Final Report Summary - LAWENDEL (Laser welding of newly developed Al-Li alloy)
Executive Summary:
The LAWENDEL project aimed to develop laser welding process for aerospace applications by combining expertise in model based process design at the University of Manchester with a systematic experimental approach by the project coordinator, Helmholtz-Zentrum Geesthacht. The objective was to study the laser weldability of a newly developed Al-Li alloys, to determine the process parameters needed to obtain consistent laser welds, and to compare the mechanical behaviour with the conventional aluminium alloys series. The study emphasized the microstructure characteristics and the mechanical properties of the weld joint to gain an understanding of the underlying factors controlling the performance of the welds. During the demonstration phase of the project, the developed LBW technology was applied for welding three stiffened flat panels out of the new Al-Li alloys, named demonstrator B1, in order to evaluate the industrial application. The skin and the stiffener were provided by the Topic Manager. The demonstrator B1 is a rectangular aluminium panel 384 mm x 742 mm (AA2198) with 4 stringers out of AA2196 spaced in equal distances. Three demonstrator panels were welded by using the optimized parameters defined from the previous phase and then inspected by NDT in order to assure the structural integrity. The innovative combination of state-of-the-art modelling and experiments enabled physics based optimization of the welding process with greatly reduced time and cost compared to traditional trial and error methods.
Project Context and Objectives:
The LAWENDEL project aimed to develop laser welding process in case of Al-Li alloys for aerospace applications by combining expertise in model based process design at the University of Manchester with a systematic experimental approach by the project coordinator, Helmholtz-Zentrum Geesthacht.
For overcoming the difficulties by laser welding of Al-Li alloys caused by their susceptibility to weld cracking the influence of chemical composition of the filler material, especially the lithium and magnesium content on the cracking susceptibility was studied. Furthermore the influence of the laser welding process (welding speed, filler wire type and feed rate etc.) on the formation of welding defects was investigated. Refinement of the microstructure is critical for reducing the crack susceptibility. LAWENDEL addressed the problems of weld porosity formation and its prevention. Improvement in the inherent weld cracking susceptibility of the Li-bearing base metals required further alloy development and consideration of the microstructural factors that control cracking susceptibility.
The laser welding process development emphasized the microstructure characteristics and the mechanical properties of the weld joint to gain an understanding of the underlying factors controlling the performance of the welds. During the demonstration phase of the LAWENDEL project, the developed LBW technology was applied for welding a stiffened flat panel out of the new Al-Li alloy, named demonstrator B1, in order to evaluate the industrial application. The skin and the stiffener were provided by Topic Manager. The demonstrator B1 is a rectangular aluminium panel 384 mm x 742 mm (AA2198) with 4 stringers out of AA2196 spaced in equal distances. Three demonstrator panels were welded by using the optimized parameters defined from the previous phase and then inspected by NDT in order to assure the structural integrity. The innovative combination of state of the art modelling and experiments enabled physics based optimization of the welding process with greatly reduced time and cost compared to traditional trial and error methods.
The LAWENDEL project had the following main five objectives:
• To develop and optimize the laser welding process for the Al-Li alloy
The development of laser welding process was accomplished by a combination of modelling and experiments. The project partner UMAN developed a model for the prediction of the weldability and the mechanical properties after welding on basis of the process parameters and initial material properties. This model was calibrated on basis of welding experiments performed at HZG for selected cases. After the experimental calibration the developed model was used for the numerically based optimization of the welding process with the goal to identify an optimum process window for reaching the best balance between good weldability and high mechanical properties after welding.
• To transfer of optimized process for welding of coupons
Laser welding of coupons was carried out by HZG on the large-scale laser beam welding facility equipped with two 3.5 kW CO2-lasers and a movable processing head, which was installed in cooperation with AIRBUS Germany. T-joints of the skin and the stringer components were welded simultaneously from both sides of the stringer. The surface of both components was prepared before welding. The filler wire and the shielding gas were supplied simultaneously from each side of the stringer. The experimental DoE study covering the identified optimum process parameter field for coupon T-joint specimens was conducted. These experiments were the basis for the validation of the calibrated models used to identify this optimum process parameter window. The microstructural model parts were validated by characterizing the manufactured welds microstructurally and micromechanically.
