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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.

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.

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.

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.

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.

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.

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).

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.

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 % .

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.

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.

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.

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.

The present paper focuses on the metallurgical and microstructural characterization of the laser beam-welded T-joints between commercially pure titanium (CP-Ti) and Ti-6Al-4V alloy. The weld regions were comprehensively studied and the mechanisms leading to the final morphology within each weld region were described. The link between microstructural features and local mechanical properties was demonstrated. Owing to different constitution, the responses of the two titanium alloys to thermal cycles imposed by laser welding are completely different. A strong interface with no dilution zone between the two alloys was observed. The cooling rate during the welding process is high enough for diffusionless martensitic transformation in the Ti-6Al-4V part of the fusion zone. In contrast, no evidence of martensite was found in the CP-Ti because of low solute content and, consequently, much higher critical cooling rate. Plausible reason for some controversy found in the literature on the resulting transformation products after laser processing of CP-Ti was given. The present findings might have important industrial implications because careful microstructural characterization revealed the real position of the skin fusion line, which is of great importance for fulfillment of the weld quality criteria.

This paper describes microstructure and microtexture development in dissimilar friction welded -TAB-Ti64 joints. The effect of friction welding parameters on microstructure and local properties are examined and discussed. It was found that the intermetallic -TiAl based alloy Ti-47Al-3.5(Mn+Cr+Nb)-0.8(B+Si) (denoted as -TAB) is more sensitive to the applied friction welding parameters used in this study. Furthermore, the bonding between these two alloys was controlled by a diffusion process during a very short process duration. Grain refinement as well microstructure transformation led to local improvement of the friction-welded joints.

This study is carried out to investigate the laser spot welds (LSWs) of advanced high strength steel sheets of 1·0 mm thickness for application to the automotive industry. Mechanical properties and failure behaviour of LSWs for transformation induced plasticity steel sheets (TRIP800) subjected to monotonic coach–peel loading are investigated by experimental and finite element (FE) simulation methods. Microstucture of LSW and fracture surfaces of the welded specimens were examined by scanning electron microscopy (SEM) to describe the microstructural features and to clarify the crack initiation mechanism respectively. Based on simulation solutions equivalent plastic strain was obtained to describe local deformation of LSW joints. Experimental results revealed a 'plug type' of failure mode of the circular LSW joints under 'peel–coach' loading condition. This type of ductile failure is the most common failure mode for spot welds used in the automotive industry. Numerical simulation of damage process w...

Dissimilar welded components joined with any kind of welding technologies gain an increasing interest due to significant improvements in engineering structures by using new materials (alloys) or new materials combinations. The present paper deals with laser welding of Al5083-H111 and Al6013-T6 and the characterization of the crystallographic texture. Fine grained Al5083 was joined with coarser grained Al6013.Whereas, Al6013 is strongly oriented dominated by a strong cube component and a much lower Goss component, the finer grained Al5083 shows a week deformation texture. The welding seam itself has a moderate cube texture with a minor fiber texture in welding direction. A small texture variation about the texture strength and the volume fraction of the fiber component was observed along the weld. Results were obtained by neutron, hard X-ray and electron diffraction.

Defect-free joints of TiAl/Ti6242 have been produced by TLP bonding using Ti, Cu foils as insert metals. The influence of the bonding parameters such as bonding pressure, bonding temperature, dwell time and the stacking sequence of the parent materials on the microstructure of the joined zone has been investigated. The results showed that bonding pressure played an important role even though there was a liquid phase during bonding. The stacking sequence of the parent materials also had a major effect on joint formation. Elimination of defects at the interface of the TiAl/ interlayer in order to obtain sound joints was a major problem. Similar interface structures were observed in all cases except that the thickness of the joined zone was different. The results of the study revealed that the surface oxide layer on TiAl had a major effect on the interface diffusion process. Based on the interface diffusion and interface structure, the joint formation process has been investigated and a mechanism for...

The goal of the present study is to understand the effects of laser shock peening (LSP)-induced residual stresses on the fatigue crack propagation (FCP) behaviour of the commonly used aircraft aluminium alloy AA2024 in T3 heat treatment condition. LSP treatment was performed using a pulsed Nd:YAG laser on compact tensile C(T)50-specimens with a thickness of 2.0 mm. LSP-treated specimens reveal a significant retardation of the fatigue crack propagation. The fatigue crack retardation effect can be correlated with the compressive residual stresses introduced by LSP throughout the entire specimen thickness. A possible application of the LSP process on a component like panel with three welded stringers representing a part of a fuselage structure was performed as well. The skin-stringer AA2024-AA7050 Tjoints were realised through stationary shoulder friction stir welding (SSFSW), a variant of the conventional friction stir welding process. In this relatively new process, the shoulder does not rotate and therefore does not contribute to the heat generation. Consequently, a reduced and more homogeneous heat input leads to a less affected microstructure and better mechanical properties. The efficiency of the LSP process has been demonstrated resulting in an increase of 200 – 400% in fatigue lifetime.

