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Ti-Al(48at.%) alloy was deposited by twin-wire plasma arc welding additive. The preheating temperature was 560, 620 and 680 °C, respectively. The results show that when the preheating temperature was 560 and 620 °C respectively, cracks appeared at the arc-starting part of the lower of the sample. The preheating temperature was 680 °C, and the sample had no cracks. The numerical simulation results show that the transient stress decreased with the increased of the preheating temperature. The transient stress at the lower of the specimen was greater than that at the middle and top, and the YZ shear stress and Z-direction stress at the front end were greater than those at the middle and end, resulted in cracks at the arcing starting part of the lower. [ABSTRACT FROM AUTHOR]

Twin-wire plasma arc additive manufacturing (TW-PAAM) is an innovative process for titanium aluminide manufacture which is low in equipment cost and high in efficiency. However, as an in situ alloying process, the composition homogeneity is typically a source of concern. The present work investigates this subject by observing the element input mode at various welding wire positions with a high-speed camera system and particle tracking method. It is found that when the welding wires are in a high position, the droplet enters the molten pool from the tip of the titanium wire, and the alloy element input interval is long, resulting in the formation of bands with high aluminium content at the edge of the deposition layer. When the welding wires are close to the workpiece surface, the droplet transfer mode switches to a mixture of dual wire-bridge transfer and Ti wire-bridge transfer. The diameter and transfer interval of the droplets decrease, as well as the heterogeneity of the deposition layer. The conclusion establishes a correlation between droplet transfer mode and composition homogeneity, which is beneficial for optimizing the TW-PAAM titanium aluminide fabrication process. [ABSTRACT FROM AUTHOR]

Laser cleaning has been widely used in manufacturing industries due to its lower pollution and higher cleaning precision and efficiency compared with traditional ways. However, laser cleaning would inevitably induce processing roughness on the workpiece surface and only a few researches have been conducted to this field. In this work, the influence of process parameters on surface roughness was explored in detail. The results showed that process parameters changed surface roughness by changing energy density and overlap ratio. And surface roughness was showed linear positive correlation with energy density. At same energy density, surface roughness will first increase and then decrease with increasing overlap ratios. The maximum roughness was in the range of 50 to 66.7%. It could be explained that after laser irradiation, the sample surface absorbed pulse energy and formed craters. The rim and craters made the sample surface coarse. Due to the depth and diameter of craters were dependent on energy density, the roughness increased with increasing energy densities. The adjacent craters will also change the height of rim and depth of crater with different overlap ratios. It led to different surface roughness. [ABSTRACT FROM AUTHOR]

The dynamic behavior of an arc plasma involves the complex heat and mass transfer process in the arc welding process, especially for pulsed gas metal arc welding (P-GMAW). The spectral diagnostics of the arc plasma is essential to both the validation of simulation results and the monitoring and control of the arc welding process. In this article, we studied the dynamic behavior and spatial spectral characteristics of the welding arc during the pulsed gas metal arc welding of the AA5083 aluminum alloy with an ER5183 wire using a high-speed camera and spectrometer. Based on the acquired spectra at different time points of the pulse, we calculated the plasma characteristics using the spectral lines emitted from the shielding gas and the metal vapor. The variation and spectral diagnosis of the arc indicated that the bright region was mainly caused by radiation from the metal vapor, which easily increased at the peak current. The measured arc temperature and electron density were elevated along with the welding current. Moreover, the ratio of metal vapor in the arc plasma also increased due to strengthened evaporation from the electrode. Along the horizontal axis, the arc temperature increased and reached a maximum in the central part of the arc. There was a potential local minimum near the electrode as the current increased. The drop of temperature near the electrode was mainly caused by the rapid evaporation and cooling effects of metal vapor. [ABSTRACT FROM AUTHOR]

