Search

In this work, TiC nanoparticles (TiCps) reinforced 7075 aluminium alloy was fabricated via an oscillating laser-arc hybrid additive manufacturing process. To show the beneficial effect of TiCps addition on microstructure and mechanical properties, a conventional 7075 aluminium alloy was also manufactured for comparison. Upon the addition of TiCps, remarkable grain refinement was achieved with a decrease in average grain size from 111.8 to 12.5 µm, and grain boundary segregation was alleviated. It was also found that many secondary phases precipitated at the grain interior in the deposit with TiCps. Owing to the refined microstructure and favourable precipitation, as-deposited 7075 aluminium alloy with TiCps showed enhanced tensile properties of ultimate tensile strengths of 346 and 355 MPa along scanning and build directions.

The currently available high-power laser presents significant opportunities for welding of thick section components in single-pass. However, weld defects frequently occur and depth of penetration is usually lower than expected in this situation. In this work, a process of laser welding under reduced ambient pressure was investigated to achieve a single-pass complete-joint-penetration weld of 20 mm-thick high-strength steel. An experimental and theoretical program of investigation was undertaken to evaluate the characteristic of this welding process. The weld appearance, porosity inside the weld metal, microstructure and mechanical properties of weld joint were examined experimentally. The temperature field was calculated numerically using a 3D heat transfer and fluid flow model. A defect-free sound weld joint of 20 mm-thick plate was obtained just using an 8-kW laser power. The weld joint showed a very high aspect ratio characteristic of electron beam welding. From base metal (BM) to fusion zone (FZ) through heat-affected zone (HAZ), the microhardness varied significantly duo to their different microstructures resulted from the various peak temperatures and cooling rates for varying distances from weld center. All the tensile test samples fractured at the BM away from HAZ and FZ, which suggested that the welded metal was stronger than BM. The calculated weld geometry agreed with the corresponding experimental results through considering the role of ambient pressure. Their good agreement indicated the validity of the potential causative variables considered in the simulations. The modelling results also showed that the weld pool had a lower peak temperature and thinner liquid weld metal around keyhole under reduced ambient pressure.

Small chain resistance butt welding often has unstable welding quality due to high production frequency and short welding time. However, there is still a lack of research on online monitoring of resistance upset butt welding. This study proposes to collect the state information in the welding process and combine with the unsupervised learning method to predict the final welding quality. The collected state information includes the dynamic resistance curve, electrode displacement curve, and upset pressure curve. And state information of 308,274 welding joints was collected. Because of the high production frequency of chains, it is difficult to obtain the quality label by the destructive detection method. The commonly used supervised learning is no longer applicable. In view of this challenge, the unsupervised learning methods, Isolation Forest and Local Outlier Factor are proposed to predict the welding quality online for the first time. Another critical problem is that the unsupervised learning model lacks evaluation criteria. This paper presented the concept of separation degree between anomalous data set and normal data set to solve this problem. The classification performance of the model is judged by comparing the separation degree. According to the separation degree calculation results, the Isolation Forest's classification performance is better than the Local Outlier Factor. Finally, according to the classification results of Isolation Forest, the correlation between state information and welding quality is analyzed. It is found that the upset pressure and dynamic resistance can reflect the change of welding quality, but the correlation between electrode displacement and welding quality is not significant.

A novel laser welding-brazing process with temporal and spatial power modulation was firstly reported to join dissimilar alloys of AA6061 and Ti6Al4V with medium thickness. The power modulation was achieved by changing laser power as the same time as laser beam oscillating. A sound welded-brazed joint with relatively uniform intermetallic compound morphology along the Ti/Al interface was obtained. A multi-physical model was also developed and utilized to understand the underlying mechanism of the benefit of the temporal and spatial power modulation. It was found that the temperature gradient along the Ti/Al interface was reduced effectively by using this novel process.

