 6 through a frame-shaped connection mechanism 7 .

A full thread screw 3 is installed at the end of the connecting screw 5; a graphite plug 4 is arranged at the joint between the connecting screw 5 and the sample mold unit.

Example 1

(1) Materials used in the experiment: A356 alloy (the alloy composition is shown in Table 1), graphite insulation material, Al-10%Sr

Modifier, Al-5%Ti-1%B refiner, DSG aluminum alloy slag and degasser, paint.

Chemical composition (wt%) of table 1A356 alloy

(2) Instruments and equipment used in the experiment: intermediate frequency furnace, 12KW resistance furnace, active load thermal cracking test device, TP1000 paperless recorder, "K" type nickel-chromium-nickel-silicon thermocouple.

(3) Preparations before the experiment:

① Before the experiment, check whether the paperless recorder, speed-regulating motor, tension-pressure sensor, "K" type nickel-chromium-nickel-silicon thermocouple and other components are working normally.

②Adjust the connecting screw to the initial position, and tighten the full-thread screw at the end of the connecting screw.

③ Mix the paint used in the experiment, the paint ingredients are zinc oxide, water glass and water.

④Connect the signal output terminal of the "K" type nickel-chromium-nickel silicon thermocouple and the "S" type sensor to the signal input terminal of the paperless recorder.

(4) Experimental operation steps:

The first step: paint the metal mold, and install the required graphite insulation material and graphite plug on the metal mold, and then preheat the metal mold, the preheating temperature is about 230 ℃.

The second step: melting. The specific process is:

① Put the weighed A356 alloy into the crucible, and put the crucible in the intermediate frequency furnace for melting;

②Transfer the melted A356 alloy (aluminum liquid) to the resistance furnace for heat preservation, and the heat preservation temperature of the resistance furnace is set to 730°C;

③ When the temperature of the molten aluminum is 700°C to 720°C, evenly sprinkle 0.5% of the total weight of the A356 alloy DSG aluminum alloy deslagging and degassing agent on the surface of the molten aluminum, and repeatedly press it into the molten aluminum for 3 to 5 times with the back of a pressure spoon. Stand for 10 minutes, scoop out a small amount of aluminum liquid and put it on the refractory brick to observe the degassing effect;

④ Add Al-5%Ti-1%B refiner with 0.5% of the total weight of A356 alloy, stir for about 3 minutes, and let stand for 10 minutes;

⑤Add 0.2% of the total weight of the A356 alloy Al-10%Sr modifier, stir for 3 minutes, and let stand for 10 minutes.

Step 3: Pouring at 730°C.

Step 4: After the alloy pouring is completed, the paperless recorder starts to collect temperature data. When the temperature drops to 610°C, turn on the power of the speed-regulating motor, and adjust the speed of the speed-regulating motor to 20r/min (at this speed, the horizontal The loading speed is 0.7mm/s), until the sample is completely cracked or the sample is completely solidified, stop the motor, stop the temperature collection when the temperature drops to 300°C, and finally open the metal mold to take out the sample. In this experiment, the rod-shaped sample was broken. By observing the surface morphology of the fracture under the scanning electron microscope, it can be proved that the fracture is a thermal crack.

The fifth step: data processing. During the experiment, the curves of measured temperature and applied load are shown in Fig. 3. From the graph given in Figure 3, it can be seen that the primary crystal phase temperature of the alloy measured in the experiment is 609.8°C, and the eutectic temperature is 564.8°C, which is slightly lower than the theoretical eutectic temperature, mainly because this experiment uses The metal mold is poured, and the overall cooling rate of the casting is relatively fast. It is a normal phenomenon that the eutectic temperature of the alloy measured by the experiment is lower than the theoretical eutectic temperature, and this phenomenon has been verified in other literatures. Through the load curve, three prominent feature points can be obtained, that is, the stage where the sample is applied with load (point B in Figure 3), the stage where cracks appear in the sample (point C in Figure 3) and the final fracture stage of the sample (point B in Figure 3). Point E in Figure 3). Area A in the figure shows the pressure of the molten liquid metal on the tension-compression sensor, area D shows a decrease in strength value due to the existence of intergranular bridging between cracks in the sample, and area F shows What is important is the friction between the broken sample and the metal mold.

Example 2

Concrete experimental procedure sees embodiment 1 for details, difference: the thermal insulation material used inside the metal mold cavity is changed into riser thermal insulation material by graphite thermal insulation material; Insulation bricks are used around the mold for insulation; the horizontal loading speed is controlled at 0.15mm/s.

