Search

This study focuses on the microstructure, tensile and fatigue properties of laser beam welded butt joints in 4 mm thick sheets of duplex stainless steel 2205. The microstructural characteristics of the joints were investigated via optical microscopy and electron backscattered diffraction. A 300 μm wide heat affect zone with increased ferrite content and a nearly fully ferritic 800 μm wide fusion zone were found. No porosity could be found with X-ray radiography. Microhardness measurements revealed increased strength in the fusion and heat affect zones of the weldments. Uniaxial tensile and stress controlled fatigue tests were performed in order to characterize the mechanical properties of specimens containing laser beam welded joints and the base material. The specimens containing weldments were stronger, but less ductile than the base material, due to the weld metal restricting deformation. The as-welded joints exhibited worse fatigue performance than the base material due to the notch at the excess weld metal. The fatigue properties of the specimens containing joints could be elevated to the base material level by a laser surface remelting treatment.

Fatigue & fracture of engineering materials & structures 44(4), 887 - 900 (2021). doi:10.1111/ffe.13400, Local laser heating treatment using a defocussed laser beam was applied to thesurface of 2-mm-thick AA2024-T3 sheets. Two different treatment designs—namely, lines and circles—as well as the positioning and number of treatmentswere investigated regarding their potential to retard the propagation of through-thickness fatigue cracks. The highest fatigue crack growth life extension of 285%was achieved by the application of four laser heating lines or two circles on eachspecimen side. The induced compressive residual stress field through the LH pro-cess is primarily responsible for an improvement in fatigue crack growth life. Anemphasis was placed on investigating the effect of the treatment on the possiblereduction of tensile and fatigue strength (S-N life). If only one line was appliedtransverse to the loading direction or only one circle was used, the reduction offatigue strength was comparable to the reduction of fatigue strength resultingfrom the stress concentration introduced due to the presence of a rivet hole., Published by Wiley-Blackwell, Oxford [u.a.]

The present study aims to investigate the extent to which the fatigue behaviour of laser beam-welded AA6056-T6 butt joints with an already existing crack can be improved through the application of laser shock peening. Ultrasonic testing was utilized for in situ (nondestructive) measurement of fatigue crack growth during the fatigue test. This procedure allowed the preparation of welded specimens with surface fatigue cracks with a depth of approximately 1.2 mm. The precracked specimens showed a 20% reduction in the fatigue limit compared with specimens without cracks in the as-welded condition. Through the application of laser shock peening on the surfaces of the precracked specimens, it was possible to recover the fatigue life to the level of the specimens tested in the as-welded condition. The results of this study show that laser shock peening is a very promising technique to recover the fatigue life of welded joints with surface cracks, which can be detected by nondestructive testing.

Titanium alloys are widely used in aerospace and automotive industries due to their excellent mechanical properties, however, the formability is limited, which is an issue during forming. In the present study, the effect of temperature and strain rate on the tensile properties of the titanium α-alloy KS1.2ASN was investigated. It was observed that there is initially no gain in ductility with increase in temperature until 400 °C, however, maximum formability is reached at maximum tested temperature of 600 °C. EBSD analysis revealed that twinning is the main deformation mechanism at room temperature, however, sliding deformation becomes more pronounced with increasing temperature. An increase in strain rate leads to a decrease in elongation, but the influence is less pronounced compared to temperature.

Crack formation process in the Ti-23Al-23Nb-1.4V-0.8Zr-0.4Mo-0.4Si (at.%) alloy based on orthorhombic titanium aluminide (Ti2AlNb), especially in the weld zone (WZ), heat-affected zone (HAZ), and base metal (BM), during laser beam welding (LBW) at room temperature was considered. It was determined that the crack spread in the WZ throughout the crystallites of β-phase. In the HAZ, cracks spread throughout the globular β-phase crystals body and along the edges of globular α2- and O-phases. The cracking stopped in BM due to a mechanism similar to shear ligament toughening. Cracking can be effectively suppressed by increasing the LBW temperature.