The influence of pure Al, Al-Si and Zn-Al weld fillers on microstructure and mechanical properties was investigated during lap laser welding-brazing of Al to Cu. Al4.2Cu3.2Zn0.7 IMC layer and plate-like Al2Cu phase were observed at Al/Cu interface with pure Al and Al-Si weld fillers. Microcrack was easy to generate in Al2Cu phase decreasing the joint strength. When the Zn-Al weld filler was adopted, the formation of Al2Cu brittle phase was limited by low Al content and the CuZn and CuZn5 IMC layers were observed at Al/Cu interface. According to the chemical potential, the diffusivity of Cu atoms in Zn-Al weld filler was much lower than that of pure Al and Al-Si weld fillers, which prevented from the formation of coppery phases near the Al/Cu interface. Si atoms tended to concentrate at Al/Cu interface. It played a role in obstructing the diffusion of Cu atoms, enhancing the Al/Cu joint strength to some extent. According to nanoindentation result, the mechanical property of the formed phases (CuZn and CuZn5) in the case of Zn-Al weld filler was better than that formed with pure Al and Al-Si weld fillers such as Al2Cu and Al4.2Cu3.2Zn0.7. The highest tensile strength was 148 MPa with Zn-Al weld filler.

The new generation of Sc and Zr modified Al alloys has been attracted wide concerns in aerospace industry, owing to the excellent mechanical performances and superior thermal stability than other normal Al alloys. By microalloying with Sc and Zr, the Al3(Sc, Zr) particle forms as the grain refiner during the solidification, which is extremely beneficial for the laser powder bed fusion (LPBF) processed Al alloys. In this study, a new type Al-14.1Mg-0.47Si-0.31Sc-0.17 Zr alloy was additively manufactured by LPBF, and the microstructure, tensile properties and thermal stability were studied in detail. By using a single melt-67° scanning strategy, the LPBF-processed specimen with a relative density of 99.4 % and tensile strength of 487.7 MPa was obtained at 160 W-200 mm/s. And this AlMgSiScZr alloy can still exhibit an excellent tensile strength of 393.9 MPa at a moderate temperature of 473 K. After the aging treatment, the tensile properties further increased due to the precipitate hardening of Mg2Si and Al3(Sc, Zr), and the maximum value (580 MPa) was reached at an aging time of 10 h. The average crystal size was almost unchanged after aging treated at 325 ℃ and 24 h, indicating this AlMgSiScZr alloy has an improved thermal stability. The AlMgSiScZr alloy is recommended to substitute some particular titanium alloys in aerospace field afterwards.

A novel Al-14.1Mg-0.47Si-0.31Sc-0.17 Zr alloy was applied in the printing process of selective laser melting (SLM), and the corresponding microstructural feature, phase identification, tensile properties and corrosion behavior of the AlMgSiScZr alloy were studied in detail. As fabricated at 160 W and 200 mm/s, the Mg content of bulk sample decreased to 11.7 wt% due to the element vaporization at high energy density, and the density of this additively manufactured AlMgSiScZr alloy was 2.538 g/cm3, which is 4.2 %∼8.5 % lighter than that of other SLM-processed Al alloys. After heat-treated (HT) at 325 ℃ and 6 h, the microstructure was almost unchanged with an alternate distribution of fine equiaxed crystals and coarse columnar crystals. Nano-sized Al3(Sc, Zr) and Mg2Si phases precipitated dispersedly in the Al matrix, and the tensile strength increased from 487.6 MPa to 578.4 MPa for precipitation strengthening and fine grain strengthening. With a fine grain size of 2.53 μm, an excellent corrosion resistance was obtained for the as-printed (AP) AlMgSiScZr alloy. While the corrosion resistance of HT sample decreased slightly for the formation of non-dense oxide layer and pitting corrosion induced by diffuse precipitation distribution. This SLM-printed AlMgSiScZr alloy with high specific strength, good thermal stability and excellent corrosion resistance has broad prospects for the aerospace and automotive applications.

