In the last decades, automotive manufacturers are facing the very hard challenge to design more and more comfortable and safe cars preserving the vehicle overall weight [1, 2]. Weight reduction is the key factor for the decrease in fuel consumption, which is fundamental both for internal combustion engine and electric vehicles. Indeed, if the former have to satisfy more and more restrictive regulations for environmental pollution and greenhouse gas emissions, the latter have to fully optimize the energy usage in order to guarantee longer ranges of autonomy per battery charge [3–7]. For these reasons, the development of lightweight solutions for the vehicle body-in-white through the application of both more strength materials and tailored blanks technologies is mandatory nowadays [8–10]. Consequently, the use of advanced high strength steels (AHSS), and in particular its subfamily of ultra-high strength steels (UHSS), is growing much more than forecast due to the capability of these grades of steel to reach lightweighting targets without the critical issues of other alternative materials (e.g. aluminum, magnesium, titanium) [11–16]. Typical UHSS problems related to limited formability and significant springback at room temperature have been overcome for press hardened steels (PHS) thanks to hot stamping, which takes advantage of the high formability properties of the austenitic phase at high temperatures [17]. The use of PHS by carmakers has exploded in the last few years, and the demand will continue to increase according to the future forecasts [18]. The most commonly employed PHS is 22MnB5, which is a low-carbon steel grade with manganese and boron alloying. PHS are widely used with an Al-Si coating layer, necessary to protect the austenitized sheet from oxide scale formation and decarburization at the surface during the hot stamping heat treatment [19, 20]. For the optimal exploitation of the light-weight potential of these steel grades, their use as tailored welded blanks (TWB, i.e. two or more sheets of different materials and/or thicknesses joined through a laser beam before stamping) is a viable solution. TWB guarantee also an optimized employment of material usage [21]. However, it is well known that Al-Si coating causes issues in the laser welding process since Al contaminates the molten pool causing the presence of ferrite into the martensite microstructure of the hot-stamped fusion zone that weakens the strength of the weld seam [21–26]. Up to now, several different methodologies have been tested attempting to resolve this problem both with and without the Al-Si layer removal. In industrial applications, the most used method consists in removing the Al-Si coating by means of ablation before welding through a specific Q-switched pulsed laser. This method can preserve efficaciously the underlying Fe-Al intermetallic layer that faces the steel substrate, continuing to guarantee minimum protection of treated area against oxidation [27]. Nevertheless, as already pointed out [21], the popularity of laser ablation does not imply necessarily that it is the most efficient way to resolve this issue. Generally, the coating removal is to be considered not desirable because entails additional steps and equipment increasing times and costs of the manufacturing process. For these reasons, the latest attempts are all oriented toward methods that avoid ablation as well as any other decoating procedure. Some of these are based on the addition of filler material in the form of a wire inside the fusion zone [28–32]. The filler wire method causes an enhanced stirring of the melt pool, improving the homogeneity of Al distribution inside the fusion zone and bringing to a more uniform δ-ferrite distribution [28]. In addition, a high-carbon content of filler wire could act as an austenite stabilizer, suppressing ferrite formation and improving martensite fraction in the hot stamping heat-treated weld seam [29]. The results reported in the literature have shown an effective improvement of the mechanical properties of the joints with filler wire compared to those welded without in hot stamped condition. The addition of filler wire also allows better control of the weld seam geometry thanks to the possibility to properly adjust the wire feeding speed. It could be noticed that by using a filler material, the cost of processing will likely increase due to two main reasons: first, the direct cost of consumables needed for the joining process, and second, the additional capital cost of acquiring the filler wire feeding equipment for the laser welding station. However, these costs result by far lower than the capital cost of acquiring and powering the additional laser source for the Al-Si layer ablation, considering that the ablation and welding steps require properties of the respective laser beams so different each other that is impossible to obtain by the same laser source. In other similar approaches, the use of Ni foils as an interlayer between the two sheets to be joined [33–35] or as an additive coating layer on the sheets [36] has been evaluated. Ni is a strong austenite-stabilizer element, acting in this way similarly to carbon filler wire. However, the application through interlayered foils does not allow direct control of the fusion zone geometry since the variation of thickness of Ni foils has not influenced the shape of welded joints [34]. Moreover, the solution of an additive Ni surface layer, which has been tested only in bead-on-plate tests [36], could be difficult and time-consuming on butt joints for real industrial applications. Other approaches have been developed trying to eliminate the Al-Si layer without a real decoating procedure but through a preheating treatment [37] or the application of a colloidal graphite coating [38, 39]. The former consists in heating the steel sheet prior to laser welding reaching a temperature of 800°C that allows the evaporation of Al-Si coating but preserving the underlying Fe-Al intermetallic layer [37]. Although it is effective, this method is time-consuming and implies anyway an additional step in the manufacturing process. In addition, the oxidative effects of a preheating treatment of large portions of steel could represent an issue in the following heating for the hot-stamping that has not been investigated yet. The latter method uses a colloidal graphite additional coating on the steel that serves both as an austenite stabilizer and an agent for in-situ ablation of the molten Al-Si coating. Indeed, the additional coating, having a vaporization temperature higher than Al-Si coating, brings to an increase of pressure of the underlying coating that finally explodes. This method is effective for bead-on-plate joints with the highest considered thicknesses of graphite coating [38]. Nevertheless, as already observed before, the application and measurement of a coating thickness in the order of 10− 4÷10− 5 m could be difficult in a real production process with butt joint configuration of different thicknesses sheets. Moreover, the final appearance of the areas adjacent to the weld seam should be investigated since violent spurts of the molten Al-Si coating ejected from the weld pool have been observed with the application of the graphite coating. Other interesting approaches consist in enlarging the laser spot on the workpiece surface through beam defocusing [40], the use of optics with higher focal length [41], with particular beam modules, e.g. collateral dual-beam module [42], or with a laser beam oscillation [43, 44]. Generally, laser welding with larger laser spots allows a wider melt pool with enhanced flows that improve the Al mixing bringing to an increased martensitic fraction of fusion zone after hot-stamping [41]. In addition, a characteristic Y-shaped weld section, instead of the common X-shaped (or hourglass-shaped) fusion zone (FZ), could be obtained through beam defocusing [40] or setting the process parameters in the narrow range of values that allows to weld in the particular closed (or blind) keyhole condition [45, 46]. Indeed, the weld pool resulting from laser welding with closed keyhole experiences less fierce flows than the one with open keyhole due to the tighter width of the flow channel around the lower part of the keyhole. This results in contamination of the weld pool due almost entirely to the top coating and restricted to the upper portion of the FZ [45]. However, the mechanical properties of these kinds of weld in butt joint configuration and hot-stamped conditions, i.e. those of a final TWB press-hardened part, should be better investigated.
In this work, a novel approach for the laser welding of as-received Al-Si-coated PHS, i.e. without any preliminary decoating procedure, is evaluated. This original patent-pending method combines the use of a characteristic filler wire with an innovative laser beam optic module. The main results of tensile tests of a wide experimental campaign have been analyzed in order to find the optimal combination of process parameters. Then, the best and the least optimal solutions have been further mechanically and chemically investigated.