• To validate optimized laser welding process through microstructural and michromechanical characterization
The microstructure of the welds was examined via optical microscopy (OM) and scanning electron microscope (SEM). The local chemical composition (except Li) was analyzed by the energy dispersive X-ray (EDX) analysis and the electron back scattering diffraction (EBSD) was used to characterize the crystal orientations (texture) and the grain size. The local Li-content was measured by inductively coupled plasma atomic emission spectroscopy (ICP-AES) with previous acid hydrolysis. The measured Si- and Li-contents was compared to calculated theoretical values that will be obtained from the cross-sectional area of the weld and the Si- and Li-content of the skin, stringer and filler wire alloys. To determine the local mechanical properties in the weld seam Vickers microhardness measurements were performed.
In order to achieve the best performance of welded joints with regard to their strength and ductility the instrumented indentation testing procedure established at HZG for the determination of local stress-strain properties was used. This procedure is based on where the force–depth curves are analysed via artificial neural networks in order to derive local stress–strain curves. The determined stress-strain curves were validated by the use of micro-tensile tests with 0.5 mm thick specimens extracted from weld seam. The described test procedures allowed distinguishing between the mechanical properties of different phases in weld seam.
• To perform required mechanical tests for evaluation of welded coupons from Al-Li alloy
The mechanical properties of the welded Al-Li specimens were compared with the mechanical properties of riveted specimens from 2024 provided by topic manager. For this purpose mechanical tests were performed on welded and riveted specimens. The mechanical performance of the welded T-joints was investigated by hoop-stress tests, pull-out tests and fatigue tests. For fatigue tests the welded and riveted specimens in the form of M(T)250 specimens were used. The geometry of tests specimens and loading conditions were specified in agreement with the topic manager.
• To optimize the laser beam welding process for a 3-stringer panel and to fabricate the demonstrator panels
The optimized laser beam welding process for the coupons was adopted for welding of the demonstrator 4-stringer panel. The welding was also carried out on the large-scale laser beam welding facility with two 3.5 kW CO2-lasers. The final quality of the welded demonstrator was checked by visual inspection and X-ray inspection.
Project Results:
The LBW process development was accomplished by a combination of modelling and experiments. For this purpose the modelling approach for the prediction of the weldability and the mechanical properties after welding on basis of the process parameters and initial material properties was developed. This model was calibrated on basis of welding experiments for selected cases. After the experimental calibration the developed models were used for the numerically based optimization of the welding process with the goal to identify an optimum process window for reaching the best balance between good weldability and high mechanical properties after welding.
The Al-Cu-Li alloys suffer from severe hot cracking problems. It is of great interest to understand to what extent the hot cracking will occur during welding and to predict the hot cracking susceptibility (HCS) from a material’s point of view. A hot tearing criterion has been developed by Rappaz, Drezet and Gremaud on the basis of a mass balance over the liquid and solid phases during the solidification process, i.e. the so called RDG model. With this model available, it is possible to predict the hot cracking susceptibility by knowing the detailed composition of the alloy.
The fusion zone of the welds feature inhomogeneity of composition. It is possible to predict the HCS by using the RDG model and draw a relative hot cracking susceptibility map over the entire fusion zone. When the laser power is 1400 W, the homogeneity of the fusion zone is in the worst situation so that the bottom of the weld has relatively high hot cracking susceptibility, i.e. larger ‘A’ value. When the laser power increases to 1720 W, the mixing homogeneity of the three materials is improved and the centre of the fusion zone has relatively uniform hot cracking susceptibility.
Microstructural investigations showed that the grains in fusion zone are refined due to the relatively concentrated energy input and rapid cooling effect. In general, post-welding aging strengthening in the fusion zone is very limited because most of the solute needed to form the precipitates is locked in the eutectic constituent during the solidification process. Outside the fusion zone is the heat affected zone (HAZ) where the heat flows into the BM. Some of the precipitates in the HAZ zone are dissolved or coarsened by the heat flow resulting in strength loss which can be seen in the microhardness profile. Unlike conventional arc welding, laser welding features a relatively small HAZ zone.