The influence of a post-weld heat treatment (PWHT), which included a solution treatment and an artificial aging treatment, on the microstructure and mechanical properties of laser welded-brazed Ti/Al joints with Al-12Si filler was investigated. The thickness of the intermetallic compound (IMC) layer at the brazing interface increased with increasing solution treatment time at 530 °C. Si atoms diffused from the weld to the IMC layer during solution treatment, forming a Si-poor region adjacent to the brazing interface. The phases in the IMC changed from Ti(Al,Si)3, nanograined Ti7Al5Si12, and Ti5Si3 in the welded-brazed sample to Ti7Al5Si12 with a high stacking fault density in the solution-treated sample. The net-shaped Al-Si eutectic microstructure in the fusion zone was affected by the PWHT. The size of the Si particles in the fusion zone increased with increasing solution treatment time, and the artificial aging treatment further coarsened the Si particles. The softening of the heat-affected zone (HAZ) in the Al base metal was eliminated by the PWHT. The tensile strength of the Ti/Al joint increased from 177 to 260 MPa. This result of this study indicate that the mechanical properties of Ti/Al welded-brazed joints can be improved by a PWHT.

The high-temperature tensile behavior of laser welded Ti-22Al-25Nb (at%) joints was investigated at 500, 650, 800, and 1 000 °C. The temperatures for tensile tests were selected according to the phase transformation sequence of Ti2AlNb-based alloys. At temperatures lower than the B2+O phase field (500 °C) and higher than the B2+O phase field (1 000 °C), the joints fracture in the base metal in ductile fracture mode. By contrast, the joints exhibit obvious high-temperature brittleness in the B2+O phase field (650 °C and 800 °C). Heat treatments were conducted with respect to the thermal history of tensile specimens. Intergranular microcracks along the grain boundary of B2 phase are found in the fusion zone after the heat treatments at 650 °C and 800 °C. The high-temperature brittleness at 650 °C and 800 °C is attributed to the B2→O transformation along the grain boundary. The stress concentration caused by the volume change of B2→O transformation also contributes to the high-temperature brittleness of laser welded Ti-22Al-25Nb joints.

The electrode state is an important factor affecting the welding quality in the process of resistance spot welding, but there is still lack of an effective method to monitor the electrode wear state. In this paper, a novel online monitoring method of electrode wear state is proposed by exploring the variation pattern of dynamic resistance. The evaluation method of time series similarity is important for dynamic resistance data processing, and two types of evaluation methods including static evaluation method and dynamic evaluation method are proposed in this paper. The static evaluation methods include shape change factor, dynamic resistance decrease ratio and peak time delay, and the dynamic evaluation method refers to the trend change factor. The welding process parameters and the welding material are kept unchanged during the experiment, the newly polished electrode is used to weld 1300 times continuously, and dynamic resistance is collected. According to the results of data processing, the change of the electrode state can be divided into three stages: The electrode state is stable when the welding number is less than 360. When the welding number is in the range of 360–800, the electrode state is in the transition stage, and the electrode state rapidly deteriorates with the increase of the welding number. When the welding number is more than 800, the electrode state is completely deteriorated, and the electrode needs to be dressed. This study may pave the way for online monitoring of electrode wear state.

In this paper, the Ti6Al4V and 6061 Al dissimilar joints were welded by laser welding-brazing process, and the effect of subatmospheric pressure and laser offset on weld formation, porosity defect, intermetallic compounds (IMCs) distribution, and tensile properties of Ti/Al dissimilar joints was investigated in detail. At a subatmospheric pressure of 101 Pa and laser offset of 0.5 mm, a sound weld formation with full penetration was obtained. The porosity defect in the fusion zone was significantly eliminated and the thickness of IMCs layer was controlled less than 2 μm due to the enhanced melt flow and uniform temperature field at low vacuum. The tensile strength reached a maximum value of 181.1 MPa with a mixed-mode of ductile and brittle fracture.

