Joining (Welding and Brazing) (LiM 2019)

Effects of titanium on grain boundary strength in the molybdenum laser weld bead zone and formation and strengthening mechanisms of parasitic brazing layers
Liang-Liang Zhang, Lin-Jie Zhang, Jian Long, Jie Ning, Jian-Xun Zhang, Suck-Joo Na

Molybdenum (Mo) has tremendous application potential in the nuclear power field, but its application is limited by the grain-boundary embrittlement of fusion-welded joints made of it. In this study, titanium (Ti) was selected as an alloying element to reduce the brittleness of laser weld beads in Mo "cladding-end plug" socket joints. Parasitic brazing was also performed to enhance the joint strength. Joints with the same strength as the base material and a hydraulic bursting pressure of 60 MPa were produced using a combination of the two methods. The analysis indicates the following. After being added to the welded zone, Ti was able to combine with the free oxygen (O), forming TiO2 and at the same time reducing the MoO2 content of the grain surface. O and MoO2 are both the main causes of the embrittlement of Mo grain boundaries. In addition, by taking advantage of the high melting point and thermal conductivity of Mo, a Ti foil pre-placed between the Mo tube and rod in the socket joint was melted, forming metallurgical bonding, which further improved the bearing capacity of the joint. These results could facilitate the application of Mo in the nuclear power field and also provide a new approach for improving the performance of socket joints made of refractory materials.

Keywords: molybdenum; titanium; laser beam welding; grain boundary embrittlement; brazing


Keyhole Brazing with Two-Dimensional Laser Irradiation Patterns
Insa Henze, Peer Woizeschke

Laser brazing is often used to join parts in the field of view because it offers good seam quality. In the case of laser brazing with aluminum- or copper-based brazing materials, the absorption is low when applying modern fiber and disk lasers with approx. 1 μm wavelength, depending on the material, since a simple Fresnel absorption takes place at the surface. The keyhole approach (formation of a vapor capillary), which is known from deep penetration laser welding, can also be used to increase absorption in brazing by carrying out a kind of deep penetration laser welding in the brazing material. To prevent a melting of the substrate material, the melting process must be limited in the wire by adjusting the penetration depth. This is realized through an oscillation of the laser beam. In this study, two-dimensional laser irradiation patterns were applied to keyhole brazing. In addition to the circular oscillation, a kind of eight-shaped strategy was compared with the transversal linear sinusoidal oscillation. The linear and the special eight-shaped oscillation strategy have turning points in the movement, which lead to local higher time-averaged power densities and varying beam velocities. These local alterations of the process parameters can cause an unwanted melting of the substrate material or a deterioration of the seam appearance. The results show that the substrate melting is primarily caused by low average interaction times between laser beam and brazing material. The melted area increases due to the existence of turning points in the oscillation pattern. The seam appearance can be improved by using higher frequencies, whereas turning points tend to cause rougher seam surfaces.

Keywords: Laser brazing; Keyhole brazing; Beam oscillation; Single-mode laser; Process efficiency


Laser brazing of zinc-aluminum-magnesium coated steel – influence of the joint geometry
S. Wachsmuth, S. Nothdurft, W. Reimann, B. Tigges, J. Hermsdorf, L. Overmeyer

Laser brazing of zinc coated steel is a widely spread technique in car body manufacturing. For visible joints at the outer surface a double flanged joint geometry is common. However, the influence of the outer bending radius of the joint geometry is rarely discussed. Furthermore, the influence of zinc aluminum magnesium coatings (ZM) on the laser brazing process has not been reported so far. The properties of laser brazed joints on electrogalvanized (EG) and ZM coatings show deviant properties. In particular, the dimensions of the smallest wetted length, which is a widely used synonymously for the smallest seam thickness in micro-
sections, are comparatively lower. Exemplary, the average smallest wetted lengths at a bending radius of 1.6 mm reach values of 0.64 mm on EG coated material and 0.49 mm on ZM coated steel respectively. Depending on the smallest wetted length the tensile strength is reduced as well. In the case described above, values of the tensile strength differ from 228 kN/mm on EG coatings to 187 kN/mm on ZM coatings. Hence, the brazed seam’s thickness is identified as a crucial factor for the strength of a brazed joint. On ZM coatings, the properties of the joints improve to 0.57 mm smallest wetted lengths and 216 kN/mm tensile strength by increasing the outer bending radius. Thus, the connection between the outer bending radius, the smallest seam thickness and the mechanical properties are demonstrated. In general the brazeability of EG coated material is superior
to ZM coated material. While increasing outer bending radii improve the properties of brazed joints on ZM material, properties of joints on EG coated material worsen.

Keywords: laser brazing; joining radius; electrogalvanized steel zinc-aluminum-magnesium coated steel


Laser Beam Vacuum-Welded Cu-Al Mixed Joints
Uwe Reisgen, Simon Olschok, Niklas Holtum

A great potential for lightweight construction lies in the substitution of copper by aluminum. It is possible to apply aluminum as a conductor and copper as a contact material. During the fusion welding of this joint, brittle intermetallic phases occur which negatively affects the mechanical and technological properties of the joint. Also is the electrical resistance of most intermetallic phase’s one order of magnitude higher than that of the pure Materials. External influential factors, such as temperature, current-feed and time bring about the change of the joining zone structure by diffusion and electro-
migration. Moreover, a temporal change of the technological properties (further increase of the
electric resistance, reduction of strength) of the dissimilar material joint will occur. Findings about this behavior will allow drawing conclusions about the expected lifetime of the joints. This paper reports on the change of properties of the mixed joints after electrical usage.