A Ti-15Mo/TiB metal–matrix composite was produced by spark plasma sintering at 1400 °C. The fractions of the elements in the initial powder mixture were 80.75 wt.% Ti, 14.25 wt.% Mo, and 5 wt.% TiB2. The initial structure of the synthesized composite was composed of bcc β titanium matrix and needle-like TiB reinforcements with an average thickness of 500 ± 300 nm. Microstructure and mechanical properties of the composite were studied after laser beam welding (LBW) was carried out at room temperature or various pre-heating temperatures: 200, 400, or 600 °C. The quality of laser beam welded joints was not found to be dependent noticeably on the pre-heating temperature; all welds consisted of pores the size of which reached 200–300 µm. In contrast to acicular individual particles in the base material, TiB whiskers in the weld zone were found to have a form of bunches. The maximum microhardness in the weld zone (~700 HV) was obtained after welding at room temperature or at 200 °C; this value was ~200 HV higher than that in the base material.

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.

Fiber laser beam welding of a CoCrFeNiMn-type high entropy alloy (HEA) produced by self-propagating high-temperature synthesis (SHS) was reported in this work. The SHS-fabricated alloy was characterized by both ∼2 times reduced Mn content in comparison with that of the other principal components and the presence of impurities including Al, C, S, and Si. The as-fabricated alloy was composed of columnar fcc grains with coarse precipitates of MnS and fine Cr-rich M23C6 carbides. Successful defect-free butt joint of the alloy was obtained using a laser power of 2 kW and a welding speed of 5 m/min. Welding resulted in changes in texture and structure of the fcc matrix. In addition, precipitation of nanoscale B2 phase particles in the weld zone was observed. A pronounced increase in microhardness from (153 ± 3) HV 0.5 (base material) to (208 ± 6) HV 0.5 (fusion zone) was observed. The B2 phase precipitation after welding was found to be in a reasonable agreement with the ThermoCalc predictions. Quantitative analysis demonstrated that the increase in hardness can be associated with the B2 phase precipitation. Possibilities of the development of HEAs with intrinsic hardening ability after laser processing are discussed.

The application of coatings to structures is generally done in order to locally tailor properties. For this purpose, powder-based laser metal deposition (LMD) can be utilized. The resulting properties of the coatings are not only affected by the LMD process parameters, but also by the substrate material. In this regard, the chemical composition as well as its initial microstructure of the substrate has a significant influence. In the present study two Al-Cu alloy substrates, 2139 and 2198, are used for the LMD of pure Al powder. The resulting coatings possess clearly different properties, although the same process parameters are used.

Research in aerospace applications includes the replacement of well-known materials by newly developed alloys or by new manufacturing methods for the existing materials. In the frame of TiB-Air project funded by the German Federal Ministry for Economic Affairs and Energy (BMWi) the development of a process chain consisting of deep drawing at elevated temperatures, chemical milling, contour machining by laser cutting and laser beam welding to produce pneumatic T-ducts used in bleed air systems is in focus. This production process of sheet metal parts could lower costs in terms of the process itself and the used materials: low alloyed Ti-alloys. Commercially pure titanium alloy (cp-Ti) is commonly used for these structures because of its balanced mechanical properties regarding tensile strength, yield strength, plastic strain and fatigue strength as well as good resistance against corrosion and oxidation. The possibility to substitute cp-Ti by low-alloyed Ti-alloys is examined in this work, by the comparison of two different low alloyed Ti-alloys, namely KS1.2ASN and Ti XT, with a cp-Ti alloy (Grade 4). Mechanical properties of the base materials, their weldability and the mechanical assessment of the laser beam welded butt joints in terms of static, cyclic and fracture mechanical behaviour is compared for sheet materials, with a thickness of 0.9 mm. Defect-free welding according to EN13919 acceptance criteria B was possible for all three alloys, no porosity problems occurred. The low strength alloy KS1.2ASN exhibited mechanical anisotropy between longitudinal direction and transverse direction in the tensile test, welded specimens of this alloy broke in the base material. Due to the tensile properties, both weld and base material of KS1.2ASN showed the least values for the fatigue strength and endurance limit strength. Ti XT and Grade 4 showed similar mechanical anisotropy and fractured in the base material, too. Fatigue strength of Ti XT is below Grade 4, but for the 50%-percentile the endurance limit strength is equal. Fracture mechanical testing showed that KS1.2ASN is a very promising alloy in the welded condition.

