Superplastic Property of the Ti–6Al–4V Alloy with Ultrafine‐Grained Heterogeneous Microstructure

Ti–6Al–4V alloy having a heterogeneous microstructure composed of ultrafine‐equiaxed‐α‐grains and fine‐lamellar‐α‐grains is investigated for microstructural changes during superplastic deformation at temperature of 700 °C. The Ti–6Al–4V alloy having an optimum fraction of fine‐lamellar‐α‐grains exhibits an excellent superplastic property and the highest elongation of 583% (tested at 700 °C 10^?3 s^?1). This is mainly due to the optimized activation of grain‐boundary‐sliding and additional accommodation mechanism associated with frequent occurrences of dynamic recrystallization and β precipitation at boundaries during deformation of the heterogeneous starting microstructure. The present result suggests the possibility that optimizing the starting microstructure so as to have an optimum heterogeneous‐microstructure serves as an additional stress accommodation mechanism and leads to a large superplastic elongation.

     

Influence of Parent Metal Microstructure and Post‐Weld Heat Treatment on Microstructure and Mechanical Properties of Linear Friction Welded Ti‐6Al‐4V Joint

Linear friction welding (LFW) of Ti‐6Al‐4V (Ti‐64) titanium alloy with a different microstructure from the previous study was conducted. The joint microstructure, tensile strength and fracture characteristics were studied. Results show that the microstructure of parent metal has a strong influence on the microstructure and properties of LFW Ti‐64 joints. Under the specific conditions in this study, unexpected spheric α grains were formed near the bondline, which leads to the steep drop of the joint tensile strength. Although the tensile strength could be improved through post‐weld heat treatment, the failure of specimens still took place across the bondline with the cleavage fracture due to the formation of basket‐weave structure at the weld center zone and the remaining of spheric α grains.     

Diffusion bonding between Ti‐6Al‐4V alloy and interstitial free steel

This work deals with diffusion bonded joints between Ti–6Al–4V alloy and interstitial free (IF) steel at different temperatures under a pressure of 5 MPa for 30 min. The effect of bonding temperature on the microstructure and mechanical properties of the joint interface was investigated using optic microscopy, a scanning electron microscope (SEM) equipped with X‐ray energy dispersive spectrometer (EDS) and shear strength measurements. The intermetallic phases such as FeTi and Fe₂Ti occurred at the interface of bonded specimens. In addition, it was seen that shear strength of bonded specimens decreased with increasing temperature due to growing intermetallics.     

The Influence of a Small Boron Addition on the Microstructure and Mechanical Properties of Ti‐6Al‐4V Fabricated by Metal Injection Moulding

The effect of boron additions on the sintering behavior, microstructural development, and mechanical properties of a Ti‐6Al‐4V alloy fabricated by metal injection moulding (MIM) was studied. The addition of boron promotes a significant refinement of the microstructure by changing the microstructure from the typical lamellar to a more equiaxed morphology. The presence of both features: α colonies and α grains were confirmed by electron backscatter diffraction (EBSD) experiments. Furthermore, the pinning effect of TiB particles on grain boundary motion enhances the densification process due to the fact that the separation of pores and grain boundaries is suppressed. As a result of the refinement of the microstructure achieved by adding 0.5 wt% boron to the Ti‐6Al‐4V alloy, excellent tensile (σ_0.2 = 787 MPa, UTS = 902 MPa and ε = 12%) and fatigue (endurance limit = 640 MPa) properties were obtained.     

Microstructure and superplasticity of laser welded Ti–6Al–4V alloy

In this paper laser beam welding (LBW) was used to join Ti–6Al–4V alloy as a pre-forming operation before superplastic deformation (SPF) process. Superplastic deformation behavior of laser welded Ti–6Al–4V alloy was investigated. The results indicated that the welded Ti–6Al–4V alloy had good superplasticity when deformed at temperature range of 870–920 °C and strain rate range of 10⁻³–10⁻² s⁻¹, and the elongation was 233–397%. The microstructure observation indicated that dynamic recrystallization happened in the weld bead, and the acicular structure of weld bead was transforming into equiaxed grains during tensile process.     

