The laser beam as a tool in manufacture and assembly not only allows a precise and reproducible joining of components to modules, but also, through innovative approaches, further enables the efficient production of seams with high surface quality, which can be used in parts of the structure that will later be visible to the customer. The Institute of Applied Beam Technology in Bremen (BIAS) presents recent advances in laser welding for light metals and metallic hybrids.
Laser beam joining encompasses laser welding and laser brazing as well as joining processes that have a double character. While laser welding, in the form of deep penetration welding or heat conduction welding with or without filler material, is generally used for the joining of identical — or at least similar — materials, laser brazing (with filler wire) is used both in these cases and when joining dissimilar materials. A special feature of double character laser joining processes that form hybrid joints is that the joining partner with the lower melting temperature is melted in the seam area (the welding character), while the higher melting joining partner remains in the solid state and is wetted with the melt of the other (the brazing character). Various laser joining processes are used in the field of light metals for aluminum and titanium alloys as well as for steel, from conventional types to new ultra-high strength types such as 22MnB5, which are increasingly being used in modern lightweight construction. The surface quality of laser joined seams varies considerably depending on the type of process; therefore, in addition to the specific combination of materials, the surface quality often plays a role in the choice of process. The following article uses examples to demonstrate the potential of new developments in the field of laser joining with a high seam quality. The actual quality of a seam is hereby not a value that can easily be determined and is also at times defined differently, according to the application. While for such applications as car roof or tailgate seams the optical appearance quality of the seam is important, when manufacturing equipment for the food industry the settlement of bacteria must be prevented in order to ensure sufficiently hygienic conditions. In the following, both high seam surface quality and the optical appearance quality of the seam always refer to a smooth and even seam surface. A suitable evaluation scheme for this was introduced by Sander und Reimann , whereby spatters, pores, undercuts and seam holes are considered exclusion criteria, while roughness, weld ripples, the homogeneity of the seam width, and the so-called seam roundness are considered threshold criteria.
Such joining of aluminum sheets in a butt joint allows gap dimensions of up to three times the sheet thickness to be bridged.
In the case of heat conduction welding, the resulting seams typically have a high seam surface quality. However, as the energy input is limited to the surface of the work piece, the seams are generally wider than they are deep, that is, the aspect ratio as a quotient of seam depth to seam width is less than one; therefore, in comparison to so-called deep penetration welding, there is significantly higher heat input and thus an increase in welding distortion. The proportion of absorbed laser energy is hereby — with the use of a laser and dependent on the wave length of the laser source, the material and the surface condition — for aluminum alloys, for example, between 5 and 15 % and for titanium alloys and steel in the region of 30 %. In addition to solid-state lasers with wave lengths between 1030 and 1080 nm, diode lasers in particular are used, whereby the wave lengths can be selected from a wider range, depending on the material. With deep penetration laser welding, the absorption coefficient can, due to the formation of a vapour capillary — also known as keyhole —, enable the absorbed laser values to be typically increased up to 90 %, depending to the process parameters and independent of the material used. In addition to the significantly higher utilisation of the available laser energy, narrow, deep seams with high aspect ratios can be created due to the keyhole formation. Hereby, laser sources that have a high beam quality are used, such as fibre lasers or disk lasers. In comparison to heat conduction welding, far higher welding speeds can be achieved with the same laser output power. The overall lower heat input and the lower temperature gradient in the thickness direction of the work piece considerably reduce welding distortion. However, these advantages have so far stood against two essential disadvantages: a lower quality seam surface and a lower gap tolerance during the joining of two sheets in a butt joint. In conventional deep penetration welding, the higher dynamic of the vapour capillaries often leads to pronounced seam ripples and fluctuations in the seam form (width, rounding) as well as the increased formation of spatters. In addition, during the creation of an I-seam the gap size is restricted to approximately one tenth of the sheet thickness, as otherwise it is very likely that the laser beam would enter the open gap, meaning no sufficient seam formation would occur. Here, a solution is provided by an adjusted process, namely laser welding with a filler wire and a beam that oscillates transversely to the welding direction.