The effect of laser beam welding on the structure and properties of a Ti1.89NbCrV0.56 refractory high entropy alloy was studied. In particular, the effect of different pre-heating temperatures was examined. Due to the low ductility of the material, laser beam welding at room temperature resulted in the formations of hot cracks. Sound butt joints without cracks were produced using pre-heating to T &ge, 600 &deg, C. In the initial as-cast condition, the alloy consisted of coarse bcc grains with a small amount of lens-shaped C15 Laves phase particles. A columnar microstructure was formed in the welds, the thickness of the grains increased with the temperature of pre-heating before welding. The Laves phase particles were formed in the seams after welding at 600 &deg, C or 800 &deg, C, however, these particles were not observed after welding at room temperature or at 400 &deg, C. Soaking at elevated temperatures did not change the microstructure of the base material considerably, however, &ldquo, additional&rdquo, small Laves particles formed at 600 &deg, C. Tensile test of welded specimens performed at 750 &deg, C resulted in the fracture of the base material because of the higher hardness of the welds. The latter can be associated with the bcc grains refinement in the seams.

The mechanical behavior of autogenously and non-autogenously laser beam-welded joints of Al-Cu-Li alloy AA2198 and the effect of post weld heat treatment are examined in this contribution. The deformation texture of the base material does not present any significant change in the macrotexture with applying different artificial ageing times. Autogenously and non-autogenously laser beam-welded joints present a decrease in yield stress and ultimate strength in the as-welded condition, approximately 45% and 36–38%, respectively, when compared with AA2198-Τ3 base material. The addition of the AA4047 filler wire increases the Si and Cu content in the grain interior and in the grain boundaries of the fusion zone of the welded joint. Micro-hardness measurements for autogenously laser-welded joints showed a decrease in hardness by 27% for the fusion zone and 42% for the heat-affected zone, when compared with the non-autogenously laser beam-welded joints. Α quality index was exploited to evaluate the tensile mechanical performance of the welded joints. It is observed that the non-autogenously welded joints always show a higher ‘quality’ than the respective autogenously welded joints and the highest quality index in terms of mechanical performance is achieved for the as-welded and the peak-aged conditions, respectively. Regardless of the post weld heat treatment condition, both autogenously and non-autogenously laser-welded specimens fractured in between the equiaxed and fusion zone during tensile loading.

Crenellation is a novel concept to heighten the fatigue resistance of the airframe structures without increasing weight. In this work the impact of materials on the effectiveness of crenellations was investigated. Two candidate alloys for future fuselage skin: AA2139 and AA2198 were selected for this task. Firstly the microstructure and texture of those materials were investigated. Then flat and crenellated panels made of the two alloys were tested under service-related biaxial loading conditions. A removable δ5 clip gauge was applied at the crack tip to monitor the crack closure behavior during the fatigue tests. After fatigue tests the fracture surfaces were examined to interpret the respective fatigue behavior. It was found that sharp textured AA2198 alloy, which also showed tortuous shear lip morphology, has higher fatigue resistance and larger fatigue life improvement of crenellations compared with the nearly randomly textured AA2139 alloy. The correlation between the sharp texture and tortuous shear lip morphology was discussed. The source of the additional fatigue resistance and the increased crenellation efficiency in AA2198 was also analyzed.

The present work investigates the effects of laser beam power, focus position and advance speed on the geometry, microstructure and mechanical properties of fiber laser beam welded Ti-6Al-2Sn-4Zr-2Mo (denoted as Ti-6242) butt joints used for high temperature applications. Detailed microstructural and mechanical studies were performed on welds produced using optimized parameters (a laser beam power of 5 kW, a focus position of 0.0 mm and an advance speed of 6.2 m/min). The Ti-6242 base material is characterized by a globular (α+β) microstructure. The heat input during laser beam welding led to the formation of a martensitic α’-phase fusion zone. The heat affected zone consisted of globular grains and acicular crystallites. These local transformations were connected with a change in the micro-texture, average grain size and β-phase content. Furthermore, the microhardness increased from 330 HV 0.3 to 450 HV 0.3 due to the martensitic transformation. The mechanical behavior of the laser beam welded Ti-6242 butt joint loaded in tension was determined by the properties of the Ti-6242 base material. The local increase in hardness provided a shielding effect that protected the Ti-6242 butt joint against mechanical damage.