CFRP and titanium joints are used in the aerospace industry. These materials are usually joined by titanium rivets which are inserted into holes drilled through both materials. Conventional riveted hybrid joints of CFRP and titanium parts fail under quasi static loading due to the uneven load distribution at the titanium rivets. Under cyclic loading, the fatigue failure occurs mainly in the titanium part because of the higher notch sensitivity. The aim of this work is the comparison of different joining concepts in terms of stiffness, strength and fatigue limit. First, laser riveting, here titanium pins are Nd:YAG laser beam welded to the Ti–6Al–4V parts. Second, conventional riveted hybrid joint is combined with adhesive bonding. Third, surface structuring of the Ti–6Al–4V parts is used to enhance friction in the riveted joint. Tensile and fatigue tests as well as fractographical examinations are performed to establish the process–property–performance relationship of the hybrid joints. Laser riveting leads to higher stiffness but equal strength, when compared to conventional riveted joints. Fatigue life is improved by the implementation of adhesive bonding and surface structuring.

Electron beam, CO 2 , and Nd:YAG laser beam welding were performed on four different aluminium alloys, namely alloys 5005, 2024, 6061, and 7020. The plate thickness was 5 mm except alloy 5005 which was 3 mm thick in all cases and Nd:YAG laser beam welding was performed on 3 mm thick plates for all four alloys. Microstructural characterization of the weld metals was made by optical and scanning electron microscopies. A very low level of porosity was observed in all EB welds due to surface cleaning prior to welding and the vacuum environment of the EB welding process. In the case of CO 2 laser beam welding, high porosity level was observed. The pores observed in laser beam welded joints were varied small to large in size. Furthermore, the loss of Mg apparently took place in electron beam welding of all the alloys, which leads to the loss of strength in fusion zone (strength undermatching). The use of filler wire in laser beam welding processes improved the base metal property (strength) degradation to some extent in some cases. However, the strength undermatching was still to be observed in almost all of the cases.

Laser beam welding is currently used in the welding of steels, aluminium alloys, thin sheets, and dissimilar materials. This high power density welding process has unique advantages of cost effectiveness, deep penetration, narrow bead and heat affected zone (HAZ) widths, and low distortion compared to other conventional welding processes. However the metallurgical and mechanical properties of laser welds and the response of conventional materials to this new process are not yet fully established. The welding process may lead to drastic changes in the microstructure with accompanying effects on the mechanical properties and hence, on the performance of the joint. The thermal cycles associated with laser beam welding are generally much faster than those involved in the conventional are welding processes. This leads to the formation of a rather small weld zone that exhibits locally a high hardness in the case of C-Mn structural steels owing to the formation of martensite. It is currently difficult to determine the tensile properties (full stress-strain curves) of the laser welded joint area owing to the small size (similar to 2-3 mm) of the fusion zone. Complete information on the tensile and fracture toughness properties of the fusion zone is essential for prequalification and complete understanding of the joint performance in service as well as for conducting a defect assessment procedure on such welded joints. Therefore, an experimental investigation into the mechanical properties of laser welded joints was carried out to establish a testing procedure using flat microtensile specimens (0.5 mm in thickness, 2 mm in width) for determination of the tensile properties of the weld metal and HAZ of the laser beam welds. Three similar joints, namely St 37-St 37, St 52-St 52, and austenitic-austenitic, and two dissimilar ferritic-austenitic joints were produced by CO2 laser using 6 mm thickness plates. The mechanical properties have been examined by microhardness survey and testing of conventional transverse tensile, round tensile, and flat microtensile specimens. The results for the microtensile specimens were compared with those for standard round tensile specimens and this clearly showed the suitability of the microtensile specimen technique for such joints.