TC4钛合金板材电子束焊接疲劳性能

研究了不同工艺电子束焊接TC4钛合金板材的疲劳性能及断口金相组织。结果表明:工艺为V=150kV、If=366mA、v=600mm/min、Ib=69mA的试样焊缝的疲劳性能最好;气孔和冷隔作为应力集中源起作用而易成为裂纹源;焊缝及母材断口均由小断块组成,断块上有典型的疲劳辉纹;工艺为V=90kV、If=1654mA、v=600mm/min、Ib=51.1mA和V=150kV、If=342mA、v=200mm/min、Ib=29mA的试样焊缝疲劳辉纹间距较母材大,即扩展速率较母材快,疲劳性能不如母材;焊缝柱状晶内的网篮状马氏体组织越粗大,裂纹扩展性能越差;裂纹的扩展方式主要是穿晶断裂,相邻柱状晶内的网篮状组织取向差别太大,穿晶扩展受阻,裂纹便改沿晶扩展。     

Mechanical Properties of Friction Stir Welded Mg/Mg‐ and Mg/Al‐Joints

In a mobile world weight reduction is a predominant target of innovative products. In this context appropriate joining techniques are necessary for the integration of lightweight metals in complex mechanical components. Friction stir welding (FSW) is a newly established well suited process to realize high‐quality lightweight metal joints in solid state. In a research project of WKK the friction stir weldability of similar joints using die casted AZ91‐Mg‐alloy and MRI‐Mg‐alloys was investigated. Additionally the joining of hybrid joints between AZ91 and AA5454 aluminum alloy was performed. To describe and optimize the FSW‐process the welding temperatures and welding forces were recorded online during the process. The investigations of the monotonic properties of AZ91/AZ91‐joints and MRI/MRI‐joints yielded in tensile strength values at the level of the parent materials. For dissimilar joints an extreme increase of the nugget hardness was measured. By SEM investigations and EDX element mappings it could be proved that this is caused by intermetallic phases positioned as thin interlayers in the contact area between the Mg‐ and the Al‐alloy. As a consequence, in hybrid joints failures occur predominantly along these interlayers. Finally, for similar and dissimilar welds corrosion experiments in 5 mass% NaCl solution were carried out. The investigations showed that the nugget area was more susceptible to corrosion then the base material. To understand the corrosion behavior the affected areas were analyzed using SEM and EDX.     

Prediction of short fatigue crack growth of Ti‐6Al‐4V

It is observed that the short fatigue cracks grow faster than long fatigue cracks at the same nominal driving force and even grow at stress intensity factor range below the threshold value for long cracks in titanium alloy materials. The anomalous behaviours of short cracks have a great influence on the accurate fatigue life prediction of submersible pressure hulls. Based on the unified fatigue life prediction method developed in the authors' group, a modified model for short crack propagation is proposed in this paper. The elastic–plastic behaviour of short cracks in the vicinity of crack tips is considered in the modified model. The model shows that the rate of crack propagation for very short cracks is determined by the range of cyclic stress rather than the range of the stress intensity factor controlling the long crack propagation and the threshold stress intensity factor range of short fatigue cracks is a function of crack length. The proposed model is used to calculate short crack propagation rate of different titanium alloys. The short crack propagation rates of Ti‐6Al‐4V and its corresponding fatigue lives are predicted under different stress ratios and different stress levels. The model is validated by comparing model prediction results with the experimental data.     

Formation of Ultrafine‐Grained Microstructure of Ti–6Al–4V Alloy by Hot Deformation of α′ Martensite Starting Microstructure

We have presented a formation of ultrafine‐grained microstructure (d_α ≈ 0.2 µm) of industrial Ti–6Al–4V alloy produced by the hot compression of a sample with the acicular α′ martensite starting microstructure. The hot‐deformation behavior was different from the case of the conventional (α + β) starting microstructure, that is, the phase transformation of α′/(α + β) during hot working enhanced the microstructural conversion, especially under the conditions of a low temperature and a high‐strain rate.     