Pure-Al, Al-5Si, and Al-12Si were selected as filler materials in dual-spot laser welded-brazed Ti/Al joints to investigate the influence of Si content on interfacial reactions and mechanical properties. The microscale and nanoscale structures of the interfacial intermetallic compound (IMC) were analyzed by transmission electron microscopy (TEM). The IMC phase at the brazing interface in joints with pure-Al was only TiAl3 with twin structure defects and stacking faults. Ti(Al,Si)3 and nano-granular Ti7Al5Si12, Ti5Si3 phases were produced at brazing interfaces in joints with Al-5Si and Al-12Si. There were orientation relationships (OR) between Ti7Al5Si12 and Ti(Al,Si)3, Ti5Si3 and Ti7Al5Si12. The interfacial reactions were clarified using TEM images and thermodynamic calculations. The formation mechanism of nano-granular phases in joints with Al-5Si and Al-12Si was determined to proceed via a mechanism that included direct reactions and precipitation. The chemical potential of Si (μSi) decreased as the Ti content increased, resulting in the aggregation of Si atoms at the brazing interface. The fracture load of joint was improved by the addition of Si element in filler metal.

Adaptive control of the resistance spot welding process has always been a hot issue in the field of resistance spot welding. Due to the significant difference in weldability of different materials, the basis of adaptive control is the identification of material stack-up (including material types, layer number of metal sheets, the thickness of metal sheet). The material stack-up identification method based on dynamic resistance was proposed for the first time in this article. Ten different types of material stack-up commonly used in the manufacture of automobile body-in-white were selected in the experiment, and a total of 550 dynamic resistance samples were collected. The dynamic resistance value was used as the feature, and the supervised classification algorithms including support vector machines, logical regression, and random forest were used to classify the dynamic resistance, to realize the recognition of material stack-up. The data preprocessing part, include signal acquisition, dynamic resistance calculation, filtering, and dimensionality reduction was introduced in detail. The results show that before dimensionality reduction, the classification accuracy of the random forest is the highest, reaching 93.9%. After dimensionality reduction, the classification performance of logistic regression is the best, and the accuracy is 96.97%. The requirement of adaptive control for the accuracy of material stack-up recognition can be satisfied.

The metal matrix composites (MMCs) manufactured by additive manufacturing (AM) are widely used in aerospace, automobile and other industries for their lightweight and excellent mechanical properties. As a novel and promising AM technology, laser melting deposition (LMD), was utilized to prepare the TiB2 particle reinforced AA7075 composites. The influence of TiB2 content on the microstructural features and microhardness of the composites was studied. The results showed that, after the incorporation of TiB2, the grain size decreased due to heterogeneous nucleation and the solid-liquid interphase pushing and capture effect. Relative to the unreinforced 7075 Al specimen, the TiB2/AA7075 composites exhibited a higher hardness. For the specimen reinforced with 4 wt% TiB2, the hardness increased to 127.8 HV, and the grain size decreased to 16.8 μm. The microstructure and hardness of the TiB2/AA7075 composites were significantly enhanced by the incorporation of TiB2.

The three-dimensional finite element method was used to analyse the effect of changing the buccal tube baseplate shape on the stress magnitude and distribution at the interface between the buccal t...

The current waveform of the heating phase, the holding current and holding time of the holding phase were optimized in this study to solve the problems of low brazing ratio and poor quality stability of constant current induction brazing. Pulse current was used to replace constant current in the heating phase to improve the uniformity of the radial temperature distribution of electrical contact. The simulation results showed that the uniformity of the radial temperature field of pulse current heating is better than constant current heating. The experiment was conducted with constant and pulse induction brazing separately. Compared with constant current induction brazing, pulsed current induction brazing has a higher brazing ratio and a wider process window for brazing time. The maximum brazing ratio of constant induction brazing is 87.62 %, and pulse induction brazing is 96.61 %. Besides, the shear strength of brazing joint can be improved by extending hold time. Different from the effect of holding time, the shear strength increases first and then decreases with the increase of holding current, and it should be selected according to the actual situation.

