A comparative study of laser beam welding and laser-MIG hybrid welding of Ti-Al-Zr-Fe titanium alloy

Ti-Al-Zr-Fe titanium alloy sheets with thickness of 4 mm were welded using laser beam welding (LBW) and laser-MIG hybrid welding (LAMIG) methods. To investigate the influence of the methods difference on the joint properties, optical microscope observation, microhardness measurement and mechanical tests were conducted. Experimental results show that the sheets can be welded at a high speed of 1.8 m/min and power of 8 kW, with no defects such as, surface oxidation, porosity, cracks and lack of penetration in the welding seam. In addition, all tensile test specimens fractured at the parent metal. Compared with the LBW, the LAMIG welding method can produce joints with higher ductility, due to the improvement of seam formation and lower microhardness by employing a low strength TA-10 welding wire. It can be concluded that LAMIG is much more feasible for welding the Ti-Al-Zr-Fe titanium alloy sheets.     

Microstructures and mechanical property of laser butt welding of titanium alloy to stainless steel

Laser butt welding of titanium alloy to stainless steel was performed. The effect of laser-beam offsetting on microstructural characteristics and fracture behavior of the joint was investigated. It was found that when the laser beam is offset toward the stainless steel side, it results in a more durable joint. The intermetallic compounds have a uniform thickness along the interface and can be divided into two layers. One consists of FeTi + α-Ti, and other consists of FeTi + Fe₂Ti + Ti₅Fe₁₇Cr₅. When laser beam is offset by 0 mm and 0.3 mm toward the titanium alloy side, the joints fracture spontaneously after welding. Durable joining is achieved only when the laser beam is offset by 0.6 mm toward the titanium alloy. From the top to the bottom of the joint, the thickness of intermetallic compounds continuously decreases and the following interfacial structures are found: FeAl + α-Ti/Fe₂Ti + Ti₅Fe₁₇Cr₅, FeAl + α-Ti/FeTi + Fe₂Ti + Ti₅Fe₁₇Cr₅ and FeAl + α-Ti, in that order. The tensile strength of the joint is higher when the laser beam is offset toward the stainless steel than toward the titanium alloy, the highest observed value being 150 MPa. The fracture of the joint occurs along the interface between two adjacent intermetallic layers.     

激光功率对钛合金双侧同步焊接温度场的影响

为研究激光功率对TC4钛合金T型结构双激光束双侧同步焊接温度场的影响,针对TC4钛合金T型结构的双激光束双侧同步焊接过程,建立了相应的有限元模型,利用有限元分析软件进行了焊接过程温度场的计算,研究了激光功率对熔池形状的影响规律,并对不同激光功率下的温度场进行了分析.结果表明:随着激光功率的增加,熔池的熔深、熔宽均有所增...     

Fatigue in laser welded titanium tubes intended for use in aircraft pneumatic systems

The pneumatic system conducts the pressurized hot air from the engine to the environmental systems of the aircrafts. In-service failures of arc-welded pneumatic parts have driven further developments of laser beam welding as an alternative method. Here, a fiber laser with 2 kW power had been employed to weld commercial purity titanium tubes with 0.5 mm wall thickness and 50 mm diameter. For comparison purposes, semiautomatic TIG welding was realized. The chosen parameters speed and laser power for laser welding were 200 W–2 m/min and 250 W–3 m/min. The laser welded tubes presented 1 mm wide weld beads composed by partially twinned α-Ti grains. The TIG welded tubes showed 5 mm wide beads composed by acicular α-grains. These observed differences had been associated with the cooling rates, which are ten times higher in the laser case. Both laser and TIG welded tubes were cycled 44,000 times in a pneumatic bench at 350 °C without failures or cracks that could release the internal pressure. After the pressurization tests, the tubes were tested for tensile and fatigue resistance. The yield stresses, tensile strengths and total elongation did not change comparing base material, TIG welded and laser welded cases. The condition 200 W–2 m/min presented superior fatigue resistance values compared to other welding conditions, and could be considered similar to the tubes in the unwelded condition. The microstructural and mechanical results had shown that the current laser technology can replace, with advantages, the arc welding for the joining of the titanium tubes.     