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.

Preferred grain orientations (crystallographic texture) of base materials could influence on mechanical properties, microstructure and residual stresses of welded joints. This should be considered for design purposes, in particular for materials having non-cubic crystal structures. A multitude of experiments have been carried out in this field of study without considering the crystallographic texture based anisotropy of base materials. In the present work, commercially pure titanium (CP-Ti) rolled sheets were laser welded along various directions with respect to the sample orientations, namely rolling direction (RD), transverse direction (TD) and 45° to RD. Three-dimensional strain profiles and the local texture around the weld were measured by neutron diffraction. Furthermore, grain orientation mapping within the base material, the heat-affected zone and the fusion zone was investigated using the electron back-scatter diffraction (EBSD) technique. The results revealed that tensile properties of the samples are different, while no significant statistical differences in residual stresses were observed.

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 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.

Weld defects - such as porosity and hot cracking - occur especially during the laser beam welding of high-alloyed Al-Zn alloys. This significantly limits the application range of these promising high-strength alloys. In the present study the laser weldability of different Al-Zn alloys was investigated regarding the used welding parameters and the chemical composition of the alloys. In addition, the novel approach of the Helmholtz-Zentrum Geesthacht for overcoming the weldability problems was applied to the different Al-Zn alloys in order to assess its capability. It was shown that the laser weldability of Al-Zn alloys deteriorates with an increasing amount of Zn, Mg and Cu. The variation of laser beam welding parameters did not lead to any improvement of weldability. Only the use of the new approach resulted in promising welding results even for the high-alloyed Al-Zn alloys.

In the present work the mechanical behaviour of laser beam welded AZ31B alloy was studied, by changing systematically the spot size of the used fibre laser system between 200 µm and 1000 µm at different power levels between 2 kW and 8 kW. Maximum welding velocities with respect to imperfections were determined. The characterization of the obtained welds - in terms of Vickers hardness, UTS, Af and weld width, resp. weld area - was correlated with the micro-texture in dependence of the different Focus Spot Diameters and Laser Beam Power levels as well as the resulting cooling rates. Highest UTS of 94% of the base material was achieved with 200 µm Focus Spot Diameter and Laser Beam Power of 4 kW at welding velocity of 100 mm/s. By increasing the Focus Spot Diameter to 600 µm, the tensile strength was reduced to 86 % of the actual strength of the base material.

In the aircraft industry double-sided laser beam welding of skin–stringer joints is an approved method for producing defect-free welds. But due to limited accessibility – as for the welding of skin–clip joints – the applicability of this method is limited. Therefore single-sided laser beam welding of T-joints becomes necessary. This also implies a reduction of the manufacturing effort. However, the main obstacle for the use of single-sided welding of T-joints is the occurrence of weld defects. An additional complexity represents the combination of dissimilar and hard-to-weld aluminium alloys – like Al–Cu and Al–Zn alloys. These alloys offer a high strength-to-density ratio, but are also associated with distinct weldability problems especially for fusion welding techniques like laser beam welding. The present study demonstrates how to overcome the weldability problems during single-sided laser beam welding of a dissimilar T-joint made of AA2024 and AA7050. For this purpose a high-power fibre laser with a large beam diameter is used. Important welding parameters are identified and adjusted for achieving defect-free welds. The obtained joints are compared to double-sided welded joints made of typical aircraft aluminium alloys. In this regard single-sided welded joints showed the expected differing weld seam appearance, but comparable mechanical properties.

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.

