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A finite element model is employed to perform a sequentially coupled thermo-mechanical analysis for enabling rapid process simulations of temperature fields, residual stresses and distortions for the production of additively manufactured parts via laser metal deposition. Experimental identification of characteristic process features such as temperature distribution, melt pool dimensions and bead geometries were used for the initial built-up and calibration of the model. The addition of material during process simulation is realised through reactivating inactive elements during the transient heat transfer analysis and through reactivating a combination of inactive and quiet elements during the mechanical analysis. The travelling heat source is geometrically bounded to precisely control the volume of its energy distribution. The results of the transient heat transfer analysis are sequentially coupled to a mechanical analysis for obtaining information on the resulting residual stresses and deformation. Based on the good agreement between numerical and experimental results of the thermal analysis, conclusions on the corresponding residual stress distributions and deformation are made. It is shown that the model represents an efficient tool for process prediction regarding thermal history, residual stresses and final-part deformations. Finally, the model is utilised to identify parameters and conditions of the process that lead to reduced residual stresses and deformations of the investigated additive part.

Up to now, only a limited amount of metallic materials is investigated for laser additive manufacturing (LAM). However, the demand to widen the application possibilities by enlarging the range of materials for LAM is growing fast. By now, titanium and aluminium alloys are in the focus of research. In contrast, magnesium alloys are rarely used in the field of additive manufacturing, although they possess a low density in combination with a high specific strength. Currently, magnesium structures are mainly produced by casting but during the last years, the use of wrought alloys also increased. A reason for the rare use of magnesium alloys for LAM technologies might be the high flammability of magnesium powders. This difficulty can be avoided by using magnesium wire for laser metal deposition (LMD). In the present study, the microstructural characteristics of a LMD processed AZ31 magnesium alloy are investigated. For this purpose, optical microscopy and scanning electron microscopy were used. With the help of EDX and EBSD analysis, a change of the chemical composition and micro texture with structure height was identified. The relationship of microstructure and local mechanical properties was investigated with the help of Vickers micro hardness testing. Based on the obtained results it can be concluded that the microstructural characteristics of laser additive manufactured magnesium alloys differ from those of titanium and aluminium alloys. Thus, a wider application spectrum of LMD and magnesium alloys can be opened up.

Powder-based laser metal deposition (LMD) of Ti alloys enables the manufacturing of geometrical complex structures with tailored properties for high-temperature applications. The most important parameters for the LMD-process are the laser power, process velocity, laser focus position, gas flow and powder feed rate. Compared to the predominantly used Ti-6Al-4V, Ti-6Al-2Sn-4Zr-2Mo shows a higher oxidation resistance and a lower creep rate. For determining suitable process conditions, a parameter study using Ti-6Al-2Sn-4Zr-2Mo powder is performed. Subsequent, the parameter transfer to a γ-TiAl alloy is examined. The assessment of the dissimilar-LMD-structures is conducted by visual inspection, radiography inspection, microstructural analysis and hardness measurements.

The theoretical fundamentals of laser weldability of metals are surveyed and relevant thermophysical parameters are identified – such as vapour pressure, keyhole pressure, beam irradiance, surface tension and viscosity. The derived approach for improving the laser weldability implies the use of a Yb fibre laser with an initial large beam diameter, a top-hat beam profile and a high laser power, which enables the formation of a large and stable keyhole during deep penetration welding. For validating the effectiveness of the developed approach, it is applied to various high-alloyed and hard-to-weld Al–Zn–Mg–Cu alloys. Defect-free welds are obtained even for AA7034 – the alloy with the highest (Zn + Mg + Cu) content commercially available. As reference, the same alloys are welded by using a conventional Nd:YAG laser with a small beam diameter, a Gaussian beam profile and medium laser power. The laser weldability deteriorates with increasing (Zn + Mg + Cu) content in terms of porosity and excess of penetration. The obtained results highlight the importance of the laser system used on the laser weldability of Al–Zn–Mg–Cu alloys.