Keywords: Laser Beam Welding; Copper; Aluminium; Mixed-Joints; Singelmode Fiber Laser


Numerical Analysis of the Local Solidification Conditions in Laser Beam Welding of Aluminum Alloys
Jonas Wagner, Christian Hagenlocher, Florian Fetzer, Constantin Böhm, Rudolf Weber, Stefan Weihe, Thomas Graf

The local solidification conditions in full penetration laser welding of the aluminum alloy AA6016 were calculated. Therefore, a tailored numerical model was developed. Two sets of parameters with a large difference in absorbed line energy per welded depth were chosen in order to get an overview of the range of solidification parameters which is covered by full penetration welding. The results were applied to a solidification structure map in order to depict the implications on the local grain structure. Under consideration of the corresponding y-coordinates an estimation of the width of the equiaxed zone of the weld is possible.

Keywords: laser beam welding; grain structure; aluminum alloys; numerical modelling; welding simulation;


Influence of the Alloy-specific Solidification Path on the Critical Strain Rate for the Formation of Hot Cracks during Laser Beam Welding of Aluminum
Daniel Weller, Christian Hagenlocher, Rudolf Weber, Thomas Graf

The amount of the main alloying elements magnesium and silicon of a 6000 series aluminum alloy significantly determines the hot cracking susceptibility during laser beam welding. Based on the phase diagram, alloy-specific solidification paths according to Scheil can be determined. From the model of Rappaz, Drezet and Gremaud, a correlation between these alloy-specific solidification paths and the resulting critical strain rates can be derived. By experimentally determining the critical strain rates for three different AlMgSi alloys this correlation could be confirmed. This experimental proof allows aluminumalloys to be classified with regard to their susceptibility to hot cracking as a function of their characteristic solidification path.

Keywords: Laser beam welding; aluminum alloys; hot cracking susceptibility; critical strain rate; solidification path


Laser welding – control of microstructure with wobble technique
Tri Le-Quang, Bastian Meylan, Kilian Wasmer

Laser beam wobble is a recently developed technique for welding application, in which the beam oscillates across the process zone at high frequency. This technique has shown the capability to weld without defects materials that are difficult to weld such as aluminium and copper. In this contribution, another advantage of this technique will be presented which is the capability to control the microstructure of welded materials. The welding experiments were performed on Ti6Al4V alloy and the microstructures were investigated by optical microscopy. It is shown that the microstructure can be modified by tuning the wobble parameters without affecting the dimension of the bonded area.

Keywords: Laser welding; wobbling; microstructure.


Application of different pulsed laser sources to dissimilar welding of Cu and Al alloys
Alessandro Ascari, Alessandro Fortunato, Erica Liverani, Adrian Lutey

The paper deals with laser dissimilar welding of aluminum and copper alloys thin sheets. In particular, the application of different laser sources was investigated, both in terms of general applicability and of weld bead characteristics. The lap welding configuration considered in the experiments was Al over Cu, in order to evaluate the role of the material combination with respect to the laser radiation and to the formation of different compounds in the fused zone. The laser sources exploited were chosen among pulsed wave ones and in particular: high brilliance quasi-CW fiber source, low brilliance long-pulse lamp-
pumped Nd:YAG one and high brilliance short-pulse Q-switched fiber one. The results were
characterized by means of tensile tests, in order to understand the mechanical characteristics of the weldments and by means of metallographic techniques, in order to understand the metallurgical properties.

Keywords: Pulsed-wave; Dissimilar welding; High-reflectivity; Lap-welding.


Influence of laser wavelength on melt pool behavior in welding of thin pure copper plate with blue diode and fiber lasers
Kento Morimoto, Masahiro Tsukamoto, Shin-ichiro Masuno, Kazuyuki Azumi, Yoshihiko Hayafshi, Nobuyuki Abe

Bead-on-plate welding of copper plate with a thickness of 50 μm were carried out with near infrared (NIR) fiber and blue diode lasers. Output powers of NIR fiber and blue diode lasers were 300 W and 50 W, respectively. Spot diameters of them were 100 μm. Welding speed was varied from 15 to 30 mm/s. The molten pool was observed with high speed video camera. For NIR fiber laser, the molten pool became larger and the hole was formed at the welding speeds in the range of 20 to 25 mm/s. For blue diode laser, the molten pool size was constant during the laser irradiation. Experimental results suggested that the absorption rate was increased as the temperature was increased for NIR fiber laser. On the other hand, in the case of the blue diode laser, they suggested that the absorption rate didn’t depend on the temperature raise.

Keywords: blue diode laser; near infrared laser; pure copper; absorption rate; molten pool


Laser Welding Technology for the ITER Toroidal Field Coil Components
Shuho Tsubota, Tomoyuki Nishiyama, Norikiyo Koizumi

The radial plate (RP) of TF coils consists of 10 segments with 120 mm in thickness of high nitrogen content austenitic stainless steel. It is important to suppress the welding deformations so as to satisfy tolerances required. In the preliminary tests, we investigated feasibility of application of high power laser welding to the thick plate welding, and found out proper conditions of welding for the RP material. The cover plates (CP) were welded onto the RP with the insulated conductor to support mechanical loads and form double pancake (DP). Since the CP were thin plates and hundreds of plates were welded onto the RP, the prevention of the welding deformations and the heat input to the insulation wound around the conductor were key issues.

