Effect of gas atmosphere on the non-metallic inclusions in laser-welded trip steel with Al and Si additions

Abstract

The present study aims to characterize the weldability of a multiphase, automotive steel containing Al and Si additions from the point of view of its tendency to form non-metallic inclusions. Laser welding tests of 2-mm-thick sheets were performed using the keyhole-welding mode and a solid-state laser. The tests were carried out in air and with the use of an argon atmosphere. The distribution, type and chemical composition of the non-metallic inclusions formed in the base metal and fusion zones were analysed. The effect of applying the protective gas on the type and amount of non-metallic inclusions was determined using light and scanning electron microscopy. The chemical composition of the identified particles was assessed using the EDS method. It was found that a protective gas has a beneficial effect on reducing the non-metallic inclusions, but only to a limited extent. The boundary between the complex oxides and the pure aluminium oxides was determined to be 2-3 μm. Keywords: TRIP steel, laser welding, non-metallic inclusions, protective gas, Ar atmosphere, oxidation Namen te {tudije je opredeliti varivost s stali{~a tvorbe nekovinskih vklju~kov v ve~faznem jeklu, ki vsebuje Al in Si, za avtomobilsko industrijo. Izvedeni so bili varilni preizkusi z laserjem na 2 mm debelih plo~evinah z V zarezo. Preizkusi so bili izvedeni na zraku in v za{~itni atmosferi argona. Analizirana je bila razporeditev, vrsta in kemijska sestava nekovinskih vklju~kov v osnovnem materialu in v coni zlivanja. Vpliv uporabe za{~itnega plina na vrsto in koli~ino nekovinskih vklju~kov je bil dolo~en s pomo~jo svetlobne in vrsti~ne elektronske mikroskopije. Kemijska sestava najdenih delcev je bila ugotovljena z metodo EDS. Ugotovljeno je bilo, da ima za{~itni plin ugoden vpliv na zmanj{anje nekovinskih vklju~kov le v omejenem obsegu. Meja med kompleksnimi oksidi in~istimi oksidi aluminija je bila dolo~ena kot 2-3 μm.

Assessment of Steel Subjected to the Thermomechanical Control Process with Respect to Weldability

“…The precipitation hardening was mainly triggered by dispersive precipitates of several nanometres in size, precipitated in the steel (in ferrite) during cooling ( Figure 2) [28][29][30][31][32]. Particles sized above twenty nanometres did not significantly contribute to precipitation hardening, yet they restrained the recrystallised austenite grain growth, favouring the formation of the fine-grained structure [28,[33][34][35][36][37]. The recrystallised austenite grain growth in the test steel was primarily restrained by precipitates of globular (Nb,Ti)C carbides of approximately 10 nm in diameter and characterised by niobium predominance [38], precipitates of (Ti,Nb)C carbides of approximately 20 nm in size, and (Ti,Nb)(C,N) precipitates of approximately 50 nm in size.…”

Austenite Decomposition and Precipitation Behavior of Plastically Deformed Low-Si Microalloyed Steel

“…[1][2][3]. One of the most interesting and evolving groups of advanced steels for the automotive industry is the multiphase transformation-induced plasticity (TRIP)-aided steels consisting of ferrite, bainite, and retained austenite [4][5][6]. These steels utilize different strengthening mechanisms to improve their strength level, but the most important is the strain-induced transformation of retained austenite into martensite [7][8][9].…”

Microstructural Aspects of Bifocal Laser Welding of Trip Steels

“…The presence of non-metallic inclusions when welding steel TRIP in the atmosphere of air and argon was explained by Grajcar et al [17]. They reported that the use of argon as the shielding gas during laser welding led to the slight reduction of non-metallic inclusions.…”

“…They have also tested the effect of welding parameters on the presence of non-metallic inclusions in the fusion zone of welded joints [17]. Advanced High Strength Steels, particularly those characterised by significantly high strength tend to be problematic during welding.…”