Gap-free fibre laser welding of Zn-coated steel on Al alloy for light-weight automotive applications

As a result of new policies related to global warming announced by the European Union, avoiding unnecessary energy waste and reducing environmental pollution levels are becoming a major issue in the automotive industry. Accordingly, the lap welding of Zn-coated steels process, which is commonly used for producing car doors, has been gradually developed to lap welding of Zn-coated steel to light materials, such as Al alloy, Mg alloy and composite materials, in order to effectively reduce the vehicle weight. In certain part of car manufacture, organic glues are used to temporally join the Zn-coated steels and Al alloys before permanent welding takes place. The stability of such temporary joining by glues needs improving. Laser “stitching” or low strength welding could be considered as an alternative. However, challenges exist in joining Zn-coated steel on Al alloy by laser welding, due to significant differences of material properties between the two welding materials. Porosity, spatter and intermetallic brittle phases are readily produced in the weld. In this study, the effects of welding speed, laser power, number of the welding passes and type of shielding gas in gap-free welding of Zn-coated steel on Al alloy were investigated using a 1 kW single mode continuous wave fibre laser. Results show that a weld with higher shear strengths in the laser stitching application and less intermetallic phases could be obtained when nitrogen gas was used as the shielding gas. The corrosion resistance and the surface finish of the weld could be improved in double pass welding, especially when argon gas was used as the shielding gas.     

Cemented carbide reinforced nickel-based alloy coating by laser cladding and the wear characteristics

Composite coatings composed of metal matrix and ceramic particles have received particular attention in surface engineering. In this paper nickel base alloy powder and WC-Co cemented carbide particles have been chosen to manufacture composite coatings by use of kW CO₂ laser. Laser processing parameters including output power and scanning speed have been optimized. X-ray diffractor, optical and electron microscopes and energy dispersion spectra have been applied to investigate the phase constituents and microstructures of the coating. Block-on-ring dry sliding wear test and rubber wheel slurry erosive abrasion wear test have been conducted to study the wear properties of the composite coating as well as the reinforcing effect of WC-Co particles. A 50–100 mm wide ring appeared along the particle side of the particle-matrix boundary, showing superior wear and corrosion resistance, higher than even the original central part of the WC-Co particle. The mechanism of its formation has been probed.     

The influence of laser welding parameters on the microstructure and mechanical property of the as-jointed NiTi alloy wires

The Nd:YAG laser welding was used to join the binary NiTi alloy wires with different compositions(Ti-50.0 at.%Ni and Ti-50.9 at.%Ni) which had the same diameter of 1 mm. The wires were welded with different parameters, including impulse width and welding current. The aim was to assess the influence of the laser-welding process on the microstructure and mechanical properties of the welded joint of binary NiTi wires. The optical microscopy (OM) and the metallographic microscopy (MM) were used to analyze the microstructure of the welded joints. The tensile test and the differential scanning calorimetry (DSC) were carried out to examine the ultimate tensile strength and the reverse martensitic transformation temperatures of the welded joints. It was found that the welding current and the impulse width had great influence on the quality of the welded joints, an optimal parameter combination would remove the pores and micro-cracks appeared in the fusion zone, and result in good mechanical properties such as higher fracture strength and elongation. The laser welding had a few effect on the reverse martensitic transformation temperatures of the welded joints.     

Influence of the filler metal on the mechanical properties of Ti-6Al-4V electron beam weldments

The influence of various filler metals on the mechanical properties of 17 mm thick Ti-6Al-4V electron beam welded joints has been analysed. Autogeneous welded joints exhibit higher toughness when compared to the parent plate but this improvement was less marked than that observed in plasma arc welded joints. To achieve better toughness, without suffering unacceptable losses of strength, different morphologies of commercially pure titanium filler metals have been employed. Using 0.50 mm thick sheet as filler metal leaded to maximum toughness but as counterpart a significant decrease in strength was observed. To obtain high toughness while maintaining a high strength level 0.25 mm sheet and 1 mm diameter filler metals are recommended. Fractographic examination of the failed specimens helped to explain the fracture behaviour of the different welded joints.     