Keywords: High power laser welding, welding deformation, ITER, Troidal Field Coil, Radial plate, Cover plate


Hybrid laser arc welding of thick plates X8Ni9 for LNG tank construction
S.Gook, A. El-Batahgy, A.Gumenyuk, M. Rethmeier

Results of experimental investigations of the relationship between laser-hybrid welding process parameters, type of the filler metal and the mechanical properties of the welds made from 9% nickel cryogenic steel X8Ni9 are discussed. The results contribute to the development and conversion in the industrial practice a new laser beam-based welding technology for the automated manufacturing of LNG tanks. The remarkable heterogeneity in the chemical composition of the weld metal as well as an insufficient impact toughness could be indicated by using austenitic filler wire. The most promising results were achieved by applying 11%Ni filler wire, which is similar to the base material. A correlation between impact toughness and wire feeding speed could be shown. The highest impact toughness was 134 J at -196°C. The laser-hybrid welds passed the tensile test. The failure stress of 720 MPa with a fracture location in the base metal was achieved for all samples tested.

Keywords: Cryogenic steel, Hybrid laser arc welding, Microstructure, Toughness, Tensile strength


Influence of Alloying Elements on Laser in Vacuum (LaVa) Welding of Nickel-Based Alloys
Reisgen U., Olschok S., Krichel T.

Nickel-based alloys have good resistance to corrosion and/or high temperatures and are therefore used in chemical industry, aviation and power generation. The development of suitable welding parameters for a specific welding task requires resources such as workpieces, personnel and above all time. The understanding of the influence of alloying elements on the seam geometry can reduce these resource requirements. The aim of this work is to determine the influence of alloy constituents on the seam geometry during laser beam welding in vacuum (LaVa) of nickel-based alloys with a seam depth of up to 15 mm at a power of 6 kW. Lava is investigated in order to exploit the advantages of vacuum known from electron beam welding, such as process stability and the higher welding depth at lower system costs.

Keywords: laser in vacuum; nickel-based alloys; weld seam geometry


Fatigue behavior of laser and hybrid laser-TIG welds of high-strength low-alloy steels
Hana Šebestová, Petr Horník, Libor Mrna, Vít Horník, Pavel Hutar, Michal Jambor, Pavel Doležal

The hybrid laser-tungsten inert gas welding technology was applied to butt weld 3-mm-thick high-strength low-alloy (HSLA) steel sheets. Although the carbon equivalent number of HSLA steel is relatively low, rapid cooling rates accompanying laser welding promote the formation of quenching microstructures in the fusion and heat affected zone of such alloy. The intent of low-current arc addition was to preheat the material to reduce the cooling rate and thus to modify the weld structure. High-cycle fatigue tests were performed to evaluate the effect of heat input on fatigue behavior of S460MC and S700MC laser and hybrid welds. Laser welding led to the dramatic drop in fatigue properties of the samples, especially for S700MC. Preliminary fatigue tests at the 350-MPa level of stress amplitude revealed that the preheating has rather a negative effect on fatigue lifetime of S460MC while increasing arc current increased the fatigue lifetime of S700MC hybrid welds. During detailed high-cycle fatigue tests allowing to construct the S-N curves, a positive effect on fatigue lifetime of S700MC has not been proved. The presence of surface notches probably influences the fatigue behavior of welded samples much stronger than local microstructural changes.

Keywords: Laser welding; Hybrid laser-TIG welding; High-strength low-alloy steel; Fatigue


Laser blank-rim melting for robust laser welding of hidden T-joints with OCT-based position control
Mittelstädt C., Seefeld T., Vollertsen F.

Laser-based blank-rim melting can be used to create reinforcements and increase the sheet thickness locally. This can be utilized to improve the robustness of laser welding of hidden T-joints, a more challenging joining task in terms of joint preparation. In this work laser welding of hidden T-joints using an OCT-based closed-loop position control is presented. Thereby, a lateral offset of 4.5 mm could be compensated and defect-free welding of hidden T-joints could be attained. Furthermore, it is demonstrated that using reinforced web-sheet edges increased the robustness of the closed-loop control significantly.

Keywords: welding; keyhole; sensing; process control, OCT


Laserbeam-Submerged Arc Hybrid Welding – a welding technique for thick metal sheets
Uwe Reisgen, Simon Olschok, Oliver Engels

Industrial welding of structural steel in 40 mm plate thickness range is generally carried out by using a submerged-arc welding process. Due to the high number of welding layers and the associated high energy input, thermal distortion is often unavoidable. In addition, large quantities of filler metal are required. The laserbeam-submerged arc hybrid welding process combines the conventional submerged-arc welding process with a laserbeam-welding process to form a hybrid process and thus offers great potential to replace the existing manufacturing process. With the layer/counterlayer technique, sheet thicknesses of 40 mm could already be joined flawlessly in just two welding layers. In this paper the hybrid character of the new joining process is emphasized. The mixing of the filler material down to the root area of the weld seam is proven. In addition, the welding results achieved with the hybrid welding process on material 1.4301 are presented and discussed.

Keywords: Hybrid Welding, Thick Metal Sheets, Submerged Arc Welding


Single-pass Hybrid Laser Arc Welding of Thick Materials Using Electromagnetic Weld Pool Support
Ömer Üstündag, Andrey Gumenyuk, Michael Rethmeier

Hybrid laser-arc welding process allows single-pass welding of thick materials, provides good quality formation of joints with minimal thermal deformations and a high productivity in comparison with arc-based welding processes. Nevertheless, thick-walled steels with a thickness of 20 mm or more are still multi-pass welded using arc welding processes, due to increased process instability by increasing laser power. One limitation factor is the inadmissible formation of gravity drop-outs at the root. To prevent this, an innovative concept of electromagnetic weld pool support is used in this study. With help of such system a stable welding process can be established for 25 mm thick steel plates and beyond. Sound welds could be obtained which are tolerant to gaps and misalignment of the welded parts. The adaptation of this system to laser and hybrid laser-arc welding process can dramatically increase the potential field of application of these technologies for real industrial implementation.