Keyhole-induced porosity can strongly weaken the joint strength during aluminum alloy laser welding. Two factors are proposed to be responsible for the porosity formation: bubble formation caused by the instability of the keyhole and bubble captured by the solidification interface. Based on numerical and experimental results, the porosity is suppressed with preheating, due to the capillary stability of the keyhole and easy escape of bubble. N2 shielding gas has a minor effect on keyhole stability, but it reduces the porosity number as N2 bubble can dissolve into the liquid aluminum alloy melt pool quickly. The laser-gas metal arc (laser-GMA) welding process can stabilize the metallic plume and enlarge the weld pool. As a result, few spatters and bubbles are formed, and bubbles are easily escaped, both of which contribute to the suppression of porosity. [ABSTRACT FROM AUTHOR]

The instability of the keyhole and bubble being captured by solidification interface are the main factors that affect the formation of porosity in laser welding of both aluminum alloy and steel. In laser welding of aluminum alloy, the more violent fluid flow behind the keyhole, the larger depth to width ratio can cause the keyhole more unstable, and susceptive to collapse that can be described as three necessary steps. The much larger solidification rate and longer escaping distance make the bubble easily captured by the solidification interface, and easily combined. All of these reasons contribute to the formation of larger number and size of porosity in laser welding of aluminum alloy. The numerical and experimental results are in good agreement. [ABSTRACT FROM AUTHOR]

Thick plates of 5083 aluminum alloy are welded with GMAW and GMAW-TW processes; the influence of weld pool convection on porosity formation, microstructure, and mechanical property of the welding joints is investigated. The results showed that due to the appearance of outward flow and 'push-pull' patterns, and intense mixing and stirring effects in GMAW-TW process, a sound weld seam with less porosity can be obtained. The less probability of bubble formation, the shorter escaping distance of the bubble, the easy splitting of bubble, the higher bubble escape velocity, and lower solidification rate are responsible for porosity suppression. Even though the grain size in GMAW-TW process is larger than that in GMAW process, the ultimate tensile strength of the joints is larger, which is caused by less porosity formation, larger number, and uniform distribution of second-phase particles in the welds. Compared with GMAW process, the GMAW-TW process possesses the advantages of higher efficiency, less weld defects, and higher mechanical property in the welding of 5083 aluminum alloy thick plates. [ABSTRACT FROM AUTHOR]

The droplet escape condition is established, a three-dimensional numerical model is built to investigate the behavior of the weld pool in fiber laser welding of 5083 aluminum alloy, the shear stress caused by the metallic vapor flow that has great influence on spatter formation is considered. A high speed photography system is used to capture the transient images of the weld pool, the spatter formation is discussed. The numerical and experimental results show that the spatters generate mainly around the keyhole. Two factors are responsible for the spatter formation: the surface tension of the molten metal around the keyhole is low. The recoil pressure caused by evaporation and the shear stress caused by metallic vapor flow accelerate the upward moving melt around the keyhole. Increasing the welding speed and placing the nozzle at the trailing position can decrease the upward momentum of the molten metal around the keyhole, and stabilize the keyhole, which are help to suppress the formation of spatter. [ABSTRACT FROM AUTHOR]

In this research, the thicknesses of Fe 2 Al 5 sub-layer ( l Fe2Al5 ) in bead welds were roughly calculated according to the formula l = Kt 1/2 . The values of constant K and residence time t were obtained through recording welding pools temperature. Fe 2 Al 5 sub-layers were also observed and their thicknesses were measured by SEM and EDS analysis. The calculated results were in close agreement with measured results, demonstrating the accuracy of these calculated Fe 2 Al 5 sub-layer thicknesses. [ABSTRACT FROM AUTHOR]

