Effect of pulsed metal inert gas (pulsed-MIG) and cold metal transfer (CMT) techniques on hydrogen dissolution in wire arc additive manufacturing (WAAM) of aluminium

Abstract

Aluminium is one of the most experimented metals in the WAAM field owing to a wide range of applications in the automotive sector. Due to concerns over reduction of strength, elimination of porosity from wire arc additive manufactured aluminium is one of the major challenges. In line with this, the current investigation presents findings on hydrogen dissolution in solid aluminium and hydrogen consumed to form porosity along with its distribution as a function of heat inputs and interlayer temperatures in a WAAM 5183 aluminium alloy. Two varieties of WAAM, pulsed metal inert gas (MIG) and cold metal transfer (CMT), were explored. Samples made with pulsed metal inert gas (pulsed-MIG) process picked up more hydrogen compared to samples produced by cold metal transfer technique. Correspondingly, pulsed-MIG samples showed increased number of pores and volume fraction of porosity than samples manufactured using the cold metal transfer (CMT) technique for different heat input and interlayer temperature conditions. However, CMT samples exhibited higher amount of dissolved hydrogen in solid solution compared to pulsed-MIG process. In addition, heat input, interlayer temperature, and interlayer dwell time also played a key role in pore formation and distribution in WAAM-produced aluminium 5183 alloy.

Appendix

Dissolved hydrogen calculations for sample C-LH-T2

• 1. Total volume of the sample pore measurement by XCT scan = 1440 mm³

  Mass of the sample considered can be calculated as

  $$ {\displaystyle \begin{array}{c}\mathrm{Mass}=\mathrm{density}\times \mathrm{volume}\\ {}=2.7\times {10}^{-3}\ \left(\mathrm{g}/{\mathrm{mm}}^3\right)\times 1440\ {\mathrm{mm}}^3\\ {}=3.888\ \mathrm{g}\end{array}} $$

• 2. Total volume of the pores found in 1440 mm³ (3.888 g) of samples volume = 0.44 mm³.

• 3. Weight of the samples tested for dissolved hydrogen = 0.402 g

  Thus, corresponding volume of the pores in samples of weight 0.402 g can be calculated as

$$ =0.402\ \left(\mathrm{g}\right)\times 0.44\ \left({\mathrm{mm}}^3\right)/3.888\ \left(\mathrm{g}\right)=0.04549\ {\mathrm{mm}}^3 $$

• 4. Total hydrogen detected after dissolved hydrogen test 0.834 ppm

  ppm to ml conversion can be as follows:

$$ 1\ \mathrm{ppm}=1.12\ \left(\mathrm{ml}\right)/100\ \left(\mathrm{g}\right) $$

Thus, 0.834 ppm are

$$ =0.834\ \left(\mathrm{ppm}\right)\times 1.12\ \left(\mathrm{ml}/100\ \mathrm{g}\right)/1\ \left(\mathrm{ppm}\right)=0.93408\ \mathrm{ml}/100\ \mathrm{g} $$

Hence, 0.93408 ml of hydrogen per 100 g of metal.

• 5. Weight of the samples for dissolved hydrogen test was 0.402 g.

  Thus, total hydrogen for 0.402 g of metal can be calculated as

$$ =0.93408\ \left(\mathrm{ml}\right)\times 0.402\ \left(\mathrm{g}\right)/100\ \left(\mathrm{g}\right)=0.003755\ \mathrm{ml} $$

Hence, 0.402 g of tested samples showed 0.003755 ml (375.5 × 10⁻⁵ ml) of total detected dissolved hydrogen.

• 6. From point (3), we know that 0.402 g of samples showed 0.04549 mm³ of pore volume. Here, we are assuming that all the pores are completely filled with hydrogen.

  Therefore, converting pore volume from mm³ to ml, we get

$$ =0.04549\ \left({\mathrm{mm}}^3\right)=4.549\times {10}^{-5}\ \left(\mathrm{ml}\right) $$

Hence, in a sample of weight 0.402 g with 0.04549 mm³ of pore showed 4.549 × 10⁻⁵ ml of hydrogen.

• 7. From point (5), we know that total hydrogen in sample was 375.5 × 10⁻⁵ ml.

  From point (6), it was clear that hydrogen in the pore was 4.549 × 10⁻⁵ ml.

  Thus, dissolved hydrogen ca ne calculated as

  $$ {\displaystyle \begin{array}{c}=\left(375.5\hbox{--} 4.549\right)\times {10}^{-5}\\ {}=370.951\times {10}^{-5}\ \mathrm{ml}\end{array}} $$

Dissolved hydrogen in the sample was 0.00370951 ml (370.951 × 10 ⁻⁵ ml).

• 8. Percentage of dissolved hydrogen with respect to total hydrogen in sample

  $$ {\displaystyle \begin{array}{c}=\left(370.951\times {10}^{-5}\right)/\left(375.5\times {10}^{-5}\right)\times 100\\ {}=98.78\%\end{array}} $$

Thus, samples C-HH-T2 showed 98.78% of dissolved hydrogen and 1.22% of hydrogen in pores.

Dissolved hydrogen values for other samples after following similar calculations are summarized in Table 11.

Table 11 Details of dissolved hydrogen values samples wise