Laser beam welding of aluminium alloys

LBW is a welding technique that joins metals by use of a laserbeam as heating source which is commonly produced by either gas or solid-state lasers. Different configurations of optical devices focus the beam to allow high welding rates at high quality welds. Laser beam welding of aluminium alloys in aerospace was established in 2000, driven by the increasing demand of a cost- and weight-saving joining alternative to riveting. The main challenges of quali-fying LBW for aerospace applications were related to process stability and process quality.

In 1996, Rapp discussed [31] basic fundamentals and influencing parameters for LBW of Al materials in light-weight applications. Klassen [32] described weld pool dynamics for Al lead-ing to weld imperfections due to changes in laser power, focus geometry and focus position.

Schinzel [33] investigated LBW of Al-alloys with Nd:YAG lasers for automotive industries.

Heimerdinger [34] analysed the influence of different process parameters on the weld quality and hot crack resistance for different Al-alloy compositions. He showed that with an increase of the ratio of laser power to focus diameterP_L/d_f (called specific power in [31]), the amount of process pores decreases.

The functional principle of SLM corresponds to conventional LBW with powder as filler ma-terial. Only the basic fundamentals of LBW that are transferable to SLM are explained in this Section. Figure 2.4 illustrates three types of welding modes that can occur in SLM. It shows graphically the differentiation between heat conduction welding, transition keyhole welding and keyhole/ deep penetration welding, which differ in their weld aspect ratioANof weld seam

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depth to weld seam width (see Equation 2.3) and the intensity of metal vapour formation, as indicated by the yellow or red cloud around the laser beam.

AN=d_d

d_w (2.3)

Figure 2.4.: Laser welding modes and aspect ratios according to [35]

The melt pool during heat conduction welding stays intact, and welding is only affected by the absorption capability of the material’s surface (Fresnel absorption). In the keyhole welding or deep penetration welding mode is the degree of energy coupling the more dominant factor.

The melt pool forms depending on the evaporation temperature of the alloying elements at high power densities rapidly a vapor capillary, the so called keyhole. The vapor is surrounded by the melt, which solidifies at the vapor’s reverse side. Multiple laser reflections are the result, leading to higher local absorption. In the literature [36, 34, 33, 37, 35], the different modes are often ranged depending on the power density. Heimerdinger [34] ranges heat conductivity welding between laser intensities ofI₀=E=10⁴W/cm²to 10⁵W/cm²and keyhole welding above an intensityI0=E>10⁶W/cm². According to Birnesser in [37] keyhole welding oc-curs already at several 10⁵W/cm²and at an aspect ratio ofA>2. Experiments in this thesis showed that in SLM processes a classification of the laser intensity and associated welding mode is even more complex as many more interference factors appear that change locally the ratio in the energy conservation of reflection, absorption and transmission (see Equation 2.1).

Beck [38] has described that the ratio of laser power to laser beam diameter (^P_d^L

f) reflects the threshold conditions between heat conductivity to keyhole welding for Al-alloys. Heimerdinger proofed in [34] that this approach is valid for welding velocities between 1.5 to 31 m/min.

Equation 2.4 presents this simplified approach which is also taken into account for the eval-uation of the SLM process (see in Section 3.2). It includes the absorption capabilityA, the welding (scan) velocityv_sand material-dependent factors.

P_L

d_f ∼T_v·L A

√Pe_Al+1 (2.4)

The dynamics of weld pools with a free surface are essentially influenced by convective flows because of different temperature and surface tension gradients in the weld, so called Marangoni convection, (see [39, 40, 31]). The velocity of the Marangoni convection flow is in the range of several meters per second and is therefore significantly higher than the scanning velocityv_s. The Marangoni convection flow for metal alloys usually occurs due to negative surface tension gradients ^d_dT^σ <0 [40], as illustrated in Figure 2.5. Complex melt movements in SLM processes are simulated and described for AM Scalmalloy in the literature [7].

Figure 2.5.: Schematic drawing of Marangoni convection for ^d_dT^σ <0

The weld pool widthd_w is depending mainly on laser focus diameterd_f and the resulting weld depthd_dis strongly dependent on the chosen scan velocity, as investigated in this thesis.

The weld seam volume is very small compared to the volume of the platform, at least for the first layer in the process. The heat can therefore be transferred rapidly to the cooler plat-form (similar heat transfer as for laser remelting, as explained in [40]). Cooling rates between (10⁴−10⁶)K/sare reported in literature, depending on SLM process parameters and alloy-dependent thermophysical properties. However, the geometry of the part and an increasing number of layers decreases the cooling rate and changes solidification processes.

During solidification of a weld, the solid and liquid interfaces play an important role and de-cide about the resulting solidification mode. Different zones exist in weld beads, which are in general distinguished as liquid zone (LZ), mushy zone (MZ) and partly melted zone (PMZ) (where liquid and solid coexist) and solid zone (SZ). Transferred to SLM processing, the (SZ) equals to the base material or plate (BM) or the additive manufactured zone (AMZ), which contains previous molten layers.

The solidification of an Al alloy weld follows constitutional supercooling (CS) as the weld exists for only a short time in liquid form, so convection or diffusion compensation at the solidification interface is avoided (see [40, 41]). Constitutional supercooling means that crys-tallisation is delayed and that the melt remains liquid although the actual temperature of the melt is below the liquidus temperature.

In general, it can be assumed that for Al-alloys, solidification during welding at high cooling rates occurs in two ways according to [42], and can be heterogeneous (columnar) dendriti-cally at a the solid interface or homogeneous equiaxed (dendritidendriti-cally) in the liquid weld pool.

However, with increasing CS rises also equiaxed grain growth on a solid interface in the MZ as described in [41].

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(a.) (b.)

Figure 2.6.: (a.) Increasing constitutional supercooling (CS) leading to different grain growth mechanism in mushy zone (MZ) (b.) Solid zone (SZ), mushy zone (MZ), and liquid zone (LZ) in a general phase diagram; both illustrations according to [42]

[39]