Adjacent to the fusion boundary within the fusion zone, a small area of fine equiaxed grains is observed in all welding conditions. The so-called equiaxed grain zone (EQZ) only occurs in Li-bearing aluminium alloys. The width of the EQZ next to the AA2196 base material increases from the outside towards the centre of the weld. Next to the EQZ, the grains grow towards the centre of the weld following the gradient of the heat flow. In the area close to the fusion boundary, micro-cracks are found in transverse to the grain growth direction. The EQZ is associated with severe cracking problem during welding. It has been reported that cracks are often found in EQZ in Varestraint test. In this study, severe cracks are found in the EQZ along the fusion boundary in the weld made with 2000 W laser. The formation of the EQZ can be explained by the recrystallization in the HAZ adjacent to the fusion zone or heterogeneous nucleation on the precipitates or dispersoids survived in the thermal cycle. Some researches show that EQZ can be eliminated by using one of the following approaches: (1) stirring welding pool, (2) change material composition and (3) change the nature of the parent metal.
The formation of pores and cracks is a common challenge during LBW of Al-Li-alloys. Based on the modelling and experimental results a good compromise was fond in case of laser power of 1720 W, so it was decided to use this parameter set for further LBW experiments.
In order to find out to what extent the weld joint is weakened, Vickers microhardness indentations were made across the weld’s cross-section through two directions: through the height of the weld from stringer to skin (Y direction) and through the width of the weld across the skin only (X direction). It was obtained that the fusion boundary has the lowest hardness which corresponds to the EQZ mentioned above. The microhardness in the grain boundary drops to about 60% of BM and recovers to about 75% of BM in the weld centre. The partial recovery of hardness in the centre of weld reflects to the casting structure which losses the precipitation hardening effects. The size of HAZs in AA2198 was calculated on the basis of the hardness change. When the laser power is 1400 W and 1720 W, the HAZ zone in the skin is about 0.8 mm wide in average. When the laser power increases to 2000 W, the HAZ zone slightly increases to about 1.0 mm outside fusion boundary. In order to compensate the weakening of the skin due to the LBW, the welding of the stringer is considered on a socket with the total thickness = thickness of the skin + the width of the HAZ.
Mechanical Properties of the Joints
Due to softening under the weld, in case of the tensile specimen with T-joint the strain is localized in the softer regions leading to a limited strain to fracture. In different T-joints welded at a laser power of 1720 W different values for the yield strength (YS) and ultimate tensile strength (UTS) were determined. A joint efficiency of at least 76 % was received, with of over 80 %. The maximal loss of strength determined in the tested laser beam welded specimens was -24%, which will lead in direct comparison to a necessary socket under the weld of 0.8 mm. It must be mentioned, that this value is only valid for 3.2 mm thickness of the sheet. If thinner sheets are welded, the heat distribution has to be taken into account, which may influence other thicknesses in a different way. The higher strength of the stringer plays no role in this loading case it is connected via a low strength zone. Just the geometry with the stringer perpendicular to the loading direction in the middle of the specimen may lead to strain concentration under the stringer.
Comparing riveted and laser beam welded specimens, the Al-Li alloys show a slightly higher elastic modulus – theoretically 1 % of Li shall increase it by 6 %. The strain to fracture of the laser beam welded joints is much larger than that of the riveted joint, but without the existence of Rp0.2 the joint is stiffer and more sensitive to strain. Theoretically, with static loading transverse to stringer, AA2198T8 skin must be 1.615 mm thick to bear the same load as 2.0 mm AA2024T3 skin. For 1 m² the weight saving would be 25 %, without taking the stringers into consideration and calculating with the following densities: AA2198 = 2.619 g/cm³ and AA2024 = 2.79 g/cm³.
For laser beam welded structures it is recommended, that the weld zone shall be reinforced by a socket. This socket will protect the weld area and the fracture should occur in the BM. The socket height was determined by comparing the fracture load of the hoop-stress test with the base materials tensile strength. This led to a difference in thicknesses of 0.7 mm. For testing this second set of hoop-stress specimens more detailed deformation analysis was done. From this set-up it is possible to determine the global strain between the outer stripes and local strains between the different stripes. The data evaluation was limited to the inner (the socket area) and the global strain. The stresses were calculated according to the thickness of the section, the shielding effect of the socket is clearly visible. All of the four tested specimens broke in the thinned region of the BM. It must be mentioned, that according to skin thickness and welding parameters, other sockets will be necessary.