Buttonhole Welding of Aluminum Sheets in a Butt Joint
This approach to the laser welding of aluminum alloys combines the proven process of a wire material feed with the use of spatial sinus-shaped beam oscillation, Figure 1. This allows welding in the focus position (smallest laser beam diameter) with keyhole formation, thus providing a higher energy absorption with simultaneously higher gap bridgeability , Figure 2. Such joining of aluminum sheets in a butt joint allows gap dimensions of up to three times the sheet thickness to be successfully bridged. In the case of a variable gap, the gap dimensions would have to be continuously measured; thus, the oscillation parameters as well as the wire feed speed could be appropriately adapted inline.
This process is currently under further development, with the aim of not only bridging large gaps, but also achieving high seam surface qualities; thus, the disadvantages of conventional deep penetration welding can be rectified. For this purpose, a combination of sheet thickness, laser power, oscillation width and oscillation frequency are selected to affect the formation and stable existence of a so-called buttonhole in the weld pool . Here, a buttonhole refers to a circular cavity that emerges directly behind the laser’s point of impact in the melt pool, Figure 3, which has been identified as the cause of the high surface quality of the seams, within a certain set of parameters . While a too low oscillation amplitude of the laser beam, or a too low laser power, can result in a seam width that is insufficient for the formation of the buttonhole, greater amplitudes can in turn lead to the cavity becoming unstable. In this case, the cavity either closes itself after a short period of existence, leading to a decline in the surface quality of the seam, or it detaches itself from its position behind the laser beam and stays behind as a hole in the solidified seam. In the case of joining 1.5 mm thick aluminum sheets, however, a process window across many kW of laser power was found, Figure 4 c, in which process-reliable buttonhole welding could be achieved . The significant increase in seam quality is demonstrated in the comparison between a conventional deep penetration welding seam and a buttonhole seam in Figure 4 a and b. In addition to an almost complete avoidance of seam ripples, an even seam width development was achieved. Thus, it can be said that the seam has a good optical appearance quality.
Two-beam Laser Brazing of Hybrid Joints with Ultra-high Strength Steel
While classic laser beam brazing using a laser beam to create visible seams between galvanised steel sheets with copper-based braze metals is the current state of technology in the automobile industry, the use of ultra-high strength press-hardened steels, such as 22MnB5, for structural components of modern vehicles is placing increasing demands on the joining process, meaning that new concepts must be developed. For example, in the joining of press-hardened steel with conventionally galvanised steel, as for a multi-material design of car bodies with different materials for passenger compartment and outer skin, the joining partners have various coatings, each with different characteristics which must be maintained. Instead of a zinc coating, which is easily wettable in the brazing process, aluminum-silicon coatings (AS layers) protect the press-hardened steel from scaling during the manufacturing process. The press hardening hereby leads to a formation of an alloy layer of the AS layer with the substrate, so that the layer ultimately comprises different intermetallic phases and oxides at the surface. The AS layer is melting at higher temperatures compared to the zinc layer and, in the classical process, is hardly wettable by a copper-based braze metal. In order to achieve a sufficient wetting of the 22MnB5 steel, and also not destroy the zinc layer of the conventional steel, an adapted process control based on two-beam laser optics was developed . Hereby, one laser beam is used for preheating, whereby a second beam melts the braze metal, Figure 5 a and b. With a suitable configuration, such a process concept enables brazing of hybrid joints with a high surface quality similar to laser brazing and without seam defects. Regarding the material combinations considered here, the highest joint strength was achieved when a galvanised steel sheet was the upper sheet in an overlap joint with a 22MnB5 sheet, Figure 5 c. The joints failed reproducibly within the weaker galvanised steel sheet. However, in order to achieve these joint strengths, the ultra-high strength lower sheet had to be locally melted during the process due to the wetting impeding layer. The removal of the layer beforehand was thus not necessary. The process and temperature control thus ensured that both the zinc layer of the upper sheet and the aluminum-silicon layer of the lower sheet remained undamaged beside the seam. In addition to the suitability of conventional copper-based brazing materials, such as CuSi3Mn1, the usability of a higher strength aluminum bronze of the CuMn13Al8 type has also been shown . With the same high quality of the seam surface, this higher strength braze metal provides a joint strength consistently above that of the lower strength joining partner — regardless of its use in the joint as the upper or lower sheet — to be reliably achieved. Two-beam laser brazing can thus expand the application spectrum of laser brazing of hybrid joints to include ultra-high strength steels, which are extremely relevant to lightweight construction, creating smooth and even seams with sufficient strength.