This paper is a continued study on laser cleaning removal of marine microbiofouling from Al alloy surfaces. According to our previous study, it is noted that the antifouling functions of the generated laser-cleaned metallic surfaces must be highlighted. In this work, the inhibition effectiveness of the laser-cleaned Al alloy surfaces was evaluated using a type of vital marine microorganism, sulfate-reducing bacteria (SRB)

The currently available high-power laser shows promising opportunities for the welding of thick plates in a single pass. However, weld-root defect frequently occurs when a high-power laser is used to join thick plates in a full-penetration mode, which has a significantly adverse effect on the serviceability of the weld joint. The purpose of this work is to understand the defect formation mechanism and reduce these defects through controlling welding parameters. In this study, the characteristics of weld root defects were investigated using a 10 kW fiber laser using a program of experiment and theoretical analysis. The corresponding defect formation mechanisms were discussed based on the bottom molten pool behaviors observed by the high-speed camera. The results showed that there were four types of weld-root appearances as follows with an increase of linear heat input from 300 J/mm to 1000 J/mm: weld-root humping (30 mm/s), sound weld (25 mm/s), weld sagging (20 mm/s) and excessive weld sagging. The remedies for reducing weld-root defects were also presented to obtain sound weld bead by optimizing welding parameters. Weld-root humping was formed due to the quasi-full-penetration keyhole. Weld sagging resulted from the imbalance of the hydrostatic pressure and surface tension in the condition of a through keyhole. It was also found that the sound weld was formed when a through keyhole and a proper molten pool size were obtained. Thus, the state of the keyhole and molten pool geometry were the major factors that affect weld-root defects.

The influence of tin foil and Ni coatings on microstructures, mechanical properties, and the interfacial reaction mechanism was investigated during laser welding/brazing of Al/Cu lap joints. In the presence of a Zn-based filler, tin foil as well as Ni coating strengthened the Al/Cu joints. The tin foil only slightly influenced the joint strength. It considerably improved the spreading/wetting ability of the weld filler; however, it weakened the bonding between the seam and the Al base metal. The Ni coating considerably strengthened the Al/Cu lap joints; the highest tensile strength was 171 MPa, which was higher by 15.5% than that of a joint without any interlayer. Microstructure analysis revealed that composite layers of Ni3Zn14–(τ2 Zn–Ni–Al ternary phase)–(α-Zn solid solution)–Al3Ni formed at the fusion zone (FZ)/Cu interface. Based on the inferences about the microstructures at the interfaces, thermodynamic results were calculated to analyze the interfacial reaction mechanism. The diffusion of Cu was limited by the Ni coating and the mutual attraction between the Al and Ni atoms. The microstructure comprised Zn, Ni, and Al, and they replaced the brittle Cu–Zn intermetallic compounds, successfully strengthening the bonding of the FZ/Cu interface.

The effect of energy density on formability, microstructure and micro-hardness of Sc- and Zr- modified 7075 aluminum alloy fabricated by selective laser melting (SLM) has been studied. It was observed that, with the increase of energy density, the density of block specimen increased firstly and then remained unchanged. When the energy density is 375 J/mm3, the roughness of surface decreased from 20.1 to 8.5. That's because the temperature of melt pool increased with the increase of energy input. What's more, the spreading and solute redistribution of melt pool were further improved. Due to heterogeneous nucleation mechanism of Al3(Sc, Zr) particles, the grain refinement is obvious and the average grain size is 0.78 μm at the energy density of 375 J/mm3. With the combination effect of grain refinement and chemical composition, the micro-hardness increased firstly and then decreased with the increase of energy density.

