Casting stress is the internal stress formed by the resistance of shrinkage (or expansion) after the casting cools into the elastic region.
The casting stress has a great influence on the quality of castings. If the total stress value of the casting exceeds the yield limit of the alloy, it will cause deformation of the casting and reduce the dimensional accuracy of the casting. If the total stress exceeds the alloy strength limit, the casting will produce cold cracks, resulting in scrap. Residual stress exists in the casting and works under alternating load. If the residual stress is in the same direction as the load, the sum of the internal and external stresses of the casting may exceed the allowable strength limit of the material and cause damage, or even cause a major accident. Therefore, it is of great practical significance to detect the residual stress of castings, study the generation and development process of residual stresses, to formulate corresponding technological measures, minimize and try to eliminate the residual stresses of castings, and improve the quality of castings.
1. Experimental purpose
Learn the method of measuring the residual stress of castings, and improve the perceptual understanding of the generation and development of residual stresses of castings.
2. Experimental principle
The casting will continue to cool after solidification, and solid shrinkage will occur. If the wall thickness of each part of the casting is different or due to factors such as process, the shrinkage of each part is inconsistent or the shrinkage is hindered, then internal stress will be generated-casting stress.
During the casting process, due to various reasons, the generation of stress in the casting is almost inevitable. Casting stress has a great influence on the quality of castings. It is the root cause of deformation and cracks in the cooling process of the casting and in the subsequent cutting process or the use of the casting. Castings used in corrosive media can also cause stress corrosion. Casting stress can be divided into mechanical stress and thermal stress according to the reasons for its formation.
1. Mechanical stress
Mechanical stress is also called shrinkage stress, which is the stress formed by mechanical obstruction during the shrinking process of castings. There are many reasons for the formation, such as the over-tightness of the sand mold, the high-temperature strength of the molding sand and the core sand is too high, and the concession is poor.
Mechanical stress is generally tensile stress. Because it is the stress generated when the casting is in the elastic state, when the cause of the stress is eliminated, such as falling sand and breaking the riser system, the stress will disappear. Mechanical stress is a temporary stress.
2. Thermal stress
The thermal stress is the stress caused by the uneven wall thickness of the casting and the different cooling rates of the parts, so that the shrinkage of the parts of the casting is inconsistent at the same time. Once this stress is formed, it will remain until room temperature. It is the main reason for casting deformation and cracking. Therefore, when designing castings, it is necessary to make the cooling speed of each part consistent to achieve simultaneous solidification, which can reduce the thermal stress of the casting.
3. Experimental equipment and equipment
ZQY casting stress dynamic tester (Figure 1-9), EX series desktop recorder, crucible resistance furnace casting tool, aluminum and its alloys, thermocouple.
4. Experimental content
Determine the residual stress value and stress development process of ZL203 aluminum-copper alloy and ZL102 aluminum-silicon alloy (schematic experiment diagram is shown in Figure 1-10).
V. Experimental procedure
(1) The main body is flat, and the three probes and the sensor are tightly connected with nuts, and there must be no looseness.
(2) The self-hardening sand mold is placed on the bracket, and the cavity and the probe must be closely matched to prevent the outflow of metal liquid.
(3) Connect the wires according to the above diagram, the sensor power supply voltage is 6 V.
(4) Adjust the desktop recorder, select one or two pens to record the stress, the range is 5mV, the zero point is selected in the middle of the recording paper, three or four pens record the temperature, and the range is 50 mV. Select the recording speed of 1200 mm / h. Put down the stylus, turn on the paper switch, and check whether the record is normal.
(5) Tighten the hydraulic bolts on both sides of the machine body so that the pre-pressure is above 1500 N (indicated by the pressure gauge).
(6) Turn on the sensor cooling water and check whether all preparations are ready.
(7) The aluminum alloy is overheated to 750 ℃, take out the small pouring ladle and quickly cast it (note that the sensor must be cold water before pouring, otherwise it will burn the sensor).
(8) Pay attention to observe whether the record is normal and the temperature and stress change, the sensor output Ⅰ404×86 kg/mV, Ⅱ403×24kg/mV, Ⅲ409×84kg/mV.
(9) The temperature drops to 120°C and the voltage is about 5 mV. After the test, turn off the recorder, loosen the hydraulic bolts, the pre-pressure is reduced to zero, loosen the probe connection nut, remove the sand mold, clean out the casting, observe flawless.