Keywords: Hybrid laser-arc welding; thick-walled structures; electromagnetic weld pool support; single-pass welding


Influence of Partial Penetration Laser Hybrid Welding Parameters on the Solidification Cracking for Thick-Walled Structures
Nasim Bakir, Jacques Biltgen, Andrey Gumenyuk, Michael Rethmeier

In this study, the impact of the important laser hybrid welding parameters regarding the solidification cracking formation is investigated. Experimentally, the welding processes were performed for specimens under critical restraint intensity, which promotes solidification cracking formation. The welding speed and the arc power were varied under the same restraint-intensity condition to study its impact on the solidification cracking phenomenon. The results showed that there is an influence of the welding speed and the arc power on the formation of solidification cracking. The welding speed shows a significant effect on the crack number; that is by decreasing the welding speed, the crack number decreased. The arc power shows a slight influence on the solidification cracking. Moreover, the experiments were accompanied by the numerical simulation to understand the behavior of the stress in the welds by varying the welding parameters. 


Investigations on laser beam welding of thick shipbuilding steel plates using a very high-power diode laser
Oliver Seffer, Sarah Nothdurft, Alexander Brodesser, Michael Hustedt, Jörg Hermsdorf, Stefan Kaierle

The thickness of a layer made by Laser Metal Deposition (LMD) on a freeform surface varies depending on the local angle of inclination and the tool path direction. When depositing multiple layers, the CAM (Computer-Aided Manufacturing) planning for the tool paths requires an accurate description of the surface topography of the previous layers to ensure a stable process. Offline path planning is commonly based on the translation of the surface in the building direction (z-offset) or in the direction of the plane normal after each layer. In this paper, troughs with freeform surfaces are filled via LMD. The surface topography is measured after each layer and its deviation against the calculated surface according to z-offset and plane normal method is evaluated. Advice is given on which method of offline path planning is preferable and if additional controlling measurements of the surface are necessary after a certain amount of layers.

Keywords: Laser Metal Deposition; CAD; CAM; Reverse Engineering; Offline path plannin


Potential of Laser Beam Welding under Vacuum
Stefan Jakobs

The first studies on the effects of reduced pressure on a laser beam deep welding process were carried out in the 1980s using the CO2-laser of the time. Beside the suppression of plasma plume buildup, also a correlation between working pressure and welding depth achieved could already be established. With the availability of solid-state lasers with high beam quality and power, also these new lasers were occasionally brought to a reduced pressure atmosphere. Most of these investigations are largely phenomenological, dealing with the effects of different pressures and beam powers. In addition, the process was developed into a usable joining process for various applications. In this lecture, the physical conditions of laser beam welding under vacuum are presented. The potentials of this process are derived and welding results on different materials such as steel, aluminum, titanium and copper are presented.

Keywords: Laser Beam Welding, Vacuum; Dissimilar Welds, Titanium, Nickel


Study of laser cold-wire welding of thick aluminum sheets along different gap size and laser-to-wire position
F. Mirakhorli, F. Nadeau, G. C.-Guillemette, R. Fakir

In recent years, there has been growing interest in joining thick aluminum sheets in various markets such as railcar,
shipbuilding and aerospace. Laser cold-wire (LCW) welding was developed in this study to join 5.0 mm thick AA6005-T6
aluminum sheets in butt joint configuration. To investigate the gap bridging ability of LCW technique, a process
parameter optimization was performed for three different gaps ranging from (I) 0.25-0.50 mm, (II) 0.50-0.70 mm and (III)
0.70-1.00 mm. The effect of a linear misalignment was also studied by applying a 0.75 mm misalignment (15% of the
sheet thickness) during joint preparation. The result of this study showed that the LCW can easily sustain the gap and
misalignment tolerances of 0.75 mm. The welded joint integrity with different gaps and misalignments was
characterized in terms of the weld bead geometry, defects, macrostructure, hardness and tensile strength.

Keywords: Laser cold-wire welding, gap tolerance, misalignment, thick aluminum sheet.


Utilization of electric arc for preheating of special steels during laser welding
Mrna L., Horník P., Šebestová H.

Laser welding is characterized by high cooling rates. We found out both experimentally and numerically that during our
welding experiments it reaches about 500 °C.s-1. Such cooling rates lead to the formation of undesirable microstructures
in some steel grades affecting weld mechanical properties. In our research, we verified the control of weld cooling rate
by preheating with electric arc of TIG torch placed close to the laser beam impact. The stabilization of discharge burning
in the neighborhood of laser-induced plasma above the keyhole was studied experimentally and optimal stand-off
position of the TIG torch towards the beam axis was found. The flow of protective gas was vis ualized by schlieren
method. Further, the effect of DC/AC arc mode on both weld and electrode was examined. The stability of low-current
(20-60 A) arc was proved during casual laser welding speed. The effect of arc preheating was examined for HSLA and
creep-resistant steels.