The effects of the laser power and the scanning speed on molten pool shape, liquid metal spreading characteristics, and surface quality of the Al-Zn-Sc-Zr alloy fabricated by selective laser melting (SLM) were studied. The results showed that at the highest laser power of 360 W and the lowest scanning speed of 400 mm/s, the width and penetration of single track reached the maximum values of 244 μm and 241 μm. The powders were fully melted, and the liquid metal had more time to spread out when the line energy input is 800 J/m. The hole and balling defects disappeared, and the surface quality of the block specimen was significantly improved. With a decrease in the line energy density, the width, penetration, and overlap ratio of the single track gradually decreased. The surface morphology became rough with less melting and spreading time of the liquid metals. When the line energy density decreased from 800 J/m to 160 J/m, the roughness of the top surface increased from 9.4 to 16.1, and the densities of the block specimen increased from 98.1% to 99.5%.

The tensile deformation behaviors of laser-welded Ti-22Al-24.5Nb-0.5Mo alloy joints have been investigated at room temperature and 650 °C using in situ tensile analysis methods. The α2 phase had a significant influence on deformation behaviors of base metal at room temperature and 650 °C. The microcracks mainly nucleated in B2/α2 phase boundaries or within α2 phase and then propagated along B2/α2 phase boundaries subsequently. Compared with the plastic fracture of base metal, the fracture modes of the fully B2-phase fusion zone at room temperature and 650 °C were quasi-cleavage and intergranular fracture, respectively. While dislocation slips became foremost deformation mode in the fusion zone at room temperature, there were a great amount of slip bands on the surface of grains caused by the slip systems. The microcracks of fusion zone at 650 °C nucleated and propagated along the grain boundaries of B2 phase. Owing to the lack of grain deformation, the cross-slip bands were in small quantities on the surface of B2 phase grains.

Real-time monitoring of the welding quality is quite important during the process of industrial laser manufacturing. In this paper, a multi-information fused neural network, combining welding parameters and morphological features of the molten pool, was proposed to predict geometric features of the weld seam. Firstly, a modified optical fiber laser coaxial monitoring platform was set up to acquire clear images of the molten pool. Then, several morphological characteristics of the molten pool were extracted. By using principal component analysis (PCA) to reduce the redundancy of these features, the welding speed, the laser power and the two PCA components acted on as the four input neurons, while the two output neurons consisted of the weld waist width (WW) and the weld back width (BW) representing weld seam quality. Before training, the genetic algorithm (GA) was adopted to optimize the initialized weights and bias of the neural network due to its globally search ability. The experiment results showed that our proposed model can effectively and steadily predict the geometric features of the weld seam with the mean absolute percentage error (MAPE) less than 1% and the mean square error (MSE) less than 10−3. Time analysis showed that the whole process time of our system containing feature extraction and neural network was less than 90 ms which can meet the time requirements of large-scale real-time thin-plate laser welding application. Our system lays a foundation on the real-time quality monitoring in the process of laser welding thin-plate butt joint.

The epitaxy behavior and stray grains (SGs) formation in the deposit during epitaxial laser melting deposition (E-LMD) of directionally solidified (DS) superalloys were investigated. Columnar dendritic structures were obtained by epitaxial solidification on the DS substrate. The deposit also remained the orientation of the substrate. The SGs at the fusion interface, which were hardly eliminated, were attributed to different SGs formation mechanisms. The SGs were divided into GB-SGs and MC-SGs by the distribution characteristics. The GB-SGs at the low-angle and high-angle grain boundaries with a new mechanism of dynamic recrystallization induced by accumulation of thermal strain and stress under repeated spatially variable heating and cooling. The MC-SGs around the carbides were related to the misoriented cellular crystal formation caused by the varied shape of the solid-liquid interface. The columnar to equiaxed transition (CET) was another mechanism of MC-SGs formation.