Heat-treatable Al-Zn alloys are promising candidates for use as structural lightweight materials in automotive and aircraft applications. This is mainly due to their high strength-to-density ratio in comparison to conventionally employed Al alloys. Laser beam welding is an efficient method for producing joints with high weld quality and has been established in the industry for many years. However, it is well known that aluminum alloys with a high Zn content or, more precisely, with a high (Zn + Mg + Cu) content are difficult to fusion weld due to the formation of porosity and hot cracks. The present study concerns the laser weldability of these hard-to-weld Al-Zn alloys. In order to improve weldability, it was first necessary to understand the reasons for weldability problems and to identify crucial influencing factors. Based on this knowledge, it was finally possible to develop an appropriate approach. For this purpose, vanadium was selected as additional filler material. Vanadium exhibits favorable thermophysical properties and, thereby, can improve the weldability of Al-Zn alloys. The effectiveness of the approach was verified by its application to several Al-Zn alloys with differing amounts of (Zn + Mg + Cu).

Crack-free dissimilar TNM-Ti6242 joints were successfully obtained by laser welding with preheating temperatures of 400 °C, 600 °C and 800 °C, in spite of the differing thermophysical properties of the used alloys. The microstructure of the joints was investigated by optical microscopy, SEM, EDX, EBSD and the local mechanical properties were determined by microhardness measurements. The microstructure coarsens with increasing preheating temperature. Homogenous mixture of both alloys in the fusion zone was achieved, even so local microsegregations of Ti, Al and Nb occurred. The fusion zone mainly consists of α2 (Ti3Al). A transition zone, present as a β/βO seam, was revealed for preheating temperatures of 400 °C and 600 °C. For preheating to 800 °C the β/βO seam was penetrated with α2 lamellas. Since the alloying elements of the other alloy were detected in the heat-affected zone, diffusion processes occurred. The average microhardness in the fusion zone of the dissimilar TNM-Ti6242 joints is independent of the preheating temperature and is higher than the base material microhardness. Comparing the microhardness in the fusion zone of similar Ti6242 and TNM joints with dissimilar TNM-Ti6242 joints, the microhardness values of the dissimilar TNM-Ti6242 joints range between the values of the similar Ti6242 and TNM joints. However, the values in the fusion zone are closer to the microhardness values of the similar TNM joint, which is caused by the mainly α2 containing fusion zone of the dissimilar TNM-Ti6242 joints.

Wire-based laser metal deposition enables to manufacture large-scale components with deposition rates significant higher compared to powder-based laser additive manufacturing techniques, which are currently working with deposition rates of only a few hundred gram per hour. However, the wire-based approach requires a significant amount of laser power in the range of several kilowatts instead of only a few hundred watts for powder-based processes. This excessive heat input during laser metal deposition can lead to process instabilities such as a non-uniform material deposition and to a limited processability, respectively. Although, numerous possibilities to monitor temperature evolution during processing exist, there is still a lack of knowledge regarding the relationship between temperature and geometric shape of the deposited structure. Due to changing cooling conditions with increasing distance to the substrate material, producing a wall-like structure results in varying heights of the individual tracks. This presents challenges for the deposition of high wall-like structures and limits the use of constant process parameters. In the present study, the temperature evolution during laser metal deposition of AA5087 using constant process parameters is investigated and a scheme for process parameter adaptions in order to reduce residual stress induced componential distortions is suggested.

Wire-based laser metal deposition enables to manufacture structures with very high deposition rates in comparison to powder-based laser additive manufacturing. However, this advantage is generally accompanied with a high energy input. Thus, an accumulation of heat within the structure can result. In addition, the heat conduction conditions can also change with increasing structure height, leading to inhomogeneous microstructural formation along the part. The present study deals with the evolution of the microstructure and hardness in laser metal deposited AA5087 wall structures. In this regard, the resulting microstructure of two samples processed at different process parameters and increased deposition rates is investigated.

Wire-based additive manufacturing has been increasingly investigated in recent years. Although it is possible nowadays to manufacture structures that are free from inner defects such as porosity and cracks using wire and arc additive manufacturing, there is still a lack of knowledge regarding wire-based laser metal deposition of aluminium and its alloys. In order to be able to produce locally tailored part properties, it is necessary to understand the process parameter to material property relationships. Using laser energy sources, it becomes possible to analyse in detail the heat input and to observe occurring microstructural evolutions. This work includes a microstructural and mechanical characterization of an AA5087 wall structure. Detailed analyses of the chemical composition, texture, and microhardness of the structure have been performed. The microstructure contains different grain orientations as well as grain shapes and sizes along the structure, resulting in locally different material properties. The results have been analysed and discussed in reference to fundamental theories such as the Hall–Petch and Orowan mechanisms.