The fatigue strength of LBW AA2198/AA2196 was determined to be 80 MPa, 23 % higher than the strength of riveted AA2024/AA7075, which was 65 MPa. The laser beam welded joints suffer here from relatively sharp inconsistencies in the weld zone where cracks can initiate more easily, as in the riveted joints with relatively large smooth holes. Theoretically, with fatigue loading (R=0.1) transverse to stringer, AA2198T8 skin must be 1.625 mm thick to bear the same load as 2.0 mm AA2024T3 skin. For 1 m² the weight saving would be 25 %, without taking the stringers into consideration and calculating with the following densities: AA2198 = 2.619 g/cm³ and AA2024 = 2.79 g/cm³
Compression Test
The compression test of laser beam welded panel with four stringers was performed to complete the mechanical characterization. No plastic deformation occurred till fracture. The skin flipped to buckling and the stringers were separated by the fracture on their whole length. The demonstrator reached a load of 365 kN than skin buckled and stringers broke away on full length.
Potential Impact:
The LAWENDEL project is oriented to gain a large benefit for the industrial application in the aerospace sector. Beginning 2011 it has been announced by the European aerospace industry that until 2030 new aircraft will be designed with a fully metallic fuselage. The continuous need to reduce fuel consumption will be solved by new engines with higher efficiency and by improved lightweight fuselage design including modern alloys and welding technologies. However, bound to the ongoing development with immanent problems in the production of carbon fuselages, the aerospace industry has very limited capacities for R&D needed for the development of next generation metallic fuselages. Through the development in this project, laser beam welding was developed for very promising lightweight Al-Cu-Li alloys, so that the range of possible industrial application for laser beam welding and its acceptance in aerospace industries will be increased. The successful project will create the basis for following technology transfer and implementation projects with industry. The demonstration of feasibility for the aluminium alloys in this project will initiate increasing interest in an extension to other alloys. The possible applications will be also interesting for automotive industry, where the laser welding faces similar challenges as in the aerospace industry.
List of Websites:
not applicable
The LAWENDEL project aimed to develop laser welding process for aerospace applications by combining expertise in model based process design at the University of Manchester with a systematic experimental approach by the project coordinator, Helmholtz-Zentrum Geesthacht. The objective was to study the laser weldability of a newly developed Al-Li alloys, to determine the process parameters needed to obtain consistent laser welds, and to compare the mechanical behaviour with the conventional aluminium alloys series. The study emphasized the microstructure characteristics and the mechanical properties of the weld joint to gain an understanding of the underlying factors controlling the performance of the welds. During the demonstration phase of the project, the developed LBW technology was applied for welding three stiffened flat panels out of the new Al-Li alloys, named demonstrator B1, in order to evaluate the industrial application. The skin and the stiffener were provided by the Topic Manager. The demonstrator B1 is a rectangular aluminium panel 384 mm x 742 mm (AA2198) with 4 stringers out of AA2196 spaced in equal distances. Three demonstrator panels were welded by using the optimized parameters defined from the previous phase and then inspected by NDT in order to assure the structural integrity. The innovative combination of state-of-the-art modelling and experiments enabled physics based optimization of the welding process with greatly reduced time and cost compared to traditional trial and error methods.
Project Context and Objectives:
The LAWENDEL project aimed to develop laser welding process in case of Al-Li alloys for aerospace applications by combining expertise in model based process design at the University of Manchester with a systematic experimental approach by the project coordinator, Helmholtz-Zentrum Geesthacht.
For overcoming the difficulties by laser welding of Al-Li alloys caused by their susceptibility to weld cracking the influence of chemical composition of the filler material, especially the lithium and magnesium content on the cracking susceptibility was studied. Furthermore the influence of the laser welding process (welding speed, filler wire type and feed rate etc.) on the formation of welding defects was investigated. Refinement of the microstructure is critical for reducing the crack susceptibility. LAWENDEL addressed the problems of weld porosity formation and its prevention. Improvement in the inherent weld cracking susceptibility of the Li-bearing base metals required further alloy development and consideration of the microstructural factors that control cracking susceptibility.