2060 and 2099 are both newly developed high-strength aluminum-lithium (Al-Li) alloys for fuselage panels of jetliners. In this work, double-sided laser beam welding of 2-mm-thick 2060-T8 and 2099-T83 Al-Li alloys was performed using AA4047 filler wire under optimized parameters. Weld zone microstructure and T-joint mechanical properties were investigated by means of scanning electron microscopy (SEM), electron backscattered diffraction (EBSD), nano-indentation and in-situ tensile test. Experimental results indicated that the equiaxed grain zone (EQZ) featured by dense large-angle (θ＞10°) grain boundaries was observed in the weld zone along the fusion lines. Besides the softening problem found in the partially melted zone (PMZ), local softening phenomenon was detected in the local enrichment region of the EQZ. During in-situ tensile tests, the crack was initiated at the weld toe and then propagated alternately in the PMZ and EQZ's local enrichment regions. Micro-voids induced the localized softening problem and the crack's intergranular expansion in the EQZ. Localized absence of silicon was most likely to cause continuous micro-voids in the EQZ.

Nanosecond laser-induced fabrication of superhydrophobic surfaces has become a research focus of functional surfaces studies in recent years. In this study, we present a novel and highly efficient method for fabricating a superhydrophobic functional aluminum alloy surface. The method is based on large spot diameter (d = 960 μm) infrared nanosecond laser high-speed (linear velocity v = 3 m/s) scanning and ablation of the surface followed by low surface energy agent chemical treatment to attain a superhydrophobic state on the aluminum alloy surface. The textured roughness surfaces show three specific micro-morphology characteristics including corrugated, mastoid and flat zones all densely covered by cauliflower-like nanostructures. The hierarchical micro- and nanostructured surfaces demonstrated outstandingly stable superhydrophobicity during long-term exposure to an ambient atmosphere. The proposed method proves that large spot diameter high-speed scanning low-cost nanosecond lasers are also suitable for fabrication of large area metallic superhydrophobic surfaces for practical industrial application.

A numerical modeling approach is proposed for the process of powder feeding-based laser welding occurring in joining similar and dissimilar materials. For this reason, a three-dimensional computational model is developed to describe the complex physical phenomena of powder transport, laser heating, and thermal fluid dynamics in the molten pool. The VOF method in conjunction with the continuum theory is employed to study the effects of powder particles on molten pool dynamics. It is found that the addition of powder with an average temperature of 1850 K results in a lot of local low- and high-temperature zones, and small vortexes in the shallow areas of the molten pool with a depth of no more than 0.3 mm. The increasing laser power causes the smaller average depth oscillation amplitude, which visually reflects the higher keyhole stability under partial penetration welding. Note that two vortexes in the opposite direction behind the keyhole exist in full penetration welding, which are mainly driven by surface tension. The increasing average impact velocity of powder particles above 2 m/s leads to the more significant fluctuation of free surface profile, which again results in the smaller keyhole entrance size and more frequent keyhole collapses. Also, the corresponding experimental tests are carried out as verification. The findings from the study provide fundamental insights into the mechanism of melt flow and guidance in process optimization during powder feeding-based laser welding.

Laser welding–brazing of Ti/Al butt joints was performed with coaxial Al–10Si–Mg powders feeding. The experimental results indicated that a sound Ti/Al butt joint could be obtained by an additive layer approach. The influence of the laser melting deposition layers on the weld appearance, interfacial microstructure and tensile properties were investigated. High-quality joints were produced when five and seven layers were deposited. The morphology and thickness distributions of the interfacial intermetallic compounds (IMC) at the brazing interface along the thickness direction of the joint varied with the number of deposition layers. Continuous serrated IMC was obtained in joints produced by seven deposition layers, and the IMC layer was distributed homogenously along the thickness direction. The maximum thickness difference of the IMC was only approximately 0.12 μm. The microstructure of the IMC layer was composed of a nanosized granular Ti7Al5Si12 phase and serrated Ti(Al, Si)3 phase. The maximum tensile joint strength reached 240 MPa, 80% of that of the aluminum base metal, and the lower tensile strength of the other joints was caused by insufficient IMC layer or a porosity defect.