Keywords: Laser welding; preheating; arc discharge


Liquid zone and spatter behavior during continuous laser welding of titanium
I. Tomashchuk, A. Chabot, P. Cottet, M. Duband, P. Sallamand, A. Mannucci

Welding lines were performed on 2 mm thick titanium plates with continuous Yb:YAG laser with 600 μm spot diameter
and different combinations of laser power (1-5 kW) and speed (1-12 m/min). High-speed imaging was performed with a
camera Phantom V9.0 with the use of interferential filter of 810 nm to visualize only self-illuminated melted zone.
The observed welding regimes were close to those previously reported for steel: regular Rosenthal regime with and
without spatter, smooth spatter-free single wave regime, quite unstable elongated keyhole regime and low energy
conduction regime. Unlike steel, the noticeable change in the morphology of weld crosscuts was not observed, but a
simple reduction of weld width with a decrease of linear energy. Important spatter formation occurred only for
Rosenthal regime under P2kW. The analysis of spatter size and speed showed the relation between laser power and
population of tiny, average and giant droplets with speed from 3.5 to 0.5 m/s.

Keywords: laser welding; titanium; keyhole regime; spatter; high-speed imaging


Pure vanadium insert for efficient joining of Ti6Al4V to 316L stainless steel with continuous Yb:YAG laser
A. Mannucci, I. Tomashchuk, A. Mathieu, R. Bolot, E. Cicala, S. Lafaye, C. Roudeix

Laser welding of titanium alloys to stainless steels remains challenging due to the formation of brittle intermetallic
compounds. Pure vanadium has full metallurgical compatibility with titanium and good solubility in iron, except the
region of sigma phase, which precipitation is avoidable by rapid cooling. This makes vanadium an attractive interlayer
material capable to enhance weld strength and ductility.
In the present study, the continuous double pass welding of 1 mm thick Ti6Al4V alloy and 316L steel through 2 mm wide
pure vanadium insert was studied. It was found that the composition of the melted zone between steel and vanadium is
a key factor determining the strength and fracture mode of the joint. In spite of absence of sigma phase, the melted
zones having >40wt% V showed brittle behavior, when the presence of <40wt% V led to ductile fracture in unmelted
vanadium insert. Best results of tensile strength reached 500 MPa.

Keywords: dissimilar welding; vanadium; titanium; steel


Influence of superimposed intensity distributions on the welding process and the spatter behavior during laser welding of steel
Michael Jarwitz, Jannik Lind, Rudolf Weber, Thomas Graf

The influence of superimposed intensity distributions on the resulting melt flows, capillary shapes, and generated
spatters during laser beam welding of mild steel was investigated using online high-speed X-ray imaging and visual high-
speed imaging. The shape of the capillary changed for different superimposed intensity distributions and the size of the
eddies in the melt pool changed for welds with different amounts of generated spatters. For a high number of generated
spatters, a higher number of trajectories of the tracer particles leading to the upper part of the melt at the rear side of
the capillary was observed, which was also the location of the generation of the spatters.

Keywords: laser welding; spatter reduction; online X-ray diagnostics; superimposed intensity distribution


Effect of gas flow on spatter formation in deep penetration welding at high welding speeds
Leander Schmidt, Klaus Schricker, Jean Pierre Bergmann, Steen Hickethier

Spatter formation is a major issue in deep penetration welding with solid state lasers at high welding speeds from 8 up
to 20 m/min. To avoid spatter formation, the use of local adjustable shielding gas flows was investigated. By using a
precisely adjustable shielding gas supply, the effect of argon was characterized by welding stainless steel (1.4301/
X5CrNi18-10/ AISI304). Different flow rates and spatial orientations were examined. The influence of the gas flow on the
keyhole formation and the resulting spatter formation was recorded by means of HV-camera and subsequently analyzed
by image processing (number, velocity and trajectory of spatters, shape of the keyhole aperture). A reduction of spatters
up to 91 % at welding speeds up to 16 m/min was reached. Finally, a description of the general mechanisms between
gas flow, keyhole fluctuations, pressure conditions and melt pool dynamics is given.

Keywords: welding; macro processing; spatter formation; low-spatter welding; influence of shielding gas


Reduction of spatters and pores in laser welding of copper hairpins using two superimposed laser beams
Oliver Bocksrocker, Nicolai Speker, Matthias Beranek, Tim Hesse

Hairpin technology is a new degree of freedom in the manufacturing of eDrives for the automotive section of eMobility
to increase the efficiency of the powertrain. Experimental investigations in laser welding of copper hairpins show that
the specific beam profile of two superimposed laser beams, using a so called “2in1-fiber” (BrightLine Weld), stabilizes
the keyhole and therefore reduces spatters and pores. High-speed videos of the process were used to quantify spatters.
The generation of pores during the welding was investigated via online xRay-imaging. The observation shows that the
opening of the keyhole is widened, if two superimposed laser beams are used. As a result, the shape of the keyhole is
stabilized over time. In addition, the shape of the keyhole enables unrestrained degassing of vaporized material. The
latter restricts bulging of the keyhole and therefore reduces the formation of spatters and pores during the hairpin
welding process close to zero.

Keywords: laser welding; copper; hairpins; electric motor; eMobility; eDrive; powertrain; spatters; pores; xRay


Modeling and simulation of laser micro welding
Christoph Schöler, Markus Nießen, Marc Hummel, Alexander Olowinsky, Arnold Gillner, Wolfgang Schulz

Laser welding using fiber lasers of high power and quality facilitate the joining of highly conductive materials while
attaining high aspect ratio welds at spot sizes in the micrometer range. Industrial applications are laser bonding of
copper contacts in power electronics and battery modules. An important quality feature is a defined weld seam
geometry which is determined by the physical transport phenomena during the process. This contribution targets the
development of a model that serves as a basis for experimental control strategies. Reasonable assumptions for reducing
the model's complexity are made by exploiting the characteristic length and time scales of the process. Laser absorption
at the metal surface and heat conduction in the material are considered as the dominant phenomena to define the
shape of the keyhole. Simulations are subjected to parameter variations and validated by experimental reference data.