The present study deals with laser beam welding (LBW) and friction stir welding (FSW) applied to high-strength aluminum alloys used in aircraft industry and displays their advantages compared with the riveting technique regarding structural integrity, weight and material savings. First of all, it is shown with respect to different applications and strength levels which high-strength aluminum alloys represent the state-of-the-art and which aluminum alloys are proposed as substitutes in the future. Furthermore, the respective joining process principles are described and demonstrated on different joint configurations, whereby mechanical and microstructural properties of laser beam- and friction-stir-welded joints are discussed and compared. The current study clearly demonstrates that these two joining techniques are not competing but complementary joining techniques in the aircraft industry. FSW, as a solid-state joining process, has the advantage that the joining is conducted at temperatures below the melting point of the materials to be joined. Therefore, improved mechanical performance of joints is expected compared to that of fusion joining processes such as LBW. Furthermore, better mechanical properties can be obtained when heat input during joining is reduced by employing stationary shoulder FSW and/or external cooling. On the other hand, LBW offers several advantages such as low distortion, high strength of the joint, and high welding speeds due to its low localized-energy input. Thus, LBW - as a high-speed and easily controllable process - allows the welding of optimized complex geometrical forms in terms of mechanical stiffness, strength, production velocity, and visual quality. Both joining processes have advantages and disadvantages, depending on joint geometries and materials. They both have the potential to reduce the total weight of the structure. The FSW process (particularly lower heat input stationary shoulder FSW process) is more advantageous in producing long-distance straight-line butt joints or overlapped joints of aircraft structures, whereas the high-speed and easily controllable LBW process allows the joining of complex geometrical forms due to its high flexibility, particularly in the new generation high strength Al-alloys (such as AA2198), the strengthening phases of which are more heat resistant.

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 % .

Titanium alloy sheets are widely used for highly loaded components in the aerospace industry as well as spacecrafts. The unique combination of high strength, outstanding corrosion resistance and thermal endurance makes titanium alloys the preferred material for applications with severe requirements. Due to the limited formability at room temperature, forming processes have to be conducted in a multitude of steps what is costly and labour intensive. Additionally, typical titanium alloy sheets show a significant anisotropy of mechanical properties and material flow. Undesired earing, wall thickness variation and residual stresses are the result. Complex shaped parts can be produced at elevated temperatures to avoid named drawbacks. The present work introduces a newly developed hot deep drawing process, applied to titanium sheets at FormTech. In comparison with conventional superplastic forming processes via gas pressure, hot deep drawing comes with a significantly reduced process time and hence, increased output over time. Titanium sheets of the work horse alloy Ti–6Al–4V were formed in a single stroke to a U-shaped component at process temperatures ranging from 750 to 890 °C. Specimens were extracted to validate the neglectable influence of the hot forming process on mechanical properties and fatigue behaviour. In conclusion, hot deep drawing of titanium sheets offers a cost efficient alternative to a gas pressure superplastic forming process, while maintaining its main benefits such as significantly improved formability, low residual stresses and tight tolerances.

A fine-grained Ti–6Al–4V sheet that has been developed for superplastic forming (SPF) was joined to a standard Ti–6Al–4V sheet using a Nd:YAG laser and alloy compatible filler wire. The microstructural and mechanical properties of dissimilar laser beam welded butt joints were investigated to determine their behavior under static and cyclic loads and for SPF. The filler wire affected the heat input and reduced the increase in the hardness within the fusion zone compared to that in the heat-affected zone. The laser beam welding process activated local microstructure transformations that were associated with local changes in the microtexture, the β content, and the grain size. The mechanical behavior of a dissimilar laser beam welded butt joint under a static tensile load was controlled by the properties of the standard Ti–6Al–4V sheet. Laser beam welded specimens showed inferior fatigue behavior. Removing the geometrical notches did not significantly improve the fatigue behavior because local microstructural and microtextural changes still created metallurgical notches. SPF was observed in the fine-grained Ti–6Al–4V sheet without crack formation in the heat-affected zones or the fusion zone. The welding seam of the dissimilar fine-grained-standard butt joint was resistant to SPF.

This article reports on the development of the rotational friction welding process for the welding of γ-TiAl- casting alloy Ti-47Al-3.5(Mn+Cr+Nb)-0.8(B+Si) to Ti6Al4V base metal and on the characterisation of the mechanical and microstructural properties of dissimilar metal friction welding. Pre- and post welding heat treatment is necessary in order to reduce the occurrence of bonding defects. The mechanical behaviour of friction welded joints under tensile loading at room temperature is primarily determined by the properties of the inter-metallic γTiAl- casting alloy base metal and not by the bonding zone itself. At the relevant service temperature of 700 °C, failure by fracture does not take place on the Ti6Al4V side of the dissimilar metal friction welded joint due to the super-plastic deformation of this alloy.

