Mechanical properties and strengthening mechanisms in laser beam welds of pure titanium

Abstract

The mechanical properties and strengthening mechanisms in laser beam welds of pure titanium were investigated. Although grain coarsening is evidently observed in the heat affected zone (HAZ) and fusion zone (FZ) compared with the base metal (BM), the tensile and hardness tests indicate that the HAZ and FZ are stronger than the BM under the welding conditions employed in the present work. The strengthening mechanism in the HAZ is ascribed to the substructure strengthening and that in the FZ is attributed to the combination of the substructure strengthening and the solute solution strengthening.     

原位钛基复合材料激光焊接过程中TiB晶须增强体的微观结构演变

本研究采用Nd:YAG激光成功地对TiB晶须和La₂O₃颗粒混杂增强的原位钛基复合材料进行了焊接。利用金相观察、X-ray衍射、扫描电镜(SEM)和透射电镜(TEM)等测试方法,研究了激光焊接过程中TiB的演变行为,探讨了激光焊接头中的物相组成,TiB的分布及形貌特征,及TiB(或La₂O₃)和基体之间的界面关系。研究结果表明,TiB依然存在于焊接接头中,未发现有害物相的形成。在接头熔化区和靠近焊接热源的热影响区中,TiB尺寸显著细化,重新分布于b柱状晶晶界形成新颖的网络状结构。而在远离熔合线的热影响区中,由于受焊接热输入影响小,仅有少量TiB晶须通过B原子的强化扩散而改变了尺寸大小。而靠近母材的TiB未有变化,保持着和母材中TiB相似的形貌特征。进一步的TEM研究证明,增强体和基体之间的界面干净,仍保持良好的界面结合关系,未发现任何不良界面反应的发生,这也表明在激光焊接过程中,增强体和钛基体之间的界面结构是比较稳定的。     

Weld zone characteristic and mechanical performance of in situ titanium matrix composites using gas tungsten arc welding (GTAW)

Abstract

In situ TiB_w and La₂O₃ reinforced titanium matrix composites were successfully fusion welded by the gas tungsten arc welding (GTAW) process and the weldability and feasibility for composites were studied, and uniform and defect free welds were produced with sound welding parameters. Microstructural observations showed that the joint has a distinctly identified weld zone characteristic. In addition, the distribution and size of TiB_w in the weld became much more homogeneous and smaller. Moreover, the welded joints exhibited good mechanical properties at ambient temperature and the strength equal to the base metal at elevated temperature. The fracture mechanism of TiB_w in the weld also was investigated.     

Microstructure and mechanical properties of SiC-nanowire-augmented tungsten composites

The effect of an addition of SiC nanowire on the microstructure and mechanical properties of tungsten-based composites is investigated in this study. SiC-nanowire-augmented tungsten composites were prepared by a spray-drying process and an in situ spark plasma sintering process. Three distinctive reaction phases, tungsten, tungsten carbide (W₂C) and rod-type tungsten silicide (W₅Si₃) were formed during the sintering process. The flexural strength was significantly increased from 706 MPa to 924 MPa in tungsten composites augmented with SiC nanowires, as was the formation of W₂C and W₅Si₃ phases. The rod-type W₅Si₃ bears significant stress by both sharing a portion of the load and providing a bridging mechanism. Furthermore, a high ablation resistance at an elevated temperature was observed for tungsten composites augmented with SiC nanowires.     

Characterization of (TiB + TiC)/TC4 in situ titanium matrix composites prepared by laser direct deposition

Preliminary characterization of microstructure and mechanical properties of (TiB + TiC)/TC4 in situ titanium matrix composites prepared by laser direct deposition is reported in this paper. The results indicate that in situ reaction occurred during laser direct deposition of coaxially fed mixed powders from TC4 and B₄C. Reinforcements of TiB and TiC with a fraction of about 25 vol.% were formed with feeding 5 wt.% B₄C. The morphology of TiB tended to be needle-like and prismatic, while TiC appeared as granular. Small amount of un-reacted B₄C with reduced size remained within the composites. A thin skull of reaction product formed around the un-reacted B₄C weakened its interface bonding with the titanium alloy matrix, resulting in less outstanding properties of the composites.     