Keywords: laser micro welding; weld seam geometry; modeling; simulation, 515 nm; green laser


Simulation of the Temperature Profile on the Cutting Edge in Laser Fusion Cutting
Ulrich Halm, Markus Niessen, Dennis Arntz, Arnold Gillner, Wolfgang Schulz

Striations on the cutting edge are one of the essential quality reducing features in laser fusion cutting. Research shows
that dynamics in the motion of the thin melt film on the cutting front have a strong effect on the formation of striations.
A highly resolved 3D simulation of the melt film dynamics provides a detailed insight into the evolution of the
temperature profile on the cutting edge. The horizontal streak analysis of the simulated temperature explains the
connection between strongly changing surface temperatures and pronounced striations. The comparison with
experimental results shows good correspondence with the tendency that a reduction of the dynamics of the
temperature profile on the cutting edge leads to a reduced striation depth. Measures to reduce the striation depth
should aim to a reduced dynamics of the surface temperature. Examples of such measures are Gaussian beam
distributions or oscillation of the beam axis into lateral direction.

Keywords: Simulation, Laser Fusion Cutting, Streak Analysis


Quantitative Analysis of the Temporal Distance between Melt Waves on the Cutting Front Apex duringLaser Fusion Cutting of Stainless-Steel Sheet Metal with 1 Micron Wavelength
D. Arntz, D. Petring, F. Schneider, S. Stoyanov, U. Halm, A. Gillner

Instabilities of the melt flow dynamics on the laser cutting front cause loss of quality due to the formation of striations on the cut flank. Only individual aspects of mechanisms of striation generation during laser cutting are understood. Further evaluations of the melt flow dynamics inside laser cutting kerfs regarding characteristic temporal and spatial variables of the cutting process and their dependences are still necessary. Within this paper the melt flow dynamics during laser cutting of 6 mm thick stainless steel using a disk laser with a laser power of 5 kW are visualized and analyzed by means of in-situ high-speed video diagnostics (>100,000 fps). For that purpose, an advanced algorithm is used to detect the temporally and spatially resolved melt waves sliding down along the cutting front apex. The cutting depth-dependent analysis of the temporal distance between the melt waves reveals only a low dependence on the cutting velocity.

Keywords: Macro Processing; Cutting; 1 micron laser; high-speed imaging; melt flow; process diagnostics


Evaluation of offline path planning for laser metal deposition on freeform surfaces
Marie-Noemi Bold, Norbert Pirch, Stephan Ziegler, Johannes Henrich Schleifenbaum

The thickness of a layer made by Laser Metal Deposition (LMD) on a freeform surface varies depending on the local angle of inclination and the tool path direction. When depositing multiple layers, the CAM (Computer-Aided Manufacturing) planning for the tool paths requires an accurate description of the surface topography of the previous layers to ensure a stable process. Offline path planning is commonly based on the translation of the surface in the building direction (z-offset) or in the direction of the plane normal after each layer. In this paper, troughs with freeform surfaces are filled via LMD. The surface topography is measured after each layer and its deviation against the calculated surface according to z-offset and plane normal method is evaluated. Advice is given on which method of offline path planning is preferable and if additional controlling measurements of the surface are necessary after a certain amount of layers.

Keywords: Laser Metal Deposition; CAD; CAM; Reverse Engineering; Offline path planning


In-situ diagnostic in laser beam welds with digital image correlation – Reduction of residual stress and distortion in laser beam welds using low-transformation-temperature (LTT) filler materials
Uwe Reisgen, Simon Olschok, Fatma Akyel

During welding the localized heat input results in high temperature gradients between the weld seam and the base material leading to thermally and transformation induced stresses. To counteract problems like residual stress and distortion, in the past few years low-transformation-temperature (LTT) materials have been successfully used as filler wire. When austenite transforms to martensite the surrounding base material prevents dilatation in the weld seam. Compressive stress builds up while reducing residual stress and distortion. Digital image correlation is used to visualize the resulting surface displacements or deformations as well as surface contractions and phase transformations during the cooling process. This visualization is key to understand and reduce the formation of residual stress and distortion in welds. The displacements after welding are always lower when using LTT filler material when compared to conventional wire, proving that LTT can be used to mitigate distortion during laser beam welding.

Keywords: low-transformation-temperature (LTT); digital image correlation; distortion, residual stress


Simulation of Solidification Microstructures under Thermal Conditions of Laser Beam Welding
M. Apel, O. Stryzhyboroda, G. Boussinot, B. Böttger

Today, the phase-field method has reached a level of maturity that allows for spatially and temporally resolved simulations of the solidification microstructure evolution in technical, i. e. multicomponent and multiphase, alloys. In this paper, we will discuss two simulation examples: (i) laser welding of low-alloyed aluminum A3003 leading to cellular-dendritic microstructures, and (ii) welding of the nickel base superalloy IN718 showing more pronounced dendritic solidification structures. The simulations provide e. g. the cellular or dendrite spacing, the solidification path, i. e. solid fraction versus temperature, or the microsegregation pattern of the alloying elements. Based on these two examples we will remark on strategies to link local microstructure simulations with macroscale simulations of the thermal field and the accessible length and time scales for the microscale simulations. Furthermore, perspectives for applications in LPBF will be briefly addressed.

