Determination Of Residual Stress In Titanium Standard Parts And Titanium Screw Casting

Aug 22, 2019

Casting stress is the internal stress caused by resistance to shrinkage (or expansion) after casting cooling into the elastic zone.

Casting stress has great influence on casting quality especially for titanium standard parts and titanium screws. If the total stress value of casting exceeds the yield limit of alloy, it will cause casting deformation and reduce casting dimensional accuracy. If the total stress exceeds the strength limit of the alloy, cold crack will occur in the casting, leading to scrap. Residual stresses exist in castings and work under alternating loads. If the residual stress is in the same direction as the load acting force, the total internal and external stress of the casting may exceed the strength limit of the material and be destroyed, or even cause serious accidents. Therefore, it is of great practical significance to detect the residual stress of casting, study the process of the generation and development of residual stress, reduce and eliminate the residual stress of casting as far as possible, and improve casting quality in order to formulate corresponding technological measures.

I. purpose of the experiment

To learn the method of measuring the residual stress of casting, to improve the perceptual understanding of the process of the generation and development of residual stress of casting.

II, experimental principle

As the casting continues to cool after solidification, solid shrinkage will occur. If the wall thickness of each part of the casting is different or the shrinkage of each part is not consistent or hindered due to the process and other factors, then the internal stress - casting stress will be generated.

In the process of casting, stress in casting is almost inevitable due to various reasons. Casting stress has a great influence on casting quality, which is the fundamental cause of casting deformation and crack in cooling process, cutting process or using process. Castings used in corrosive media can also lead to stress corrosion. The casting stress can be divided into mechanical stress and thermal stress according to the reason of its formation.

1. Mechanical stress

Mechanical stress, also known as shrinkage stress, is the stress caused by mechanical obstruction in the process of casting shrinkage. The formation of a lot of reasons, such as sand ground too tight, molding sand and core sand high temperature strength is too high, poor retreat.

Mechanical stresses are generally tensile stresses. Because it is the stress produced when the casting is in the elastic state, the stress will disappear when the cause of stress is eliminated, such as falling sand and breaking the casting riser system. Mechanical stress is a temporary stress.

2. Thermal stress

Thermal stress is the stress caused by the unevenness of the casting wall thickness and the different cooling speed of each part, so that the shrinkage of each part of the casting is not consistent in the same time. Once this stress is formed, it remains at room temperature. It is the main reason of casting deformation and cracking. Therefore, when designing the casting, the cooling speed of each part should be as consistent as possible to achieve simultaneous solidification, which can reduce the thermal stress of the casting.

III.Experimental equipment and apparatus

ZQY casting stress dynamometer,EX series table recorder, crucible resistance furnace casting tool, aluminum and its alloy, thermocouple.

IV. Experimental contents

The residual stress of ZL203 al - cu alloy and ZL102 al - si alloy were measured.

5. Experimental procedures

(1) the main engine shall be laid flat and the three probe heads shall be tightly connected with the sensor with nuts without looseness.

(2) the self-hardening sand casting mold should be placed on the bracket, and the mold cavity and the probe should be closely coordinated to prevent the outflow of metal liquid.

(3) connect the wires according to the diagram above, and the power supply voltage of the sensor is 6v.

(4) adjust the table recorder, choose 1 or 2 strokes to record the stress, the measuring range is 5mV block, the zero point is chosen in the middle of the recording paper,3 or 4 strokes to record the temperature, and the measuring range is 50 mV block. Record speed 1200 mm/h. Put down the recording pen and turn on the paper switch to check whether the recording is normal.

(5) tighten the hydraulic bolts on both sides of the body to make the pre-pressure above 1500 N (indicating by pressure gauge).

(6) turn on the cooling water of the sensor and check whether all preparations are ready.

(7) when the aluminum alloy overheats to 750℃, take out the rapid pouring with a small casting ladle (note: before pouring, the sensor must be supplied with cold water, otherwise it will burn out the sensor).

(8) observed records are normal and the change of temperature and stress, 404 x 86 kg/mV, sensor output Ⅰ Ⅱ 403 x 24 kg/mV, Ⅲ 409 x 84 kg/mV.

(9) the temperature drops to 120℃ and the voltage is about 5 mV. After the test, turn off the recorder, loosen the hydraulic bolt, reduce the pre-pressure to zero, loosen the connecting nut of the probe, remove the sand mold, clean out the casting, and observe whether there is any defect.

(10) clean up the experimental site.