The laser welding process development emphasized the microstructure characteristics and the mechanical properties of the weld joint to gain an understanding of the underlying factors controlling the performance of the welds. During the demonstration phase of the LAWENDEL project, the developed LBW technology was applied for welding a stiffened flat panel out of the new Al-Li alloy, named demonstrator B1, in order to evaluate the industrial application. The skin and the stiffener were provided by Topic Manager. The demonstrator B1 is a rectangular aluminium panel 384 mm x 742 mm (AA2198) with 4 stringers out of AA2196 spaced in equal distances. Three demonstrator panels were welded by using the optimized parameters defined from the previous phase and then inspected by NDT in order to assure the structural integrity. The innovative combination of state of the art modelling and experiments enabled physics based optimization of the welding process with greatly reduced time and cost compared to traditional trial and error methods.
The LAWENDEL project had the following main five objectives:
• To develop and optimize the laser welding process for the Al-Li alloy
The development of laser welding process was accomplished by a combination of modelling and experiments. The project partner UMAN developed a model for the prediction of the weldability and the mechanical properties after welding on basis of the process parameters and initial material properties. This model was calibrated on basis of welding experiments performed at HZG for selected cases. After the experimental calibration the developed model was used for the numerically based optimization of the welding process with the goal to identify an optimum process window for reaching the best balance between good weldability and high mechanical properties after welding.
• To transfer of optimized process for welding of coupons
Laser welding of coupons was carried out by HZG on the large-scale laser beam welding facility equipped with two 3.5 kW CO2-lasers and a movable processing head, which was installed in cooperation with AIRBUS Germany. T-joints of the skin and the stringer components were welded simultaneously from both sides of the stringer. The surface of both components was prepared before welding. The filler wire and the shielding gas were supplied simultaneously from each side of the stringer. The experimental DoE study covering the identified optimum process parameter field for coupon T-joint specimens was conducted. These experiments were the basis for the validation of the calibrated models used to identify this optimum process parameter window. The microstructural model parts were validated by characterizing the manufactured welds microstructurally and micromechanically.
• To validate optimized laser welding process through microstructural and michromechanical characterization
The microstructure of the welds was examined via optical microscopy (OM) and scanning electron microscope (SEM). The local chemical composition (except Li) was analyzed by the energy dispersive X-ray (EDX) analysis and the electron back scattering diffraction (EBSD) was used to characterize the crystal orientations (texture) and the grain size. The local Li-content was measured by inductively coupled plasma atomic emission spectroscopy (ICP-AES) with previous acid hydrolysis. The measured Si- and Li-contents was compared to calculated theoretical values that will be obtained from the cross-sectional area of the weld and the Si- and Li-content of the skin, stringer and filler wire alloys. To determine the local mechanical properties in the weld seam Vickers microhardness measurements were performed.
In order to achieve the best performance of welded joints with regard to their strength and ductility the instrumented indentation testing procedure established at HZG for the determination of local stress-strain properties was used. This procedure is based on where the force–depth curves are analysed via artificial neural networks in order to derive local stress–strain curves. The determined stress-strain curves were validated by the use of micro-tensile tests with 0.5 mm thick specimens extracted from weld seam. The described test procedures allowed distinguishing between the mechanical properties of different phases in weld seam.
• To perform required mechanical tests for evaluation of welded coupons from Al-Li alloy
The mechanical properties of the welded Al-Li specimens were compared with the mechanical properties of riveted specimens from 2024 provided by topic manager. For this purpose mechanical tests were performed on welded and riveted specimens. The mechanical performance of the welded T-joints was investigated by hoop-stress tests, pull-out tests and fatigue tests. For fatigue tests the welded and riveted specimens in the form of M(T)250 specimens were used. The geometry of tests specimens and loading conditions were specified in agreement with the topic manager.
• To optimize the laser beam welding process for a 3-stringer panel and to fabricate the demonstrator panels
The optimized laser beam welding process for the coupons was adopted for welding of the demonstrator 4-stringer panel. The welding was also carried out on the large-scale laser beam welding facility with two 3.5 kW CO2-lasers. The final quality of the welded demonstrator was checked by visual inspection and X-ray inspection.