Weight reduction is the main driving force in automotive and aircraft structural design. As a result, magnesium alloys, with their high potential for lightweight construction, have attracted a considerable amount of industrial attention. The determining criterion for the structural applications of magnesium alloys is the availability of efficient joining technologies for the construction of lightweight structures and the availability of reliable data for the assessment of their damage tolerance behaviour. Laser beam welding (LBW), as a high-speed and easily controllable process, allows the welding of complex geometric forms that are optimised in terms of mechanical stiffness, strength, production velocity and visual quality. The work accomplished in this study addresses the challenges of the LBW process for typical joint configurations using the magnesium alloy AZ31HP: butt joints, T joints and overlap joints. LBW processes were developed for use with a 3.3-kW Nd:YAG laser to optimise the mechanical performance of such joints with respect to tensile strength, fatigue, fatigue crack propagation and mechanical fracture behaviour. The relationships between the LBW process and the microstructural and mechanical properties of welds were established. Compared to state-of-the-art aerospace alloys, AZ31HP demonstrates that magnesium alloys have potential for use in structural applications, with AZ31HP being comparable to AA2024T351 and AA6061T6. Welded AZ31HP exhibits better crack resistance than the base material, so fully welded integral structures made from magnesium alloys can be used in lightweight construction.

Laser beam welding of butt joints made of the newly developed high-strength Al-Zn alloy PA734 is conducted. A new approach of the Helmholtz-Zentrum Geesthacht is used to solve the problems of weldability and softening. The results of the fatigue, fatigue crack propagation and fracture toughness tests are discussed relating to the microstructural characteristics and the mechanical properties of the laser welded joints and compared to the base material. The obtained data can be used for the assessment of the damage tolerance behaviour of the laser welded integral aircraft structures made of Al-Zn alloys.

Dissimilar welds of aluminium alloy AA6056 and titanium alloy Ti6Al4V were produced by a novel technique. AA6056 sheet was machined at one end to a U-slot shape, enabling the intake of the Ti6Al4V sheet. The Al-alloy U-slot was then butt welded by split laser beam without using a filling wire, thus making a weld by melting only the Al-alloy. Thereby the intermetallic brittle phase TiAl3 formed at the weld interface and affected mechanical properties. As a continuation of the previous work, the joint design was modified by chamfering Ti6Al4V to reduce the formation of interfacial TiAl3. It is shown in this work how this seemingly insignificant joint modification has refined microstructure and increased hardness and strength. The most impressive feature was the improved resistance to fatigue crack propagation whereby the fracture type in the fusion zone of AA6056 adjacent to the weld interface changed from partially intercrystalline to completely transcrystalline. Possible metallurgical processes leading to the property improvements are discussed.

The increasing interest of the aircraft industry in reduction of structural weight of aircrafts has resulted in the development of lightweight and high-strength Al-Li alloys as well as in the introduction of laser beam welding to the manufacturing process. The objective of this study is the investigation of the influence of variations in the chemical composition on local mechanical properties, like micro-hardness and micro-tensile strength, of CO 2 laser beam welded skin-stringer joints made from AA2196 and AA2198. Additionally the influence of the welding process on weld chemistry is studied in view of the improvement of the weld quality.

A new surfacing method based on the laser melting was investigated in this study in order to improve the fatigue crack propagation performance of high strength al-alloy and its laser beam weld. The main objective was to reduce the Fatigue Crack Propagation (FCP) rate and increase the fatigue life of Al-alloy welded structure. The fatigue crack propagation rate of Al-alloy 6056 was assessed using CT-100 specimen including base material, base material with Local Laser Surfacing (LLS), laser beam weld and laser beam weld with LLS respectively. The results show a significant reduction in fatigue crack propagation rate with LLS treatment because the fatigue crack retardation phenomenon is obvious when the fatigue crack close to or meet the laser surfacing lane. The fatigue life of BM with LLS specimen is higher than that of BM specimen. For the LBW, there shows same results. That is FCP rate of the specimen of laser beam weld with LLS treatment shows significant reduction compared with the specimen of laser beam weld without LLS treatment. The facture surface and microstructure of LLS and LBM were observed also in order to explain the basic mechanism of LLS improving the fatigue life.

Laser beam welding of similar and dissimilar joints made of 7075-T6 and 5182-O sheets is conducted. A new approach of the Helmholtz-Zentrum Geesthacht is used to solve the problems of weldability and softening. The microstructural characteristics and the mechanical properties of the joints are evaluated. With this information it is possible to initially assess the formability of the tailored blanks. Finally the formability of the welds will be validated via a sheet metal forming process. Owing to the good formability of the base materials, the results obtained for the welded joints are compared with base material properties.