钛合金激光焊及其接头的显微组织与力学性能

采用光纤激光器对3.5 mm厚TC4合金板材进行焊接,并对焊接接头的显微组织与力学性能进行分析测试,确定了试验条件下的最佳激光焊参数。结果表明,焊缝熔合区组织主要为针状α′马氏体及少量β相,热影响区由α相及少量α′马氏体组成;接头区域的显微硬度在熔合区变化平缓,而热影响区的硬度下降明显。在激光功率为4.0 kW、焊接速度为3.0 m/min时,获得接头的力学性能最佳,焊缝强度与母材本身的强度接近。接头拉伸断口表面存在大量韧窝,呈明显的韧性断裂特征。     

Microstructure‐oriented characterisation of the cyclic deformation behaviour of Ti‐6Al‐4V

In this paper, the cyclic deformation behaviour of the titanium alloy Ti‐6Al‐4V is characterised in uniaxial stress‐ and total‐strain‐controlled load increase and constant amplitude tests at ambient temperature by means of mechanical stress‐strain hysteresis and temperature measurements. The measured physical values obviously show a pronounced interrelation with the underlying fatigue processes and represent the actual fatigue state. In selected experiments the influence of elevated temperatures on the cyclic deformation behaviour was investigated. Using the plastic strain amplitude and the change of the specimen temperature with the physically based lifetime calculation “PHYBAL” an excellent accordance with experimentally determined lifetimes could be realised. Microstructural changes were evaluated by transmission electron microscopy in defined fatigue states, additionally, the fracture surface was analysed by scanning electron microscopy.     

Fracture toughness and fatigue crack growth in Ti‐6Al‐4V friction stir welds

Friction stir welding of titanium holds the promise for producing joints with microstructures and mechanical properties that are more comparable to wrought material than traditional fusion welding processes. Extensive data exist on the microstructure and static mechanical properties of titanium friction stir welds, but very little are available on the durability (fatigue) and even less on the damage tolerance (fracture toughness and fatigue crack growth). This paper presents the results of an investigation into the damage tolerance of friction stir welds made in 6 mm thick Ti‐6Al‐4V after a post‐weld heat treatment. It was found that the apparent fracture toughness was lower than the wrought base material, 7–25% depending on the crack orientation relative to the weld, but the crack growth performance (ΔK vs. da/dN) of the weld in the absence of weld‐induced residual stresses was identical to the base material.     

Verhalten laserschockverfestigter und festgewalzter Randschichten der Ti‐Legierung Ti‐6Al‐4V bei schwingender Beanspruchung unter erhöhten Temperaturen

Residual stress stability and near‐surface microstructures in high temperature fatigued mechanically surface treated Ti‐6Al‐4V It is well known that mechanical surface treatments, such as deep rolling, shot peening and laser shock peening, can significantly improve the fatigue behavior of highly‐stressed metallic components. Deep rolling is particularly attractive since it is possible to generate, near the surface, deep compressive residual stresses and work hardened layers while retaining a relatively smooth surface finish. In the present investigation, the effect of deep rolling on the low‐cycle and high‐cycle fatigue behavior of a Ti‐6Al‐4V alloy is examined, with particular emphasis on the thermal and mechanical stability of the residual stress states and the near‐surface microstructures. Preliminary results on laser shock peened Ti‐6Al‐4V are also presented for comparison. Particular emphasis is devoted to the question of whether such surface treatments are effective for improving the fatigue properties at elevated temperatures up to ～450 °C, i.e., at an homologous temperature of ～0.4 T/T_m (where T_m is the melting temperature). Based on cyclic deformation and stress/life (S/N) fatigue behavior, together with the X‐ray diffraction and in situ transmission electron microscopy observations of the microstructure, it was found that deep rolling can be quite effective in retarding the initiation and initial propagation of fatigue cracks in Ti‐6Al‐4V at such higher temperatures, despite the almost complete relaxation of the near‐surface residual stresses. In the absence of such stresses, it is shown that the near‐surface microstructures, which in Ti‐6Al‐4V consist of a layer of work hardened nanoscale grains, play a critical role in the enhancement of fatigue life by mechanical surface treatment.     

Local strain accumulation in fatigue crack propagation process of Ti‐6Al‐4V alloy

Fatigue crack growth behaviours of the titanium alloy Ti‐6Al‐4V, with two different microstructures, at different maximum stresses were identified by digital image correlation technique. Full‐field strains were monitored around fatigue cracks after consecutive cycles in fatigue crack growth experiments. Results indicated that the Ti‐6Al‐4V alloy with a bi‐modal microstructure had a better fatigue resistance than that with a primary‐α microstructure. Typical behaviours of small cracks and the evolution of multi‐scale fatigue cracks were clarified. The strain accumulations around the micro‐notch and fatigue crack increased with increasing number of load cycles. On the basis of von Mises strain mapping, it was found that crack growth rate could be characterized by crack‐tip plastic zone size.