• Fe 2 Al 5 is the main factor influencing the Fe-Al IMC interface thickness. • Fe 2 Al 5 without Si has obvious (0 0 1) preferential orientation tendency. • Established Fe 2 Al 5 thickness prediction model with calculation error under 10 %. • Si addition is more effective in controlling Fe 2 Al 5 growth compared to energy. Further development of the car lightweight in automotive industry significantly depends on the reliability of the Al/steel dissimilar joint, since more and more Al alloy parts are used in the car manufacturing. It has been proved that the Al/steel dissimilar joint strength can be effectively increased when the intermetallic compound (IMC) growth, especially the Fe 2 Al 5 layer thickness, is controlled at certain level. Because the Fe 2 Al 5 is the dominant phase of the entire IMC layer in the Al/steel dissimilar joint, and cracks usually initiate at this location when damage is conducted. Therefore, for the automotive industry assembly line, the prediction model monitoring the Fe 2 Al 5 layer thickness of each Al/steel weld seam is desired to ensure the dissimilar joint quality and reliability. In the present research, a numerical dynamic growth model based on element diffusion laws is established to calculate the IMC layer thickness of Fe 2 Al 5 phase during the variable polarity cold metal transfer (VP-CMT) welding of aluminum to steel in the function of welding heat input and location. To build up effective connection between the welding heat input and the actual temperature history of Al/steel interface, a finite element analysis (FEA) model is customized for the present VP-CMT process. The Fe 2 Al 5 layer thickness calculation results are further validated using electron back-scattered diffraction (EBSD) characterization under scanning electron microscopy (SEM), confirming a 2.3 % deviation between the model prediction and actual layer thickness. [ABSTRACT FROM AUTHOR]

Abstract: 5052 aluminum alloy sheets and galvanized mild steel sheets were joined in lap configuration by alternate-current double pulse gas metal arc welding with pure Al, Al–5Si, Al–12Si and Al–4.5Mg (wt%) filler wires. The effect of alloying elements on the microstructure of intermetallic compounds (IMC) layers formed between weld seam and steel, and tensile strength of the resultant joints were investigated. The thickness of IMC layer in all samples varied along the cross-section of the joint, the intermediate part of the IMC layer was thicker than the head and root parts. The diffusion of Si into Fe2Al5 sub-layer could restrain the growth of Fe2Al5 sub-layer and IMC layer, and joint's mechanical property improved with the increasing Si content in Fe2Al5 phase. Due to the high hot crack sensitivity of Al–4.5Mg alloy, cracks generated at the root of joint made with Al–4.5Mg filler, resulting in poor mechanical property. [Copyright &y& Elsevier]

Abstract: 5052 Al alloy sheets and galvanized mild steel sheets were joined by gas metal arc welding with three different current modes, including direct-current pulse gas metal arc welding (DPG), alternate-current pulse gas metal arc welding (APG) and alternate-current double pulse gas metal arc welding (ADG). The effect of current mode on size, distribution and volume fraction of pores generated in Fe–Al dissimilar materials lap joint was quantitatively studied. EDS result showed that pores in Fe–Al joint were mainly caused by trapped zinc metal vapor from galvanized steel. Volume fraction of pores in joints made by APG and ADG processes was larger than that in joint resulted from DPG process. Moreover, pores in joints made by APG and ADG processes had smaller diameter, and tended to distribute in the middle of the weld seam. On the contrary, pores with large diameter were inclined to distribute close to upper weld surface of the joint resulted from DPG process. These results are attributed to the difference of arc stirring force and linear heat input in these three processes caused by different current modes. [Copyright &y& Elsevier]

Abstract: This paper studies Al–steel lap joints resulted from direct-current pulsed gas metal arc welding (DPG) and alternate-current double-pulse gas metal arc welding (ADG). Base metals being welded are 1mm thick 5052 Al alloy sheet and 1mm thick galvanized mild steel sheet with filler metal being 1.2mm 4047. Effect of the different welding processes on joint properties is studied through characterization of the steel/Al interphases and mechanical strength. It is found that the thickness of intermetallic compound layer composed of Fe2Al5, FeAl3and Fe3Al, and the diffusion of Fe element from steel to weld seam are dependent on the amount of welding heat input. Lap shear samples of Al–Fe joints resulted from the ADG process fractured at Al base metal or weld seam with average ultimate tensile strength (UTS) of 201MPa. On the other hand, all lap shear samples of the joints resulted from DPG process fractured along the interphase between the Fe2Al5 sub-layer and the steel sheet with average UTS of 115MPa. ADG process offers higher joint strength than that offered by DPG process due to thinner intermetallic compounds layer caused by the reduced heat input, illustrating that cyclic polarity switching and a secondary low frequency current pulse can effectively reduce the heat input in joining Fe–Al dissimilar materials, which means the ADG process is much more feasible for making robust Fe–Al lap joints. [Copyright &y& Elsevier]

