Test of titanium standard parts and titanium screws casting residual stress

Casting stress is the internal stress formed by the resistance to shrinkage (or expansion) after the casting is cooled into the elastic region.

Casting stress has great influence on casting quality. If the total stress value of the casting exceeds the yield limit of the alloy, it will cause the casting deformation and reduce the dimensional accuracy of the casting. The total stress value exceeds the alloy strength limit, the casting will produce cold crack, resulting in scrap. Residual stress exists in the casting and operates under alternating loads. If the residual stress is consistent with the direction of the load force, the sum of the internal and external stresses of the casting may exceed the allowable strength limit of the material and be destroyed, or even cause a major accident. Therefore, detecting the residual stress of castings, studying the generation and development process of the residual stress, in order to formulate corresponding technological measures, minimize and try to eliminate the residual stress of castings, and improve the quality of castings, has very important practical significance.

I. Purpose of the experiment
Learn the method of measuring the residual stress of castings and improve the perceptual understanding of the generation and development process of the residual stress of castings.

Second, experimental principle
During the continuous cooling of castings after solidification, solid shrinkage will occur. If the wall thickness of each part of the casting is different or due to factors such as process, the contraction of each part is inconsistent or hindered, then the internal stress - casting stress will be generated.

During the casting process, due to various reasons, the stress in the casting is almost inevitable. Casting stress has a great impact on the quality of castings, which is the root cause of deformation and cracks in the cooling process and in the later cutting process or the use of castings. Castings used in corrosive media can also cause stress corrosion. According to the cause of its formation, the casting stress can be divided into mechanical stress and thermal stress.

1. Mechanical stress
Mechanical stress, also known as shrinkage stress, is the stress formed by mechanical obstruction during the shrinkage of castings. There are many reasons for the formation, such as too tight sand ramming, high temperature strength of mold sand and core sand, poor yield and so on.

Mechanical stress is generally tensile stress. Since it is the stress generated when the casting is in an elastic state, the stress will disappear as soon as the cause of the stress is eliminated, such as falling sand and breaking the gating system. Mechanical stress is a kind of temporary stress.

2. Thermal stress
Thermal stress is the stress caused by the uneven wall thickness of the casting and the different cooling rates of each part, so that the contraction of each part of the casting is inconsistent at the same time. This stress, once formed, remains at room temperature. It is the main reason for casting deformation and cracking. Therefore, when designing castings, it is necessary to try to make the cooling rate of each part consistent, achieve simultaneous solidification, which can reduce the thermal stress of the castings.

3. Experimental equipment and equipment

ZQY casting stress dynamic tester (FIG. 1-9),EX series bench recorder, Crucible resistance furnace casting tool, aluminum and its alloys, thermocouple.
Iv. Experimental content
The residual stress value and stress formation development process of ZL203 aluminum-copper alloy and ZL102 aluminum-silicon alloy were measured (Fig.1-10).

5. Experimental steps
(1) The host is placed flat, the three probe and the sensor are connected tightly with the nut, and there should be no loosening.
(2) The self-hardening sand mold is placed on the bracket, and the cavity and the probe should be closely matched to prevent the outflow of metal liquid.
(3) Connect the cable according to the above diagram, and the sensor power supply voltage is 6V.
(4) Adjust the bench recorder, select 1 or 2 strokes to record the stress, the range is 5mV, the zero is selected in the middle of the recording paper,3 or 4 strokes to record the temperature, the range is 50 mV. Record speed select 1200 mm/h. Put down the record pen, turn on the paper switch, and check whether the record is normal.
(5) Tighten the hydraulic bolts on both sides of the body, so that the pre-pressure is above 1500 N (pressure gauge indication).
(6) Turn on the sensor cooling water and check whether all the preparatory work is ready.
(7) The aluminum alloy is overheated to 750 ° C, and is quickly poured with a small ladle (note that the sensor must be given 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 changes, the sensor output I 404× 86kg /mV, II 403×24kg/mV, III 409×84kg/mV. (9) The temperature drops to 120℃, the voltage is about 5 mV, the test is over, the recorder is turned off, the hydraulic bolt is loosened, the pre-pressure is reduc
ed to zero, the probe connecting nut is loosened, the sand mold is removed, the casting is cleaned out, and the defects are observed. (10) Clean up the experiment site. Fill out the experiment report (A).