Project Results:
The LBW process development was accomplished by a combination of modelling and experiments. For this purpose the modelling approach for the prediction of the weldability and the mechanical properties after welding on basis of the process parameters and initial material properties was developed. This model was calibrated on basis of welding experiments for selected cases. After the experimental calibration the developed models were used for the numerically based optimization of the welding process with the goal to identify an optimum process window for reaching the best balance between good weldability and high mechanical properties after welding.
The Al-Cu-Li alloys suffer from severe hot cracking problems. It is of great interest to understand to what extent the hot cracking will occur during welding and to predict the hot cracking susceptibility (HCS) from a material’s point of view. A hot tearing criterion has been developed by Rappaz, Drezet and Gremaud on the basis of a mass balance over the liquid and solid phases during the solidification process, i.e. the so called RDG model. With this model available, it is possible to predict the hot cracking susceptibility by knowing the detailed composition of the alloy.
The fusion zone of the welds feature inhomogeneity of composition. It is possible to predict the HCS by using the RDG model and draw a relative hot cracking susceptibility map over the entire fusion zone. When the laser power is 1400 W, the homogeneity of the fusion zone is in the worst situation so that the bottom of the weld has relatively high hot cracking susceptibility, i.e. larger ‘A’ value. When the laser power increases to 1720 W, the mixing homogeneity of the three materials is improved and the centre of the fusion zone has relatively uniform hot cracking susceptibility.
Microstructural investigations showed that the grains in fusion zone are refined due to the relatively concentrated energy input and rapid cooling effect. In general, post-welding aging strengthening in the fusion zone is very limited because most of the solute needed to form the precipitates is locked in the eutectic constituent during the solidification process. Outside the fusion zone is the heat affected zone (HAZ) where the heat flows into the BM. Some of the precipitates in the HAZ zone are dissolved or coarsened by the heat flow resulting in strength loss which can be seen in the microhardness profile. Unlike conventional arc welding, laser welding features a relatively small HAZ zone.
Adjacent to the fusion boundary within the fusion zone, a small area of fine equiaxed grains is observed in all welding conditions. The so-called equiaxed grain zone (EQZ) only occurs in Li-bearing aluminium alloys. The width of the EQZ next to the AA2196 base material increases from the outside towards the centre of the weld. Next to the EQZ, the grains grow towards the centre of the weld following the gradient of the heat flow. In the area close to the fusion boundary, micro-cracks are found in transverse to the grain growth direction. The EQZ is associated with severe cracking problem during welding. It has been reported that cracks are often found in EQZ in Varestraint test. In this study, severe cracks are found in the EQZ along the fusion boundary in the weld made with 2000 W laser. The formation of the EQZ can be explained by the recrystallization in the HAZ adjacent to the fusion zone or heterogeneous nucleation on the precipitates or dispersoids survived in the thermal cycle. Some researches show that EQZ can be eliminated by using one of the following approaches: (1) stirring welding pool, (2) change material composition and (3) change the nature of the parent metal.
The formation of pores and cracks is a common challenge during LBW of Al-Li-alloys. Based on the modelling and experimental results a good compromise was fond in case of laser power of 1720 W, so it was decided to use this parameter set for further LBW experiments.
In order to find out to what extent the weld joint is weakened, Vickers microhardness indentations were made across the weld’s cross-section through two directions: through the height of the weld from stringer to skin (Y direction) and through the width of the weld across the skin only (X direction). It was obtained that the fusion boundary has the lowest hardness which corresponds to the EQZ mentioned above. The microhardness in the grain boundary drops to about 60% of BM and recovers to about 75% of BM in the weld centre. The partial recovery of hardness in the centre of weld reflects to the casting structure which losses the precipitation hardening effects. The size of HAZs in AA2198 was calculated on the basis of the hardness change. When the laser power is 1400 W and 1720 W, the HAZ zone in the skin is about 0.8 mm wide in average. When the laser power increases to 2000 W, the HAZ zone slightly increases to about 1.0 mm outside fusion boundary. In order to compensate the weakening of the skin due to the LBW, the welding of the stringer is considered on a socket with the total thickness = thickness of the skin + the width of the HAZ.