Microstructure and mechanical properties of GTA weldments of titanium matrix composites prepared with or without current pulsing

The effects of current pulsing on the microstructure, hardness and tensile properties at different temperatures of gas tungsten arc (GTA) weldments of titanium matrix composites were studied. Full-penetration butt joints were made with or without current pulsing. Optical microscopy, hardness test and scanning electron microscopy were employed to evaluate the metallurgical characteristics of welded joints. Tensile properties of weldments at different temperatures were studied and correlated with the microstructure. The results exhibit that current pulsing leads to the refinement of the weld microstructure and TiB whisker and the redistribution of reinforcements resulting in higher hardness, tensile strength and ductility of weldments in the as-welded condition.     

Microstructures and mechanical properties of the in situ TiB-Ti metal-matrix composites synthesized by spark plasma sintering process

In situ synthesized TiB reinforced titanium matrix composites have been synthesized by spark plasma sintering (SPS) process at 950-1250 °C, using mixtures of 15 wt% TiB₂ and 85 wt% Ti powders. The effects of the sintering temperature on densification behavior and mechanical properties of the TiB-Ti composites were investigated. The results indicated that with rising sintering temperatures, relative densities of the composites increase obviously, while the in situ TiB whiskers grow rapidly. As a result, bending strength of the TiB-Ti composites increases slowly at the combined actions of the factors referred above. Fracture toughness of the composites is improved remarkably due to the large volume fraction of Ti matrix, the crack deflection, pull-out and the micro-fracture of the needle-shaped TiB grains. The results also suggested that TiB-Ti composite sintered at 1250 °C by SPS process exhibits the highest relative density of 99.6% along with bending strength of 1161 MPa and fracture toughness of 13.5 MPa m^1/2.     

Hybrid effect of TiBw and TiCp on tensile properties of in situ titanium matrix composites

In order to study the hybrid effect of in situ TiB whisker (TiBw) and TiC particle (TiCp) on the tensile properties of titanium matrix composites (TMCs), TMCs with various TiBw/TiCp volume ratios of 4:1, 1:1 and 1:4 were prepared by reactive hot pressing of blended powders of Ti, B₄C, and graphite. Room tensile testing results exhibited that the composite with TiBw/TiCp volume ratio of 1:1 experienced an increase in the ultimate tensile strength of 25 and 50%, respectively, over the composite with ratio of 4:1 (based on Ti–B₄C) and pure Ti. The hybrid strengthening effect still existed at elevated temperature but it was not as obvious as that of room-temperature. Meanwhile, the ductility of the composites was decreased with decreasing TiBw content. The results suggest that the hybrid effect exists both at room and at elevated temperatures and it should be taken into account to prepare TMCs with good mechanical properties.     

Effect of welding parameters on morphology and mechanical properties of Ti-6Al-4V laser beam welded butt joints

The influence of welding speed and laser power on weld quality of 1.6 mm thick Ti − 6Al − 4V sheets autogenously laser beam welded in butt configuration using a Nd-YAG laser was studied. The joint quality was characterized in terms of weld morphology, microstructure and mechanical properties. An underfill defect, controlling the whole weld geometry, was observed both at the weld face and root surface. In dependence of the specific heat input, this defect showed a maximum, which separates two different welding regimes: keyhole welding, at low heat input, and a welding regime where heat conduction around the keyhole is predominant, at high heat input. Influence of the underfill radius on the weld fatigue life was also assessed.     

Effect of brazing parameters on microstructure and mechanical properties of titanium joints

This study investigates the influences of brazing temperature and time on microstructures and mechanical properties of commercially pure (CP) titanium. Bonding was performed in a high-vacuum furnace using Incusil-ABA (Ag–27.2Cu–12.5In–1.25Ti, wt.%), as filler metal. Brazing temperatures employed in this study were 710, 750, and 800 °C. At the same time, the investigated holding times at the brazing temperatures were 5, 30, and 90 min. Microstructure and phase constitution of the bonded joints were analyzed by means of metallography, scanning electron microscope (SEM) and X-ray diffraction pattern (XRD). An intense diffusion of Ti to the interface and a strong reaction between the braze alloy and the base metal were observed especially at a temperature of 800 °C. A number of intermetallic phases such as TiCu, Ti₂Cu, Ti₃In, Cu–In, and TiAg have been identified. Both brazing temperature and holding time are critical factors to control the microstructure and hence the mechanical properties of the brazed joints. The optimum brazing parameter was achieved at a temperature of 750 °C and a holding time of 90 min.     