With deep penetration laser welding, the amount of absorbed laser energy can typically be increased up to 90 %.
One-beam Laser Brazing with Reflection Induced Preheating
The description of two-beam brazing given here has already indicated the importance of an appropriate preheating. In this regard, a disadvantage inherent in conventional laser brazing with one beam lies in the fact that the wire shields part of the substrate from the laser beam irradiation, so that insufficient preheating of the substrate surface can take place, Figure 6 (left). This disadvantageous feature in the configuration of conventional laser brazing can be overcome, besides the above-described two-beam solution, with a new type of wire laser beam configuration in the form of a one-beam solution, Figure 6 (right). Hereby, the incident angle of the laser beam is flatter than that of the dragging feed of the wire. This leads to the laser beam being partially absorbed at the wire’s surface as well as to its reflection onto the work piece surface during the process of the wetting. Hence, the energy of the single laser beam is not only used to melt the braze metal, it also simultaneously serves to preheat the base sheet. Thus, for one-beam laser brazing of overlap joints, comparable wetting lengths, wetting angles and surface qualities, Figure 7, can be achieved as with two-beam brazing, however with approximately 30 % higher process efficiency . In addition to fillet welds on overlap joints, a suitable process window for seam formation for brazing speeds of up to 9 m/min could also be achieved for flanged seam joints, Figure 8.
Double-sided Joining of High Strength Aluminium-Titanium Connections
The abovementioned double character laser joining of metallic hybrid joints between aluminum and titanium unites the characteristics of both laser welding and laser brazing. In order to successfully thermally join this combination to create high strength connections, the formation of hard, rather brittle, intermetallic phases in the seam must be limited to a thin compound layer a few micrometers thick along the interface. A double-sided laser beam process has been particularly shown to be suitable for this purpose, whereby a primary laser beam is divided onto two processing heads and thus simultaneously conducted from both sides onto the joining zone. In the heat conduction based process the aluminum sheet melts in the joining area and wets the edge of the titanium sheet. Recent investigations have shown that, with suitable process control, a targeted design of the titanium edge geometry can be achieved without significantly affecting the process or compound layer formation. Thereby it is possible to transfer proven approaches from other joining technologies to the laser joining of hybrid joints, such as double sided scarfing in adhesive connections. Hereby, a model based on the material and geometry parameters enables an analytical prediction of the influence of the titanium plate edge geometry on the resulting joint strength . Through a double-sided 15° scarfing of the titanium edge, Figure 9, that is, by applying a wedge shape to the titanium, the force transmission during quasi-static tensile loads can be significantly increased. Using this edge shape, the connection failures were always ductile and occurred within the heat affected zone of the aluminum sheet. In contrast to the current state of technology, the seam thickness hereby corresponds to the aluminum sheet thickness, thus there is no thickening. Through this process, which does not use brazing flux and has a comparatively smooth wetting process, seams with an extremely high surface quality can be formed, so that post-processing steps may be avoided.
In summary, it can be seen that laser joining offers a wide spectrum of various processes which can, according to the demand profile and material combination, create seams with high surface qualities.
The authors would like to thank the DFG - Deutsche Forschungsgemeinschaft (German research foundation) for funding of the projects “Sichtnahtqualität beim Laserstrahltiefschweißen durch Erzeugung einer Schmelzbadkavität” (VO530/101-1) and “Thermisches Fügen integraler CFK-Metall-Verbindungen” (VO530/46-2, FOR1224) as well as the BMWi and the AiF for the support of the following two IGF projects. The projects IGF 17.430 B and IGF 18.386 N of the “Forschungsvereinigung Schweißen und verwandte Verfahren des DVS, Aachener Straße 172, 40223 Duesseldorf” were supported through the AiF in the framework of the program of support for the Industrielle Gemeinschaftsforschung (IGF) from the Federal Ministry for Economic Affairs and Energy (BMWi) on the basis of a resolution from the German Bundestag. The technical committee FA 6 “Strahlverfahren” of the research association Schweißen und verwandte Verfahren e. V. of the DVS is also thanked for the content-related support of the IGF projects.
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