The further strengthening of the Al/steel dissimilar joint is significantly meaningful for car lightweight development in automotive industry. It has been experimentally proved that the use of Si containing aluminum alloy filler material such as ER4043 grade welding wire can effectively improve the joint strength by reducing the Fe Al intermetallic interface thickness. However, the specific functioning mechanism of Si has not been clarified explicitly thus controversy still exists. In the present research, an Al/steel dissimilar joint is generated using the variable polarity cold metal transfer (VP-CMT) technique. To address the suppression function of Si on the Fe Al intermetallic interface growth clearly, the focused ion beam (FIB) is conducted to extract the testing sample right at the interface, which is subsequently investigated under TEM. According to the obtained results, the intermetallics formed at the Fe Al interface are generally Fe 4 Al 13 phase on the aluminum side and Fe 2 Al 5 phase adjacent to the steel side. Also, rod-shaped Fe 5 Si 3 precipitates are confirmed in the Fe 2 Al 5 matrix near the steel side, rather than Si solid solutions or ternary Fe-Al-Si phases stated in the previous researches. As indicated by the dislocation distribution in the Fe 2 Al 5 matrix, the generation of Fe 5 Si 3 precipitates considerably suppresses the brittle Fe 2 Al 5 fracture. Such phenomenon clarifies the Si element functioning mechanism of improving the Al/steel dissimilar joint quality. • Focused ion beam is conducted to extract the Fe Al interface from the joint. • Existence state of Si in Fe Al interface is confirmed to be Fe 5 Si 3 precipitate. • Fe 5 Si 3 is the key to suppress intermetallic interface growth of Al/steel joint. • Fe 5 Si 3 can suppress the Fe 2 Al 5 matrix fracture by precipitate strengthening. [ABSTRACT FROM AUTHOR]

• The initial and complete cleaning thresholds of 2 μm oxide layer were 12.7 J/cm2 and 25.5 J/cm2, respectively. • Thermal stress coupling effect led to the separation of substrate and oxide layer. • Phase explosion and impact effect led to the splattering and removal of oxide. • Energy density affects plasma characteristic by changing plasma temperature and electron density. Through comparing the cleaning effect of aluminum alloy at different energy densities, the model of materials thermodynamics was established, illuminating the cleaning mechanism and plasma behavior. The research showed laser cleaning could remove oxide layer completely. The initial cleaning threshold was 12.7 J/cm2 and complete cleaning threshold was 25.5 J/cm2. At low energy density, phase explosion caused by laser ablation was the main cleaning mechanism. At high energy density, besides phase explosion, impact effect induced by evaporation pressure caused the splattering and removal of oxide layer. Plasma lifespan had positive correlation with energy density. Its lifespan in 25.5 J/cm2 and 51.0 J/cm2 were about 6 μs and above 10 μs, respectively. The model of materials thermodynamics showed that at low energy density, substrate evaporation pushed out molten oxide layer and formed pulse craters. At high energy density, transient energy absorption caused thermal stress coupling effect and separated substrate with oxide layer. The impact effect induced by evaporation pressure resulted in the removal of oxide layer. The vapor of Al absorbed laser energy and formed plasma. The high energy density enhanced the laser-plasma coupling. It caused high temperature and intensive electron density. The time of plasma cooling and atomization became longer. [ABSTRACT FROM AUTHOR]

This paper concerns the dynamic variation in plume velocity and keyhole size during the laser welding process of 5083 Al-alloy. The keyhole size and plume velocity were measured from high-speed images through imaging process methods and the particle image velocimetry (PIV) method. The correlation between the oscillation in plume velocity and keyhole size was discussed based on the pressure balance inside the keyhole. Results show that the keyhole keeps opening and collapsing during the welding process. During the time the keyhole is open, the size of the keyhole continuously oscillates until it collapses. Plume velocity increases to a maximum at the initial stage of keyhole's opening, and then varies in synchronisation with the oscillation of the keyhole size. It is considered that local evaporation occurs on the humps of the keyhole front wall, which results in the oscillation in plume velocity. The oscillation in plume velocity then leads to the variation in the pressure balance in the keyhole and drives the keyhole to expand or shrink accordingly. [ABSTRACT FROM AUTHOR]