WCp/Fe metal matrix composites produced by laser melt injection

Laser melt injection (LMI) was used to produce WC particles (WC_p) reinforced metal matrix composites (MMCs) layer on the mild steel. During the LMI process, different parameters were applied, and the processing window of this technique was obtained. The MMCs layers were studied by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The SEM result reveals that none macro-defects except few pores can be found in the MMCs layer, and the WC_p distribute uniformly in the layer. In addition, some new phases can be found in the MMCs layer, where Fe₃W₃C is the predominant phase. At the same time, the amount of dissolved WC_p plays a key role in the microstructure of the MMCs layer. The WC particle dissolved into the melt pool leads to the appearance of reaction products in the matrix, such as various primary Fe₃W₃C dendrites, and the liquid WC remained on the solid WC particle results in the formation of a thicker reaction layer.     

Microstructures and mechanical properties of Al 2519 matrix composites reinforced with Ti-coated SiC particles

Ti-coated SiC_p particles were developed by vacuum evaporation with Ti to improve the interfacial bonding of SiC_p/Al composites. Ti-coated SiC particles and uncoated SiC particles reinforced Al 2519 matrix composites were prepared by hot pressing, hot extrusion and heat treatment. The influence of Ti coating on microstructure and mechanical properties of the composites was analyzed by scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). The results show that the densely deposited Ti coating reacts with SiC particles to form TiC and Ti₅Si₃ phases at the interface. Ti-coated SiC particle reinforced composite exhibits uniformity and compactness compared to the composite reinforced with uncoated SiC particles. The microstructure, relative density and mechanical properties of the composite are significantly improved. When the volume fraction is 15%, the hardness, fracture strain and tensile strength of the SiC_p reinforced Al 2519 composite after Ti plating are optimized, which are HB 138.5, 4.02% and 455 MPa, respectively.     

TC11钛合金搅拌摩擦焊接头微观组织和力学性能

本文在不同转速下开展了TC11钛合金搅拌摩擦焊试验,分析了接头的微观组织和力学性能,建立了焊接工艺、接头组织与性能之间的内在联系。接头SZ为完全的β转变组织,其细小晶界α相和晶内片层状α+β相以及针状α′相的双重强化作用,致使SZ硬度最高。HAZ和TMAZ均由残余初始α相和β转变组织组成,但后者具有一定的流变特征,且距SZ越近,转变组织占比越大,强化效果越明显。随着转速的上升,SZ中晶界α相和晶内片层状α+β相的尺寸增加,晶内针状α′相的含量降低,这引起了其硬度的降低。与硬度规律相同,各转速下接头拉伸均断裂在BM,且各转速下SZ抗拉强度明显高于BM,并随转速上升而下降。此外,SZ中细小的晶界α相和晶内片层状α+β相,也导致各转速下SZ延伸率高于BM,使其展现出良好的塑性,且随着转速上升,因晶界α相和晶内片层状α+β相尺寸增加,SZ延伸率有所降低。     

Microstructure, mechanical and wear properties of laser processed SiC particle reinforced coatings on titanium

Laser processing of Ti-SiC composite coating on titanium was carried out to improve wear resistance using Laser Engineered Net Shaping (LENS™) — a commercial rapid prototyping technology. During the coating process a Nd:YAG laser was used to create small liquid metal pool on the surface of Ti substrate in to which SiC powder was injected to create Ti-SiC metal matrix composite layer. The composite layers were characterized using X-ray diffraction, scanning and transmission electron microscopy equipped with fine probe chemical analysis. Laser parameters were found to have strong influence on the dissolution of SiC, leading to the formation of TiSi₂, Ti₅Si₃ and TiC with a large amount of SiC on the surface. Detailed matrix microstructural analysis showed the formation of non-stoichiometric compounds and TiSi₂ in the matrix due to non-equilibrium rapid solidification during laser processing. The average Young's modulus of the composite coatings was found to be in the range of 602 and 757 GPa. Under dry sliding conditions, a considerable increase in wear resistance was observed, i.e., 5.91 × 10^− 4 mm³/Nm for the SiC reinforced coatings and 1.3 × 10⁻³ mm³/Nm for the Ti substrate at identical test conditions.     