In recent decades, laser metal deposition, as a part of additive manufacturing, developed into a promising methodology in industrial fields. In recent years, there has been an increased interest in the processability of lightweight high-strength structural materials, such as aluminium alloys. However, in terms of wire-based laser metal deposition, there is still a lack of knowledge with regard to the processability of aluminium alloys. In this research, the process development for wire-based laser metal deposition of a 5087 aluminium alloy (AlMg4.5 MnZr) has been conducted. It is observed that pre-heating is beneficial in terms of porosity and distortion reduction. Within optimized parameter ranges, it is possible to control the geometric shape, dilution, and aspect ratios of the deposited layers in a systematic way. Accordingly, defect-free layers with tailored geometrical features can be processed and adapted to specific process requirements.

Ti6Al2Sn4Zr2Mo exhibits improved oxidation and creep properties compared to Ti6Al4V. Laser beam welding (LBW) is an approved process to receive narrow weld seams at high welding speeds with low heat input. Almost distortion free complex shaped structures can be joined with optimal parameters. For the optimisation of the LBW process the most relevant parameters are the welding speed, the laser input power and the gas shielding strategy. Using a fibre laser, the laser radiation is attenuated by a welding plume the so-called metal-vapour cloud (MVC). The MVC has a large influence on the laser input power. Therefore, an approach for reducing the MVC by optimising the shielding strategy using an additional gas flow in opposite welding direction is examined. Utilizing high-speed camera records, the effectiveness of the approach is assessed. Welded samples are evaluated by visual and radiographic inspection, metallographic assessment as well as microhardness measurements with regard to weld seam geometry, defects, microstructure and local mechanical properties. The obtained results are correlated to the used laser welding parameters.

AA2198 is a relatively new light-weight and high-performance Al-Cu-Li alloy considered for aviation and space applications. However, Al-Cu-Li alloys generally exhibit severe weldability problems for all fusion-welding techniques, such as laser-beam welding. In particular, porosity formation and hot cracking are observed for the laser-beam welding of these alloys. A common remedy for hot cracking is the use of an appropriate filler wire with a high Si content. In the present study, three different approaches for improving the hot cracking susceptibility of AA2198 laser beam welded without any filler material are presented. For this purpose, pre-heating of the weld samples to elevated temperatures, pre-loading of the weld samples perpendicular to the welding direction, or an optimization of the laser-beam welding parameters were conducted. The autogenously welded samples were assessed with regard to the resulting total crack length and their mechanical properties. It was demonstrated that all of the presented approaches led to a reduction of hot cracking. However, the largest effect was observed for the use of low levels of laser power and welding velocity. The mechanical properties of the optimised autogenously welded samples are only marginally inferior as for the samples laser welded with the Al-Si filler wire AA4047.

The additive production of metallic components with high-throughput is usually associated with high process temperatures and slow cooling rates. This typically results in strongly oriented columnar grain growth along the building direction of the structure having exceedingly large grain sizes. As a result, such structures show typically low strength and anisotropic mechanical behaviour in as-deposited condition. Consequently, post-processing is commonly performed to homogenize and eventually increase the mechanical properties of the deposited structures. In this regard, precise control of the applied process energy allows a modification of the local temperature distribution and cooling conditions during the additive manufacturing process, which strongly influence the resulting solidification microstructure. The aim of the present study is the development of an approach that allows to influence the solidification conditions in wire-based laser metal deposition of an Al-Mg alloy through specific adjustments of the laser irradiation. It was found that significantly different solidification microstructures in as-deposited condition can be achieved by adjusting the laser beam irradiance within a range resulting in conduction mode welding conditions while keeping the heat input constant. The application of high laser beam irradiances, close to the transition to keyhole mode welding, results in structures with a homogeneous large-grained solidification microstructure exhibiting a degree of anisotropy of around 12 % between building direction and the direction of deposition. In contrast, the use of low laser beam irradiance close to the lower limit of stable melting, results in structures with a significantly refined microstructure. Consequently, an increase of yield strength of up to around 20 % and microhardness of up to 13 % , as compared to structures processed with high laser beam irradiance, could be obtained. Moreover, the anisotropy of the as-deposited structure was reduced to a degree lower than 2 % .