To meet the future demands of the aerospace industry with respect to safety, productivity, weight, and cost, new materials and joining concepts have being developed. Recent developments in the metallurgical field now make it possible to use laser-weldable Al-alloys of the 2xxx series such as AA2198 with a high structural efficiency index due to their high strength and low density. AA2198 holds the promise of providing a breakthrough response to the challenges of lightweight design in aircraft applications. Laser beam welding as an efficient joining technology for fuselage structures is already established in the aircraft industry for lower fuselage panels because the welded panels provide a higher buckling strength and lower weight compared with the classical riveted designs. The key factor for the application of laser-welded AA2198 structures is the availability of reliable data for the assessment of their damage tolerance behavior. In the research presented, the mechanical properties with regard to fatigue and fatigue crack propagation of laser beamwelded AA2198 joints and four-stringer panels were investigated. It was found that the fatigue endurance limit of laser beamwelded AA2198T3 is approximately 25 % below the endurance limit of the base material. With regard to the fatigue crack propagation behavior, the laser beam welded four-stringer panels with T-joints show a fatigue life increased by a factor of 1.7 compared with the base material. This work shows that high-quality laser beam welds of AA2198 can be produced on a large scale using the laser beam welding facilities of the Helmholtz-Zentrum Geesthacht.

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.

Laser beam welding was used to produce butt joints from the CoCrFeNiMn-type high entropy alloy. The alloy in the initial condition had an fcc single-phase coarse-grained structure. Laser welding resulted in the M7C3-type carbides precipitation in the fcc matrix. The carbide particles precipitation resulted in a considerable increase in microhardness from 150 HV 0.5 for the as-sintered condition to 205 HV 0.5 in the fusion zone. Laser beam welding had a negligible effect on both static mechanical properties and fatigue behaviour of the alloy. The endurance limit of either type of specimens (i.e. with and without welding seam) was 200 MPa. Fracture of all specimens with the laser beam welded seams occurred in the base material area during both tensile and fatigue testing. Weak effect of welding on static/fatigue behaviour of the alloy can be attributed to the higher hardness of the fusion zone, resulting in strain localization in the base material area. An increase in load resulted in activation of secondary slip systems and formation of deformation twins in fatigue specimens.

Investigations were conducted on the γ-TiAl-based alloy Ti–42Al–2.5Cr–1Nb–0.7Si–0.5B (at. %) to assess the influence of in situ and conventional post-weld heat treatment on the microstructure and microtexture transformations induced by laser beam welding. It was found that in situ post-weld heat treatment at 800 °C was important to inhibit weld seam cracking. EBSD and HEXRD results indicated that the welding zone mainly consisted of coarse α 2 “textured colonies”. These textured α 2 are found to be in Burgers orientation relationship with their parent β grains. After 1 h of conventional post-weld heat treatment at 1200 °C followed by furnace cooling, the textured colonies are refined. The α 2 grains nucleate heterogeneously on borides so that the sharp texture of the weld zone was broken down.

Post-weld heat treatment is carried out on the laser beam welded γ-TiAl-based alloy Ti-48Al-1Cr-1.5Nb-1Mn-0.2Si-0.5B (at. pct). The macro/microstructure and mechanical properties of both as-welded and heat-treated specimens are investigated by radiography, SEM, and tensile tests. Moreover, high energy synchrotron X-ray diffraction is performed to measure the residual stresses and evaluate the microstructure evolution. It is found that the residual stresses are distributed in a three-peak shape in the region of the weld zone and heat-affected zone of the as-welded specimen due to the microstructural transformation and heat softening. The residual stresses are largely relieved after the heat treatment. The heat-treated specimens have a near fully lamellar microstructure and show balanced mechanical properties of strength and ductility. The diffraction shows that the phase transformation from α2 to γ takes place under tensile load at 1023 K (750 °C), and the grain size and lamellar spacing are refined in the weld zone. Finally, the fracture mechanisms are found to be controlled by the local stress concentration-induced strain misfit between α2 and γ phases in the near γ grains and delamination and debonding in the lamellae. Boride ribbons of 5 μm in the near fully lamellar microstructure are found not to be detrimental to the tensile properties.