Keywords: solidification; microstructures; phase-field simulation;


Numerical investigations of the temperature distribution around the capillary and its effect on the stability of the capillary rear wall
Peter Berger

For some years now, three-dimensional numerical investigations have been used to gain insights into many phenomena of laser beam welding. However, two-dimensional investigations may be better suited to clarify basic interaction phenomena. The problem to be particularly emphasized here is welding at high speed and with relatively large capillaries. The influence of different parameters on the shape of the weld pool is shown, including the influence of different material properties at different Péclet numbers. Different flow conditions around and behind the capillary are also considered. As a result, it was found that at high Péclet numbers zones with high temperatures and low temperature gradients occur behind the capillary. These can destabilize the capillary back wall, which could explain the transition from a circular to an elongated capillary with increased welding speed.

Keywords: Laser-beam welding; capillary; stability; numerical simulation


Mitigation of Laser Keyhole Girth Weld Start/Stop Defects Through Application of Different Laser Termination Regimes
Wai Jun Lai, Supriyo Ganguly, Wojciech Suder

Laser keyhole initiation and termination defects such as keyhole cavities due to keyhole collapse is a well-known issue in deep penetration laser and hybrid laser welding. Industry can apply such productive manufacturing processes for longitudinal butt welding where run on and off tabs are used to avoid start/stop defects, however, this is not possible for girth weld applications where start/stop defects remain within the workpiece. This study shows that laser keyhole termination defects are more prominent compared to initiation defects in autogenous laser bead-on-plate overlap welding and control of keyhole dynamics is vital to achieve controlled closure of the keyhole during laser termination. Experiments were conducted using two different laser termination regimes at the weld overlap: ramping down of laser power, and gradually increasing the focal distance whilst maintaining a constant laser power output, demonstrating successful keyhole closure could be achieved, eliminating defects associated with abrupt termination of the laser.

Keywords: Laser keyhole welding; Start/stop defects; Laser defocusing; Laser keyhole termination; Keyhole closure


Beam Shaping BrightLine Weld – Latest Application Results
P. Haug, S. Weidgang, J. Seebach, N. Speker, T. Hesse, S. Bisch

This paper presents latest results on welding of steel, aluminum and highly reflective materials such as copper using the new TRUMPF beam shaping technology BrightLine Weld. The technology is based on applying a TruDisk thin-disk solid-state laser with a so called 2in1 fiber. In combination with a novel system of variable laser power coupling into the inner as well as the outer fiber core, an application-tailored laser power distribution is created. This enables a new degree of freedom through beam shaping for laser keyhole welding. The process benefits are significantly higher achievable feed rates, minimal spatter formation and highest weld seam qualities. For full penetration welding, it is even possible to reduce spatter formation on both sides of the weld seam. Endurance strength tests performed show that the weld seam characteristics of the high-speed welds fulfill state of the art requirements.

Keywords: laser welding; disk laser; powertrain; beam shaping


Laser Welding of Copper Using Superposed Green and Near-Infrared Laser Radiation
Lazar Tomcic, Michael K. Kick, Martin Haubold, Andreas Ganser, Michael F. Zaeh

Joining of copper materials is gaining in importance due to the increasing electrification in the automotive sector. Conventional joining technologies suffer from low dynamic strength or high specific weight of the welded structure. Laser beam welding seems to be a suitable alternative without the aforementioned drawbacks. Copper shows a high reflectivity for laser light at near-infrared wavelengths, which is emitted by state-of-the-art beam sources. Therefore, near-infrared laser beam welding does not seem to be a promising alternative. In contrast, copper shows a high absorptivity for laser radiation at green wavelengths. Since green beam sources can only provide a power up to several hundred Watts, the weld depth is limited. To overcome this limitation, a continuous wave beam source at a green wavelength with an output of thousand Watts is coupled to a near-infrared disk laser. In this paper, an investigation for such an attempt is presented using different laser powers as well as the relative alignment of both laser beams. The findings provide knowledge for an alternative joining method of copper materials that is based on laser beam welding.

Keywords: green laser radiation; near-infrared laser radiation; laser hybrid welding; copper; macro joining;


New Opportunities for Copper Processing with Kilowatt Blue Laser Sources
Simon W. Britten, Sörn Ocylok, Markus Rütering

The laser based copper processing gained significance in the last years due to the increasing demand for the interconnection of highly conductive connectors in the field of electric vehicles. In contrast to laser sources emitting in the near-infrared wavelength range of 1μm, blue laser sources emitting at 450nm can increase the process efficiency of up to factor 20 for the melting of copper. This high absorption level allows new process approaches, which were formerly however restricted by the conventionally low power level of blue laser system. With the availability of a kilowatt laser system with 450 nm wavelength, a major step has been achieved to reach industrial relevant power levels. In this contribution we will review the latest developments regarding the blue laser system technology and give an overview regarding welding applications on copper and dissimilar material combinations.