TC4钛合金激光焊接/超塑成形组合工艺研究

本文将超塑成形(SPF)和激光焊接技术(LW)相结合,开发激光焊接/超塑成形组合工艺(LW/SPF),研究TC4钛合金激光叠焊接头的超塑性变形行为,并对超塑性变形前后的显微组织进行分析.结果表明,TC4钛合金激光叠焊接头能够承受焊板的变形,试样在母材断裂,并通过四层板的模拟件研制验证了超塑成形/激光焊组合工艺工业应用的可行性.     

Bubble flow and the formation of cavity defect in weld pool of vacuum electron beam welding

Seen from gas-liquid two-phase-flow system, the gas phase and liquid phase of bubble flow in weld pool are studied by means of isolated phase based on the conservation of mass and momentum. The two-dimensional fractional flow model of bubble flow in weld pool of vacuum electron beam welding is developed. And the gas distribution and the phenomenon of bubble flow in weld pool of AZ91D magnesium alloy are simulated to analyze the formation and distribution of cavity defects. The results show that the possibility of gas escape in fully penetrated weld pool is much greater than non-penetrated weld. It appears that the probability of cavity defects is lower than non-penetrated weld to some extent. The formation of typical cavity defects is closely related to the flow pattern and flow characteristics of the bubble flow in deep penetration weld pool of vacuum electron beam welding. Higher liquid flow rate is more conducive to the escape of gas in molten metal, so that the final porosity in weld is low.     

Elasticity and resistivity study on the electromigration effects observed in aluminum–silicon–copper alloy thin films

The early electromigration (EM) processes in the Al–Si(Cu) thin films several tens of nanometers thick deposited on Si reed substrates were investigated by means of the simultaneous anelasticity and electrical resistivity measurements below 360 K. The grain growth, the shortening of a and the probable lengthening of a take place during the EM tests at the current density of 10⁸ A/m², where a and a denote the atomic plane spacing normal to and the one parallel to the film surface, respectively. The activation energy, E_GB, for the grain growth is found to be as low as 0.32 eV, possibly suggesting that E_GB in very thin nanometer-thick films is much lower than that found in thin micrometer-thick films. The increase in the Young’s modulus of the Al–Si(Cu) thin films takes place during the EM tests, suggesting that the grain growth is responsible for it. The decrease in Q⁻¹ observed at 330 and 360 K may be explained by a decrease in the grain boundary regions too. The increase in Q⁻¹ found during the EM tests at 300 K is possibly associated with an increase in a certain anelastic process in the grain boundary regions.     

Structure‐property investigations on a laser beam welded dissimilar joint of aluminium AA6056 and titanium Ti6Al4V for aeronautical applications Part I: Local gradients in microstructure,hardness and strength

Untersuchungen zu Struktureigenschaften von laserstrahlgeschweißte Mischverbindungen aus Aluminium AA6056 und Titan Ti6Al4V für Anwendungen in der Luftfahrt Teil I: Lokale Gradienten in Mikrostruktur, Härte und Festigkeit Durch eine spezielle Stossvorbereitung wurden laserstrahlgeschweißter Mischverbindungen aus den Blechwerkstoffen AA6056 und Ti6Al4V hergestellt und zwar ohne die Verwendung von Zusatzwerkstoffen. Die große Differenz der Schmelztemperaturen erlaubt das selektive Erschmelzen des Aluminiumwerkstoffs, der wieder um den Titanwerkstoff benetzt, sodass es zur Ausbildung einer mechanisch‐stabilen und tragfähigen Verbindung kommt. Die Al‐Legierung wurd ein den Wärmebehandlungszuständen T4 und T6 verschweißt, um den mikrostrukturellen Einfluss auf die Eigenschaften der Verbindungen untersuchen zu koönnen. Die Prozessfolgen sahen vor, dass beim Schweißen im Zustand T4 eine Warmauslagerung, beim Schweißen im Zustand T6 eine Kaltauslagerung definierter Dauer folgte. Die Charakterisierung lokaler Eigenschaftsgradienten hinsichtlich Gefüge, Mikrohärte und Festigkeit waren grundlegend für die Untersuchungen zum Ermüdungsrissausbreitungs‐ und Bruchverhalten der Mischerbindungen. Dabei wurden mögliche Bereiche, von denen Bruchversagen ausgehen könnte, identifiziert. Es hat sich gezeigt, dass die Eigenschaftsänderungen fast ausschließlich auf die Aluminiumseite beschränkt blieben. An der Grenzfläche zwischen Ti6Al4V und AA6056 wurde zudem eine schmale intermetallische Reaktionsschicht nachgewiesen. Diese lokalen Eigenschaftsänderungen im Gefüge, in der Härte und Festigkeit auf der Al‐Seite sowie der intermetallische Phasensaum in Verbindung mit geometrischen Unterschieden sind im Rahmen der Untersuchungen als mögliche kritische Bereiche identifiziert worden.     