The curves of the measured temperature and applied load are shown in Fig. 4. In this embodiment, due to the reduction of the overall cooling rate of the metal mold, the experimentally measured primary crystal phase temperature (615.5°C) and eutectic temperature (576°C) are closer to the theoretical temperature of the alloy. It can be seen from the curve of the applied load that there are 3 characteristic points and 3 characteristic regions similar to Example 1. The critical load values of the two examples cracked at high temperature are different. This is mainly due to the fact that the overall cooling rate of the metal mold is reduced in Example 2. During the process of actively applying load, the sample is not completely solidified, that is, the interdendritic The strength is not fully established before being pulled, that is to say, the sample is pulled when the solid phase ratio is less than 0.9.

Although the specific implementation of the present invention has been described above in conjunction with the accompanying drawings, it is not a limitation to the scope of protection of the present invention. Those skilled in the art should understand that on the basis of the technical solution of the present invention, those skilled in the art do not need to pay creative efforts. Various modifications or deformations that can be made by labor are still within the protection scope of the present invention.

Claims (8)

1. an aluminium alloy hot tearing experimental apparatus for testing under the imposed load condition initiatively is characterized in that, mainly comprises the sample mould unit, connecting screw rod, data test unit, frame type bindiny mechanism, the load applying unit that connect successively; Described sample mould unit, connecting screw rod, data test unit, frame type bindiny mechanism, load applying unit are positioned at equipment base top; Data acquisition unit also is connected with the data test unit, and described sample mould unit comprises metal die; Described metal die comprises two metal half modules; Described metal die inside is provided with die cavity; Described metal die top is provided with temperature and surveys mouth.

2. aluminium alloy hot tearing experimental apparatus for testing under a kind of active imposed load condition according to claim 1 is characterized in that, in the described die cavity insulation material is installed; Described insulation material is riser head heat-preserving material product or graphite material.

3. aluminium alloy hot tearing experimental apparatus for testing under a kind of active imposed load condition according to claim 2 is characterized in that described data test unit comprises load test mechanism and temperature test mechanism; Described load test mechanism is " S " type sensor, and " S " type sensor links to each other with the sample mould unit by connecting screw rod; Described temperature test mechanism is " K " type nickel chromium-nickel silicon thermocouple, and the temperature that " K " type nickel chromium-nickel silicon thermocouple directly inserts metal die top is surveyed mouthful.

4. aluminium alloy hot tearing experimental apparatus for testing under a kind of active imposed load condition according to claim 3 is characterized in that, described data acquisition unit adopts recording instrument without paper; The described recording instrument without paper data sampling cycle is 0.1S.

5. aluminium alloy hot tearing experimental apparatus for testing under a kind of active imposed load condition according to claim 4 is characterized in that, described load applying unit comprises trapezoidal screw, bearing seat, shaft coupling and the buncher that connects successively; Described trapezoidal screw links to each other with " S " type sensor by frame type bindiny mechanism.

6. aluminium alloy hot tearing experimental apparatus for testing under a kind of active imposed load condition according to claim 5 is characterized in that, described connecting screw rod end is equipped with full silk screw rod; Described connecting screw rod and junction, sample mould unit are provided with the graphite plug.

7. aluminium alloy hot tearing experimental apparatus for testing under a kind of active imposed load condition as claimed in claim 6, it is characterized in that, the center line of the output shaft of buncher, frame type bindiny mechanism, " S " type sensor, connecting screw rod, metal die all is positioned at the same level line, the right alignment≤0.15mm during assembling.

8. the method for aluminium alloy hot tearing experimental apparatus for testing test aluminium alloy hot tearing under a kind of active imposed load condition as claimed in claim 7 is characterized in that, may further comprise the steps:

(1) checks before the experiment whether recording instrument without paper, buncher, " S " type sensor, " K " type nickel chromium-nickel silicon thermocouple and shaft coupling work;

(2) preheating metal die makes the temperature of metal die remain on 200～250 ℃;

(3) connecting screw rod is adjusted to initial position, and the full silk screw rod of connecting screw rod end is tightened;

(4) install; Then the signal input data line of insulation material, graphite plug and recording instrument without paper prepares to carry out the alloy cast;

(5) behind the alloy casting complete, according to alloy species, Hot-Crack Tendency of Alloy size, select suitable alloy graining time and motor speed, carrying out load loads, until the complete drawing crack of sample or sample stop the work of buncher and recording instrument without paper after solidifying fully, data behind the recording instrument without paper record change PC over to and process, and last opening metal mould takes out sample.

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