Keywords: Copper, laser welding, electric vehicles, battery, blue laser, 450 nm wavelength


Dissimilar laser beam welding of press hardened stainless martensitic steels and a cold worked stainless TWIP steel
Martin Dahmen, Berkan Deniz, Stefan Lindner, Dirk Petring

Combinations of ultra-high strength and supra-ductile steels promise a multitude of benefits for steel construction in vehicle manufacturing. Laser beam welding is an appropriate tool for joining both materials due to its concentrated energy input. Results of research on laser beam welding of two press hardened steels, X46Cr13 (1.4034) and 37MnB5 (1.5538), and a dual phase steel DP980 (1.0944) to an X30MnCrN16-14 (1.4678), cold formed to 1000 MPa yield strength, will be reported. The resulting predominantly austenitic microstructure enables crack-free welding. Because of the cast structure in the weld material a loss of strength has to be considered. Hardness slopes in the heat affected zone of TWIP steel and significant softening in the press-hardened steel in conjunction with an inhomogeneous solidification structure in the fused zone leads to a complex strength profile in the weld zone. Insights will be given on microstructures, local chemical composition and hardness of the fused zone.

Keywords: Laser beam welding, TWIP steel, martensitic steel, microstructure, alloy composition, hardness


Laser welding of dissimilar steels: Maraging (18Ni) and 300 M
Deivid Ferreira da Silva, Milton Sérgio Fernandes de Lima, Rafael Humberto Mota de Siqueira, Antonio Jorge Abdalla

In this work welded joints made of dissimilar steels: Maraging (18Ni) and 300M-ESR steel by the laser beam welding process (LBW) was evaluated. After the laser welding, different heat treatments were applied with the purpose to approximate the hardness of both steels in the fusion zone and heat affected zone. Subsequently, the joints were submitted to tensile and microhardness test for mechanical properties evaluation. Metallographic analyses of the welded joints were also performed and the microstructure and mechanical properties were correlated. The heat treatments applied after welding proved to be convenient for improvement of the hardness and the tensile strength. The increase in tensile strength due to the heat treatment applied after the welding was about 400 MPa. The laser welding process proved to be efficient to achieve the union of the proposed dissimilar steels and the heat treatment applied contributed to improve the mechanical properties.

Keywords: maraging steel; dissimilar welding; 300M steel; laser beam welding (LBW), microstructural characterization


Micro Laser Lap Welding of Thin SS304 Sheet
S. Patel, A. Aggrawal, A. Kumar, V.K. Jain

Laser micro welding involves very small melt pool, and rapid cooling and solidification of molten metal. In this study, lap joint experiments of 100 μm SS304 thin sheet were carried out using a continuous wave fiber laser. The set of process parameters leads to acceptable lap joint is identified. Scanning electron and optical microscopy, micro hardness and tensile shear tests of the welded lap joints were carried out to examine the metallurgical and mechanical properties. Thereafter, a 3D computational model has been developed to understanding the process physics and the influence of various process parameters on the weld bead profile quantitatively. The phenomenon of heat transfer, fluid flow, melting, solidification and evaporation are incorporated into the model. The model is used to describe the free surface thermo-fluid behavior, melt pool characteristics and the solidification characteristics. The simulated results of melt pool are in good agreement with the experimental measurement.

Keywords: Micro Laser Welding; Lap Joint, Stainless Steel; Modelling and Simulation.


The effects of weld parameters on fatigue strength of a pulsed laser welding Ni-base alloy thin sheet with filler wire
Jingling Yu, Jun Liu, Dongsheng Chai, Bo Cheng, Fangyong Niu, Guangyi Ma, Dongjiang Wu

Welded nickel-based alloy Hastelloy C-276 thin sheets were widely used in nuclear industry. The fatigue life of laser weld joints with filler wire has attracted considerable interest because of the cyclic loading caused by the working condition. In the present work, fatigue strength could be predicted by weld parameters with the help of Respond surface method (RSM) and Finite element method (FEM). The relationship of weld parameters and weld profile was established by RSM experiment. Changes in weld profile would lead to the different stress concentration effect which affect the fatigue strength most. The results showed that the weld joint has better fatigue strength when the weld geometry is symmetry. Weld speed has the most significant influence on fatigue strength, and has influence on optimization of other parameters. The risk of fatigue crack can be minimized by optimization of weld parameters.

Keywords: Hastelloy C-276, Laser weld with filler wire, Respond surface method, fatigue strength


Applied machine learning for predicting the weld seam geometry based on the example of laser-assisted metal-plastic joining
Klaus Schricker, Marcus Glaser, Jean Pierre Bergmann

In this paper, supervised neural networks and support vector machines are used to predict the weld seam geometry in laser-assisted metal-plastic joints. The informative value was maximized by generalizing the training data to geometrical properties, material data and process parameters. The meaningfulness is determined by 10-fold cross-validation during the training process and different amounts of training data. Finally, referencing investigations on novel parameters were carried out to evaluate the informative value of the applied machine learning methods.

Keywords: Joining, System Technology and Process Control, Fundamentals and Process Simulation, Machine Learning, Neural Nets


Laser welding of thermoplastics – Improving the weld strength of short glass fiber reinforced thermoplastics
Gereon Bussmann, Andreas Schollmayer, Mattias Ölscher, Marcus Scholl, Ulrich Russek

Laser welding of short fiber reinforced thermoplastics is well-established in industrial production. However, until now only the polymer matrices within the joiningplane experience a connection; the glass fibers do not participate in the joining. Therefore, the achievable weld seam strength is limited to bulk strength, not to the much higher short fiber reinforced polymer strength. As a consequence weld seam areas have to be large enough to withstand expected forces / tensions. Additionally, larger weld areas asking for appropriate design, higher laser power and longer processing time. By applying adapted irradiation methods, a movement of short glass fibers from one into the other joining part has been achieved while increasing weld seam strength by about 30 %.

Keywords: Laser welding of thermoplastics, short fiber reinforced thermoplastics, weld strength improvement