African Fusion November 2017

Element

C

Mn

Si

S

P

Ni

Cr

Cu

Fe

Base metal Filler metal

0.16 0.08

0.78 1.52

0.30 0.033 0.035 0.28

0.30

0.30 0.30

Balance Balance

0.10 0.034 0.022

–

–

Table 3: Chemical composition of the base and filler metals (wt-%).

camera (2000 frames s -1 ) was used to observe the arc shape. The welding parameters are given in Table 1. The experimental design is shown in Table 2. Each gas constituent (CO 2 /He) was varied within its range while the other component remained constant. The shielding gas flow rate was 40 ℓ/min. Q235 and H08Mn2Si were used as the base and filler metals, respectively. The diameter of the wire was 1.2 mm. The chemical compositions of base metal and filler metal are given in Table 3. The properties of shielding gas have effects on arc shape. Figure 2 gives the arc shape in helium content. Increasing the heliumcontent decreases the arc length (La). When the helium content exceeds 20%, the arc length is short. Helium has a higher thermal conductivity than argon. With the increase of helium content in shielding gas, increas- ing the thermal conductivity increases the cross-sectional area through which heat can flow, thereby leading to a broader arc core. This typeof arc produces awider penetrationprofile, which improves fusion characteristics especiallywhenwelding inside the groove, and this is particularly beneficial for narrow gap welding to ensure sidewall fusion. The arc voltage-current properties are important to wire melting inGMAW, and shielding gas compositionaffects the arc properties. However, it’s hard tomeasure theMAG-arc voltage- current properties, so the influence of helium-additions to argon GTA-arc properties was investigated. Murphy et al considered that general conclusions of GTA welding are relevant to GMA welding, since the effects of dif- ferent gas mixtures on the arc properties will be similar [9]. Hence, the changing trends of arc properties is considered to also be applicable to the MAG arc with argon-based shielding gas inwhich theCO 2 content was fixedat 10%andGMAwelding was replaced with GTA welding to conduct the measurement of arc voltage and current. The arc voltage in different current and different helium content was measured using GTA welding with an arc length of 3.0mmand a shielding gas flow rate of 15 ℓ/min. The results are presented in Figure 3. It can be seen that, for the same current, the arc voltage increases with increasing of helium content, and the arc voltage-current curve is offset upwards. On one hand, helium has a low density and it disperses from the centre to the periphery of the arc. More heat is transferred to the periphery when the shielding gas has a higher helium content, so the potential gradient of the arc Results and discussion Arc characteristics in different helium content

Figure 2: The arc shape variation with helium content.

Figure 3: The arc voltage-current characteristics for different helium contents.

He (%) 25 Voltage (V) 26.5 26.99 26.91 26.96 27.16 26.97 Current (A) 308.7 323.3 317.1 308.5 304.4 302.6 Table 4: Arc voltage and current in different helium content. increases to compensate for the energy loss. On the other hand, heliumhas a higher ionisation energy, so the potential gradient increaseswith increasing heliumcon- tent. Therefore, for the same currentwitha constant arc length, the arc voltage increases with increasing helium content. Wire melting characteristics in different helium content The output mode of the GMAS power source is constant volt- age. The corresponding arc voltage and current in different helium content are given in Table 4. It can be seen that the arc voltage remains constant. 0 5 10 15 20

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November 2017

AFRICAN FUSION