Mechanical Properties of the Joints
Due to softening under the weld, in case of the tensile specimen with T-joint the strain is localized in the softer regions leading to a limited strain to fracture. In different T-joints welded at a laser power of 1720 W different values for the yield strength (YS) and ultimate tensile strength (UTS) were determined. A joint efficiency of at least 76 % was received, with of over 80 %. The maximal loss of strength determined in the tested laser beam welded specimens was -24%, which will lead in direct comparison to a necessary socket under the weld of 0.8 mm. It must be mentioned, that this value is only valid for 3.2 mm thickness of the sheet. If thinner sheets are welded, the heat distribution has to be taken into account, which may influence other thicknesses in a different way. The higher strength of the stringer plays no role in this loading case it is connected via a low strength zone. Just the geometry with the stringer perpendicular to the loading direction in the middle of the specimen may lead to strain concentration under the stringer.
Comparing riveted and laser beam welded specimens, the Al-Li alloys show a slightly higher elastic modulus – theoretically 1 % of Li shall increase it by 6 %. The strain to fracture of the laser beam welded joints is much larger than that of the riveted joint, but without the existence of Rp0.2 the joint is stiffer and more sensitive to strain. Theoretically, with static loading transverse to stringer, AA2198T8 skin must be 1.615 mm thick to bear the same load as 2.0 mm AA2024T3 skin. For 1 m² the weight saving would be 25 %, without taking the stringers into consideration and calculating with the following densities: AA2198 = 2.619 g/cm³ and AA2024 = 2.79 g/cm³.
For laser beam welded structures it is recommended, that the weld zone shall be reinforced by a socket. This socket will protect the weld area and the fracture should occur in the BM. The socket height was determined by comparing the fracture load of the hoop-stress test with the base materials tensile strength. This led to a difference in thicknesses of 0.7 mm. For testing this second set of hoop-stress specimens more detailed deformation analysis was done. From this set-up it is possible to determine the global strain between the outer stripes and local strains between the different stripes. The data evaluation was limited to the inner (the socket area) and the global strain. The stresses were calculated according to the thickness of the section, the shielding effect of the socket is clearly visible. All of the four tested specimens broke in the thinned region of the BM. It must be mentioned, that according to skin thickness and welding parameters, other sockets will be necessary.
The fatigue strength of LBW AA2198/AA2196 was determined to be 80 MPa, 23 % higher than the strength of riveted AA2024/AA7075, which was 65 MPa. The laser beam welded joints suffer here from relatively sharp inconsistencies in the weld zone where cracks can initiate more easily, as in the riveted joints with relatively large smooth holes. Theoretically, with fatigue loading (R=0.1) transverse to stringer, AA2198T8 skin must be 1.625 mm thick to bear the same load as 2.0 mm AA2024T3 skin. For 1 m² the weight saving would be 25 %, without taking the stringers into consideration and calculating with the following densities: AA2198 = 2.619 g/cm³ and AA2024 = 2.79 g/cm³
Compression Test
The compression test of laser beam welded panel with four stringers was performed to complete the mechanical characterization. No plastic deformation occurred till fracture. The skin flipped to buckling and the stringers were separated by the fracture on their whole length. The demonstrator reached a load of 365 kN than skin buckled and stringers broke away on full length.
Potential Impact:
The LAWENDEL project is oriented to gain a large benefit for the industrial application in the aerospace sector. Beginning 2011 it has been announced by the European aerospace industry that until 2030 new aircraft will be designed with a fully metallic fuselage. The continuous need to reduce fuel consumption will be solved by new engines with higher efficiency and by improved lightweight fuselage design including modern alloys and welding technologies. However, bound to the ongoing development with immanent problems in the production of carbon fuselages, the aerospace industry has very limited capacities for R&D needed for the development of next generation metallic fuselages. Through the development in this project, laser beam welding was developed for very promising lightweight Al-Cu-Li alloys, so that the range of possible industrial application for laser beam welding and its acceptance in aerospace industries will be increased. The successful project will create the basis for following technology transfer and implementation projects with industry. The demonstration of feasibility for the aluminium alloys in this project will initiate increasing interest in an extension to other alloys. The possible applications will be also interesting for automotive industry, where the laser welding faces similar challenges as in the aerospace industry.
List of Websites:
not applicable