The effect of postweld heat treatment (PWHT) on 2.6‐mm‐thick Ti‐6Al‐4V butt joints that were welded using a continuous‐wave 8‐kW ytterbium fibre laser was studied in terms of the microstructure, microtexture, number of welding defects, microhardness, residual stress distribution and high cycle fatigue (HCF) properties. Five types of heat treatments in the temperature range of 540–920°C are investigated. The main reasons leading to fatigue life deterioration after the laser welding process are discussed, and possible guidelines for further improvement of the HCF behaviour by a subsequent suitable type of PWHT are provided. Low‐temperature annealing (T 750°C) lead to the transformation of a strong martensitic structure in the fusion zone (FZ) into more ductile coarse lamellar, which is more beneficial for fatigue performance. A suitable type of PWHT can increase the fatigue limit of a laser‐welded Ti‐6Al‐4V butt joint by 10%; however, a slight decrease in static strength should be considered. The effect of stress relief at elevated temperatures is studied.

Rotary friction welding (RFW) was used in the current study to join the dissimilar metals AA7020-T6 aluminium alloy and 316L steel. Neutron diffraction was performed to investigate the texture gradient around the weld line and to map the residual stress over the whole specimen. The texture analysis showed a weak shear component near the bond line of AA7020-T6-T6 side which indicated a plastic deformation of AA7020-T6 during welding. The shear bands were also observed in optical microstructures. Relatively high tensile residual stresses were observed near the bond line on the AA7020-T6 side, which were in-homogeneously distributed from the perimeter to the rod centre, while high compressive residual stresses were found in the sample centre at the bond line in the 316L steel.

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.

International journal of fatigue 98, 223 - 233 (2017). doi:10.1016/j.ijfatigue.2017.01.042, Laser shock peening (LSP) was performed on aluminium alloy AA2024 with a thickness of 2.0 mm. Microstructural studies using the electron back scatter diffraction (EBSD) technique were performed toquantify the micro-texture changes in the material through LSP. A residual stress analysis was performedusing synchrotron radiation and a hole drilling technique. Fatigue crack propagation (FCP) tests were performedto investigate the retardation effect of LSP residual stresses. Load versus crack opening displacementcurves were analysed to obtain the corrected values of load opening levels considering the effects ofthe residual stresses. Specimens with the LSP treatment reveal a significant retardation of the FCP. Thepresence of compressive residual stresses caused the crack closure effect, which increased the level ofcrack opening load and therefore reduced the effective load range. An original methodology to considerLSP-induced residual stresses on the FCP behaviour was proposed., Published by Elsevier, Oxford

To improve innovative joining and deposition technologies for the construction of dissimilar joints, precise knowledge of the local mechanical properties of materials must be obtained. In the present article a comparative study of the tensile properties and fatigue behaviour in case of flat standard and micro-specimens of base materials Inconel 625, Inconel 718 and Ti-6Al-4 V was accomplished. The aim of the study was to develop an efficient method for the investigation of the local mechanical properties by the use of micro-specimens subjected to electro discharge machining treatment and to obtain reliable tensile and fatigue test results. By the miniaturization of specimens a significant effect of roughness on mechanical properties was obtained. By considering a correction for the effective load-bearing cross-section the data obtained from micro-specimens are within 3% error with respect to standard specimen results. The results showed that the proposed technique can be successfully used to determine the tensile and fatigue properties of a small material volume.

This paper describes a way to improve the microstructure and mechanical properties of welding seams by in situ and conventional post-weld heat treatments for laser beam welding of the Ti–45Al–5Nb–0.2C–0.2B alloy. The seams are crack-free with reduced longitudinal residual stress and higher elongation to fraction after post-weld heat treatment. The welding zone consists of α2 after welding, transforms to a massive γ during in situ post-weld heat treatment, and finally forms a convoluted microstructure after conventional heating. The phase composition across the welding zone is discussed.