Structure‐property investigations on a laser beam welded dissimilar joint of aluminium AA6056 and titanium Ti6Al4V for aeronautical applications. Part II: Resistance to fatigue crack propagation and fracture

Investigations were continued on the dissimilar laser beam welds of AA6056 and Ti6Al4V, fabricated by inserting Ti‐sheet into the profiled Al‐sheet and melting AA6056 alone. By using microstructure, hardness and strength as the criteria, sites exhibiting non‐uniform microstructure and localized plastic deformation due to strength mismatch were investigated in two orientations: ? crack parallel to the weld and ? crack perpendicular to the weld for fatigue crack propagation and fracture toughness at room temperature. Effect of temper of AA6056 on these properties was studied for two conditions; welding in T4 followed by post weld heat treatment T6, and welding in T6 and naturally aged for a defined period. The orientation “crack parallel to the weld” was investigated in 3 locations on the side of AA6056: the interface and the two changeovers on the Al‐side. Firstly, between the fusion zone and the heat affected zone (3 mm from the interface) and secondly, between (primary) heat affected zone and towards the base material (7 mm from the interface). Although brittle intermetallic TiAl₃ had been formed at the interface, uncontrolled separation or debonding at the interface was not observed. Insofar the bond quality of the weld was good. However, the ranking of interface was the lowest since fatigue crack propagation was relatively faster than that in the fusion zone and heat affected zone, and fracture toughness was low. Therefore, unstable fatigue crack propagation is observed when the crack propagates perpendicular to the weld from AA6056 towards Ti6Al4V. The results have shown that the dissimilar joints exhibit improved performance when laser beam welded in the T6 condition.     

A study on numerical simulations and experiments for mass transfer in bubble mode absorber of ammonia and water

An absorber is a major component in the absorption refrigeration systems, and its performance greatly affects the overall system performance. In this study, both the numerical and experimental analyses in the absorption process of a bubble mode absorber were performed. Gas was injected into the bottom of the absorber at a constant solution flow rate. The region of gas absorption was estimated by both numerical and experimental analyses. A higher gas flow rate increases the region of gas absorption. As the temperature and concentration of the input solution decrease, the region of gas absorption decreases. In addition, the absorption performance of the countercurrent flow was superior to that of cocurrent. Mathematical modeling equations were derived from the material balance for the gas and liquid phases based on neglecting the heat and mass transfer of water from liquid to gas phase. A comparison of the model simulation and experimental results shows similar values. This means that this numerical model can be applied for design of a bubble mode absorber.     

A simple model for falling film absorption on vertical tubes in the presence of non-absorbables

Most water–lithium bromide (LiBr) absorption chillers have a purge system to remove non-absorbable gases that cause a reduction in cooling capacity. Generally, the non-absorbables are originated in corrosion/passivation processes inside the machine, but leaks can also be a source of concern. However, since leaks must be corrected immediately to avoid machine deterioration, this study is mostly aimed at the non-absorbables evolved during operation. This paper analyses the effect of inlet non-absorbable air concentration, outlet purge velocity, absorber pressure and cooling water temperature on the falling film absorption process inside a vertical tube absorber, based on a simple transport coefficient model. This model consists of three ordinary differential equations solved with as method for initial-value problem, and a set of auxiliary equations. The study shows that the effect of non-absorbables can be significant, and furthermore provides a quantitative framework to aid in purge design. The nominal working conditions in this study were a solution Reynolds number of 100, an absorber pressure of 1.3 kPa, a cooling water temperature of 35 °C and an inlet solution concentration of 62% LiBr by weight. The results indicate that a minimum vapour velocity is required to sweep the non-absorbables along the absorber towards the purge, thus preventing reduced absorption fluxes. At a cooling water temperature of 35 °C, an inlet air concentration of 20% (by mole) resulted in a 61% reduction in mass absorption flux.