Temperature measurement and control of electron beam welding process in hydraulic actuator

 

Keywords: hydraulic actuator, electron beam welding, temperature control

 

Preface

 

The hydraulic actuator is a component of a certain type of engine in China. It consists of parts such as a cover, a liner, a piston and a rubber Seal. The cover and liner are assembled by vacuum electron beam welding. Its structural characteristics determine the following special requirements for the welding process:

 

    1 ) The temperature rise of the component should be small

Rubber Seals are used in many places in the hydraulic actuator. According to the design requirements, the welding process is 20mm away from the center of the weld. The maximum temperature during the welding process cannot exceed 100 °C. If the temperature rise is too high, it will burn and even burn the seal ring, causing the seal to fail.

 

     2 ) There is no splash in the inner cavity

 

Once the back of the weld is spattered or the weld is detached, flowing into the inner cavity of the hydraulic actuator will cause the inner surface of the cylinder to be strained or the seal ring to be damaged during the movement of the piston, which will not seal, resulting in the entire hydraulic actuator. scrapped.

 

     3 ) Welding deformation is small

 

The wall thickness of the hydraulic actuator is only 2mm , which is easy to deform during the welding process, which causes the piston of the hydraulic actuator to be unable to move normally. Therefore, the design requires that the ellipticity should not exceed 0.02mm .

 

In summary, on the one hand, the assembly has strict requirements for welding deformation. In addition, since the rubber seal is assembled near the inner cavity weld during the combined welding, the temperature of the assembly process is strictly limited, which brings special difficulties to the welding process. . Splash and deformation can be controlled by reasonable selection of joint form and control of welding process parameters, while the strict control technology of component temperature rise lacks accumulation and experience. Therefore, strict control of component temperature in welding process becomes the key to successful assembly of the component. Need to focus on experimentation and exploration.

 

     Optimization of temperature measurement method in vacuum chamber

 

Optimization of temperature measurement method

 

Between 200 and 100 BC, the ancient Greek Philippine and Hilong each produced a temperature detector based on the principle of air expansion. Since then, the development of temperature measurement technology has varied and varied. The classification and characteristics of commonly used temperature measurement methods are shown in Table ( 2 ). It can be seen from the comparison of the above table that the expansion method, the pressure method, the electric resistance method and the thermal radiation method have their limitations due to the need to measure the temperature under vacuum. The use of K -type thermocouple for temperature measurement has the advantages of good linearity, fast response, high precision, wide temperature range, suitable recording and control, and moderate price. Especially on-site implementation is simple and operability is strong. We use K -type galvanic heat to measure the temperature of the workpiece during the electron beam welding process.

 

     Classification and characteristics of commonly used temperature measurement methods

 

         Implementation of temperature measurement method

 

Since it is necessary to measure the temperature of the workpiece in the vacuum chamber, it is first necessary to solve the problem of the connection between the thermocouple and the workpiece. In the actual processing, we use capacitor energy storage welding to solder the thermocouple to the specified temperature measurement part, and use the capacitor energy storage welding metal foil strip to fix the compensation wire of the thermocouple.

 

The compensation wire is connected to the reserved terminal of the vacuum electron beam welder. The corresponding terminal block outside the vacuum chamber is connected to the DR020R digital temperature circuit tester through the compensation wire . The recorder can automatically measure, display, monitor and record the workpiece temperature during the welding process. It is also possible to read and print the temperature values ​​of the workpieces in the welding process as measured by the thermocouple.

 

In the actual welding, it should be noted that the compensation wire connecting the thermocouple inside the vacuum chamber should be wound around the workpiece two times, and then the appropriate margin is reserved before connecting to the reserved terminal of the vacuum electron beam welding machine to prevent the workpiece from rotating during the welding process. The thermocouple is broken, which affects the normal temperature measurement and welding work.

 

     Control of workpiece temperature during vacuum electron beam welding

 

In practice, the control of the temperature rise of the workpiece is achieved by using the heat-dissipating tooling and the preferred welding parameters. A pair of semi-cylindrical copper heat sinks with a thickness of about 30 mm are used to closely fit the outer surface of the hydraulic actuator to assist in heat dissipation.

 

First, the process test is carried out by means of a dummy piece. The preferred welding parameters are as follows:

 

Acceleration voltage: 150KV

 

Welding current: 7.5mA

 

Focusing current: 2088mA

 

Welding speed: 20 mm / s

 

After welding, visual inspection, X- ray and fluorescence inspection of the simulated parts are carried out, and there are no defects such as incomplete penetration, cracks and pores. At the same time, the metallographic anatomical test was carried out, and the reverse surface was well formed without spatter.

 

The actual measurement result is: the temperature rise of the near seam area is 62 °C 20 mm from the center of the weld , and the temperature rise control of the workpiece effectively meets the design requirements.

 

On the basis of the test, we carried out the welding of the actual components. In total, nearly 100 parts of the assembly were welded, no scrap, and the ovality of the barrel after welding was less than 0.005 mm, and the geometrical dimensions met the design drawings.

 

in conclusion

 

The method of measuring temperature when assembling the hydraulic actuator by electron beam welding is economical and practical, and the process of workpiece temperature control is simple and effective. The processed parts have an increase in the ellipticity of the cylinder by no more than 0.005mm , and the temperature during the welding process is about 62 °C from the weld at 20mm . The temperature rise control of the workpiece effectively meets the design requirements and has been put into mass production. This method is also suitable for the welding of other parts that have special requirements for the temperature of the components during the welding process.