Gas tungsten arc welding of network structured titanium matrix composite

The microstructure and property evolution of TiBw/Ti6Al4V composite was investigated after gas tungsten arc welding process. The results showed that the heat affected zone (HAZ) exhibited unchanged reinforcements, while the fusion zone (FZ) exhibited refined TiBw with refined network scale. Meanwhile, the matrix in HAZ presented the transformed β microstructure, while the refined columnar grains existed in FZ. Moreover, increasing heat input can further decrease the refined network size. Consequently, the ultimate tensile strength of the welded composite reaches 98% that of the parent composite (PC); while the yield and tensile strength at 500°C are even higher than those of the PC. The refined microstructure and the transformed β phase also lead to noticeable enhancement of the microhardness.     

Steel to titanium solid state joining displaying superior mechanical properties

Joint configuration with Ti inserts, which allows the increase of the bonded area of similar materials at the expense of that of dissimilar ones, is suggested. The force to failure for the specimens with the suggested configuration was about two fold higher than that for the specimens with the planar interface. Microstructural analysis of the joints, fracture characterization, mechanical testing and finite elements simulation were conducted in order to elucidate the mechanical behavior of the suggested joint configuration. Failure of joints with Ti-inserts is governed by the brittle behavior of TiC and α-Ti layers, located at the steel–Ti interface, with cracks initiation within the interfacial layer. The cracks propagate first into the brittle α-Ti phase and then cause the fracture in the Ti/Ti bond. The results of finite elements analysis are in good agreement with the analysis of the fracture surfaces.     

Microstructures and mechanical properties of BP/7A04 Al matrix composites

The microstructures and interface structures of basalt particle reinforced 7A04 Al matrix composites (BP/7A04 Al) were analyzed by using OM, TEM, SEM and EDS, and the mechanical properties of 7A04 Al alloy were compared with those of BP/7A04 Al matrix composites. The results show that the basalt particles are dispersed in the Al matrix and form a strong bonding interface with the Al matrix. SiO₂ at the edge of the basalt particles is continuously replaced by Al₂O₃ formed in the reaction, forming a high-temperature reaction layer with a thickness of several tens of nanometers, and Al₂O₃ strengthens the bonding interface between basalt particles and Al matrix. The dispersed basalt particles promote the dislocation multiplication, vacancy formation and precipitation of the matrix, and the precipitated phases mainly consist of plate-like η (MgZn₂) phase and bright white band-shaped or ellipsoidal T (Al₂Mg₃Zn₃) phase. The bonding interface, high dislocation density and dispersion strengthening phase significantly improve the mechanical properties of the composites. The yield strength and ultimate tensile strength of BP/7A04 Al matrix composites are up to 665 and 699 MPa, which increase by 11.4% and 10.9% respectively compared with 7A04 Al alloy without basalt particles.     

Microstructures of electron beam welding joint of CNTs/ Al composites

Carbon nanotube(CNT) reinforced aluminum metal matrix composites were welded by electron beam welding and the microstructures of welded joints were investigated.The result showed that the interfacial reaction happened between the CNTs and Al matrix,which resulted in producing brittle Al4C3 compounds in electron beam welds.The extent of interfacial reaction varies gradually in the depth and width direction.The length of the reactants Al4C3 became short duo to the temperature gradient in the molten pool.The quantity and size of Al4C3 compounds increased with the increase of beam current and the decrease of welding speed in the middle zone of weld.However,no needle-like phase Al4C3was observed in HAZ.     

WCp/Ti-6Al-4V graded metal matrix composites layer produced by laser melt injection

The laser melt injection (LMI) process was explored to produce WC particles (WC_p) reinforced Ti-6Al-4V metal matrix composites (MMC). In particular monocrystalline WC powder was used as injection particles to avoid the intercrystalline cracking often observed in granular or cast WC_p reinforced MMC. WC_p were injected into the extended part of the melt pool just behind the laser beam. The process allowed for the minimization of the WC_p dissolution caused by the direct irradiation of the laser beam, and the decomposition reaction between WC_p and Ti melt. Different parameters were applied, and a processing window of LMI was obtained. WC_p exhibit a graded distribution along the depth direction of the MMC layer. New phases such as TiC and W₂C are observed in the MMC layer, in which TiC is the predominant phase. TiC grains present a continuous decrease in both amount and size with the distance from the surface to the bottom of the MMC layer. Two types of reaction layers around WC_p can be distinguished, namely an irregular reaction layer and a cellular reaction layer. The growth and final morphology of reaction layers are most likely being dominated by the composition of the neighbouring melt pool. A gradual hardness distribution in the depth direction of the composites layer is observed. Moreover, the transition from the MMC layer to the substrate also exhibits a gradual change in the hardness.