The Weld Formation And Nondestructive Detection Of The Micro-porosity Of Thick TC4 Titanium Alloy With Electron Beam Welding

In this paper, the TC4 titanium alloy with 30 mm thickness was welded by electron beam welding. The influence of process parameters on the weld appearance was studied,the nondestructive detection of the electron beam weld was conducted and the formation mechanism of micro-porosity in the weld was analyzed. The cross section appearance of the weld was divided into different types and the nondestructive testing such as X-ray and ultrasonic water-immersion focusing was conducted respectively. The distribution characteristics of the micro-porosity in the weld of different types were analyzed, the formation causes of the micro-porosity was studied.The results showed that the beam current and the welding speed are mainly affect the weld penetration and the weld width, there is no significant impact on the cross section appearance type. The weld penetration and the weld width increased with the increasing of the beam current, but the up weld width has a significant increasing rather than the weld penetration when the beam current is less than a certain value. The residence time of the electron beam on the workpiece and the heat input per unit time decreased with the increasing of the welding speed, so the weld penetration and the weld width decrease, the weld turns to unpenetration from penetration when the welding speed reaches a certain value. The working distance has a different effect on the weld appearance compared to the beam current and the welding speed, the weld appearance in different working distance can be divided into four types, they are bell pattern, nail pattern, funnel pattern and chock pattern. The microstructure of the weld zone is coarseβ columnar crystal, the size of the columnar crystal decreased from the top to the bottom of the weld, and the growth direction is different in different area of the weld,the columnar crystal in the up of the weld growing upward along the weld center line while the ones in the bottom of the weld growing perpendicular to weld center line. The microstructure inside the β columnar crystal is α’ acicular martensite, which is formed under the condition of rapid cooling. The microstructure of the HAZ is the mixture of αphase and β phase and α’ acicular martensite.The X-ray has a worse detection result compared to the ultrasonic water-immersion focusing in this experimental condition. The porosity of the four different type welds are different, the chock pattern weld has a minimum porosity with 1.89 percent while the nail pattern weld has a maximum porosity with 20.15 percent.The micro-porosity in the electron beam weld can be classified into two types: one is hydrogen porosity with a circular shape and smooth inner wall, and the other one is process porosity with a irregular shape and rough inner wall. The cause of hydrogen porosity is the hydrogen produced from surface moisture, grease and oxide during the welding process and the change of the hydrogen solubility in titanium alloy is another reason for the formation of porosity. While the process porosity is caused by the keyhole instability, the impact of the metal vapor in the bottom of the keyhole leads to the collapse, and the metal vapor is involved into the hole until the metal solidification.The micro-porosity is mainly in the upper portion of the weld, which adversely affect the mechanical properties of the weld.