The wire-based direct energy deposition of metallic lightweight materials such as titanium or aluminium alloys has recently received increasing attention in industry and academia. However, high-throughput deposition is mostly associated with process-limiting phenomena such as the development of high temperatures resulting in poor surface quality as well as coarse and unidirectional solidification microstructures. In this regard, laser systems, which are already widely used in industrial processes, allow for a great variety in the controllability of energy inputs, thereby enabling the control of process temperatures and resulting microstructures. The subject of the current study is the detailed elucidation and evaluation of important features such as the development of temperature gradients, resulting cooling rates and thermal cycles for different laser beam irradiances. Significant heat accumulation and process instabilities as well as inhomogeneous thermal profiles along the length and height of the parts were observed at a high laser beam irradiance. In contrast, lower laser beam irradiance resulted in a more stable process with increased cooling rates, which favourably influenced the refinement of the solidification microstructure.

The anisotropy effect on tensile mechanical properties of an Al-Cu-Li (2198) alloy with regard to thickness of the specimens under different ageing conditions was investigated. Occurring size effects between macro and micro (0.5, 3.2 and 5.0 mm thickness and 10 and 50 mm gauge length) tensile specimens was discussed. The mechanical behavior of AA2198 was examined by taking into account the experimental results from micro-flat and standard tensile specimens. Higher thickness specimens showed higher elongation at fracture values and slightly lower yield stress properties. Anisotropy seems to be higher at T3 condition, while the lowest was noticed at the peak-ageing condition. The results showed that the micro-flat tensile specimens in T3 condition presented slightly lower yield stress (10 MPa difference) and essentially lower elongation at fracture values (more than 40 % decrease), when compared with the respective of higher thickness specimens. It was also shown that thicker (5.0 mm) specimens exhibit slightly higher tensile ductility properties (almost 17 %) and slightly lower tensile strength properties than the respective 3.2 mm thickness specimens. There is evidence of relative difference in mechanical properties due to the rolling process in the two sheet directions (L and T directions); such anisotropy difference seems to be marginal at the peak-ageing condition.

Two approaches were used for laser beam welding of similar and dissimilar joints of AA7075 and AA5182 that aim to overcome the weldability problems of high-strength Al-Zn-Mg-Cu alloys. The first approach implies the use of vanadium foil as additional filler material, while the second implies the use of a fiber laser with a large beam diameter and a top-hat beam profile. Although both approaches result in an improved weld quality, in terms of weld appearance, porosity and cracking, the resulting mechanical properties differ considerably. The addition of vanadium leads to a local increase of microhardness in the fusion zone. However, the tensile strength of these joints is lower as for fiber laser welded joints. In direct comparison fiber laser welded joints exhibit also higher formability as the joints welded with vanadium foil. The highest formability is obtained for dissimilar joints with the medium-strength Al-Mg alloy. Due to the unavoidable softening in the weld zone of heat treatable aluminum alloys, the formability of the joints is inferior in comparison to the base materials. In addition, the positive effect of post-weld heat treatment, surface milling and warm forming on the resulting mechanical properties of similar and dissimilar joints is discussed.

This work deals with the effects of laser beam power, focus position and advance speed on the geometry, microstructure and mechanical properties such as the tensile strength and microhardness of autogenously fibre laser beam welded Ti-6Al-2Sn-4Zr-2Mo (denoted as Ti-6242) butt joints used for high temperature applications. The Ti-6242 sheet employed here is characterized by a globular (α+β) microstructure. Laser beam welded butt joints consisted of a martensitic fusion zone, inhomogeneous heat affected zones and equiaxed base materials. The microhardness increased from 330 HV 0.3 in base material to 430 HV 0.3 in fusion zone due to the martensitic transformation. Butt joints showed the base material level of strength in tensile test. The local increase in microhardness provided a shielding effect that protected the Ti-6242 butt joint against mechanical damage during the static tensile load test. The predicted critical total underfill depth that does not reduce the tensile strength of the weld was determined to be 25% of the specimen thickness.