PSI - Issue 68

T.N. Examilioti et al. / Procedia Structural Integrity 68 (2025) 756–761 T.N. Examilioti et al./ Structural Integrity Procedia 00 (2025) 000–000

757

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Keywords: Laser Beam Welding, Al-Cu-Li, Mechanical Performance, Quality Index

1. Introduction Aluminum alloys have long been exploited to aerospace and automotive industries due to their high strength-to weight ratio and excellent corrosion resistance. Recent advancements in material science focus on developing lighter aluminium alloys with enhanced specific mechanical properties and improved weldability, particularly for aircraft structures where weight savings are critical (Montgomery, 2007). Among these innovations, third-generation aluminium-copper-lithium (Al-Cu-Li) alloys, such as aluminium alloys (AA) 2198 and 2196, have appeared as promising materials, offering superior mechanical properties and enhanced damage tolerance, (Dursun and Soutis, 2014). This is mainly due to the lithium (Li) addition, which reduces density and increase elastic modulus, thereby improving the overall mechanical performance (Lavernia and Grant, 1987), (Prasad et al., 2003). These alloys are aged-hardened, with their precipitate microstructure and mechanical properties significantly influenced by heat treatment conditions (precipitation hardening), including artificial ageing (Kumar et al., 1996). Although Al-Cu-Li alloys face significant weldability challenges, such as porosity and hot cracking during fusion welding, laser beam welding (LBW) has become a preferred joining technique in commercial aircraft. To address these issues, the exploitation of filler materials such as Silicon (Si), Magnesium (Mg), and Copper (Cu) can essentially improve mechanical strength and toughness in welded joints. Studies indicate that Mg and Si filler materials, effectively reduce porosity and hot-cracking. Zhang et al. (2016) and (2015), showed that LBWed AA2060 joints with the use of Al-Si filler material achieved a tensile strength of about 83 % higher than the autogenous welded joint. In contrast, the exploitation of Al-Mg filler material resulted in slightly lower mechanical properties than the previous filler material, i.e. at the 63 % higher ultimate tensile strength than the respective value of the autogenous welded joint. Similar, Ning et al (2017), used Al-Cu filler material which resulted in a tensile strength of about 49 % of the base material (BM), attributed due to eutectic phase formation in the fusion zone due to the high cooper content in this zone. The present investigation focuses on the global evaluation of the tensile mechanical performance of LBWed joints. In this regard, the concept of quality index will be exploited that comprises the mechanical strength and tensile toughness capabilities of the welded joints. Several cases studies will employed, like different filler materials, different ageing conditions or even different sheet AA2198 thickness to evaluate the overall “quality” of the LBWed joints for tensile mechanical performance. 2. Experimental procedure AA2198 sheets in T3 temper (solution heat treated, cold worked, and naturally aged) with nominal thickness of 3.2 mm and 5.0 mm were used. The sheets were laser beam welded (LBWed) in a butt joint configuration, with two different filler materials AA4047 (Al-Si) and AA2319 (Al-Cu). The weight percentage chemical composition of used materials, given in Table 1 .

Table 1. Chemical composition of investigate materials (in wt.-%).

Alloy

Si

Fe

Cu

Mn 0.1 0.15

Mg 0.32

Li

Zn 0.1 0.2

Zr

Ag

Ti

Al

AA2198 AA4047 AA2319

0.03 12.0 0.20

0.05

3.35

0.99

0.14

0.27

0.30

Bal. Bal. Bal.

0.8

0.3

0.1

- -

- -

- -

- -

0.30

6.50

0.20-0.40

0.02

0.10

Different artificially ageing heat treatments were performed to investigate the effect on the mechanical properties. The sheets were post-weld heat treated (PWHTed) at 170 o C for different ageing times according to the study of Alexopoulos et al. (2016). Several ageing times were selected namely, 3 h, 48 h and 98 h, to correspond to all ageing conditions, including under-aged (UA), peak-aged (PA) and over-aged (OA) conditions, respectively.