TC4 Side Research Articles

Microstructure and mechanical behavior of an annealed automatic gas tungsten arc weld joint of TA16 and TC4 titanium alloys

The joining of TA16 (Ti-2Al-2.5Zr, wt%) and TC4 (Ti-6Al-4V, wt%) titanium alloys was carried out using an automatic gas tungsten arc welding technique. The microstructures of different zones in the annealed weld joints were investigated, and the mechanical properties of the annealed weld joints were assessed using microhardness, uniaxial tensile and fracture toughness tests. An asymmetrical hardness profile across the annealed weld joint was obtained in the dissimilar titanium alloys weld joint with a higher hardness value in the heat-affected zone of TC4 side. The fracture location of welded tensile specimens was always at the heat-affected zone of TA16 side. Test results showed that the fracture toughness of the heat-affected zone of TA16 side was the highest, that of TC4 base material was the lowest, and the fracture toughness of other zones were as follows: TA16 base material > fusion zone > heat-affected zone of TC4 side. Scanning electron microscope examinations were performed on the fracture surface in order to identify the relevant fracture mechanisms of different zones in the annealed weld joint.

Microstructure and mechanical properties of cold metal transfer welding-brazing of titanium alloy (TC4) to stainless steel (304L) using V-shaped groove joints

Wire feed speeds of 3.5, 4.5, and 5.5 m/min and offset positions of 1 and 2 were employed for this study with an ERCuSi-A weld wire. The microstructures of the joints, which include a Cu/Ti interface layer consisting of Ti2Cu, TiCu, and AlCu2Ti, a Cu-matrix seam consisting of Cu and petal-shaped Fe-Si-Ti intermetallics, and a Cu/Fe interface layer consisting of α-Fe and Cu, were studied. The formation enthalpy calculated from the Miedema model can explained the microstructure evolution mechanism. The interface thickness and ultimate tensile strength were found to increase with wire feed speed. The highest tensile strength of the joint was 294 MPa, fracturing at the Cu/Ti interface. Offsetting the welding torch to the TC4 side increased the amount and size of the Fe-Si-Ti intermetallics, degrading the tensile strength. Four fracture modes were proposed to differentiate the crack propagations in the joints, which were determined by the interfacial bonding strength and the Fe-Si-Ti intermetallics in the weld seam.

Microstructure and mechanical properties of laser beam welded TC4/TA15 dissimilar joints

The microstructure and mechanical properties of laser beam welded dissimilar joints in TC4 and TA15 titanium alloys were investigated. The results showed that the coarse columnar grains containing a large amount of acicular α and martensite α′ were present in the fusion zone (FZ), some residual α phases and martensite structure were formed in the heat-affected zone (HAZ) on TC4 side, and bulk equiaxed α phase of the HAZ was on TA15 side. An asymmetrical microhardness profile across the dissimilar joint was observed with the highest microhardness in the FZ and the lowest microhardness in TA15 BM. The orders of yield strength and ultimate tensile strength were as follows: TC4 BM > TC4/TC4 similar joint > TA15 BM > TA15/TA15 similar joint > TC4/TA15 dissimilar joint, and increased while hardening capacity and strain hardening exponent decreased with increasing strain rate from 1×10−4 s−1 to 1×10−2 s−1. The TC4/TA15 dissimilar joints failed in the TA15 BM, and had characteristics of ductile fracture at different strain rates.

Vacuum brazing of C/C composite to TC4 alloy using nano-Al2O3 strengthened AgCuTi composite filler

Vacuum brazing of C/C composite to TC4 alloy was performed using nano-Al2O3 strengthened AgCuTi composite filler. The microstructure and main properties of the composite filler were characterized. The typical interfacial microstructure of brazed joint is TC4/diffusion layer/Ti–Cu intermetallic layers/Ag(s,s)+Cu(s,s)+TiCu/TiC/C/C composite. The effects of brazing temperature and addition of nano-Al2O3 on microstructure of brazed joints were investigated by SEM, EDS, TEM and XRD. The diffusion layer, Ti–Cu intermetallic layers and TiC layer thicken with the increasing temperature while the thickness of brazing seam decreases. The TiCu phase in brazing seam excessively develops and almost replaces the Ag(s,s) and Cu(s,s) when brazing temperature reaches 920°C. The addition of nano-Al2O3 improves the performance of brazed joints by inhibiting the growth of Ti–Cu layers adjacent to TC4 side and offering dispersive nucleation sites for TiCu phase in brazing seam. The joint brazed at 880°C for 10min exhibits the highest shear strength of 27.8MPa. The fracture positions of brazed joints are also discussed in this paper.

Low Temperature Diffusion Bonding of Ti-6Al-4V to Oxygen Free Copper with High Bonding Strength Using Pure Ag Interlayer

Abstract Diffusion bonding of Ti-6Al-4V (TC4) to oxygen free copper (OFC) at low temperature was carried out using silver foil as interlayer. Results show that the interlay can prevent the formation of brittle Ti-Cu compounds and improve the interfacial microstructure and bonding strength of the TC4/OFC joint. Moreover, adding the silver interlayer greatly lowers the bonding temperature. The composition of the joint from the TC4 side to the OFC side is TC4 substrate, AgTi compound, Ag interlayer, Ag-Cu solid solution and OFC substrate. The tensile strength of the joint bonded at 700 °C for 1 h under 10 MPa bonding pressure has an excellent value of 150 MPa, higher than those previously reported in literatures. Fracture failure takes place in the Ag/OFC interface and ductile fracture can be observed on the fracture surface. It is suggested that the ductile properties of the AgTi compounds are superior to that of Ag-Cu solid solution in this joint.

Strain-controlled fatigue properties of linear friction welded dissimilar joints between Ti–6Al–4V and Ti–6.5Al–3.5Mo–1.5Zr–0.3Si alloys

The purpose of this study is to evaluate the microstructure, microhardness and fatigue properties of linear friction welded (LFWed) dissimilar joints between Ti–6Al–4V (TC4 according to Chinese classification) and Ti–6.5Al–3.5Mo–1.5Zr–0.3Si (TC11) titanium alloys. A significant microstructure change across the dissimilar joint occurs after linear friction welding (LFW), with martensite in the weld zone (WZ) and small recrystallized grains in the thermo-mechanically affected zone (TMAZ) on the TC4 side. A characteristic asymmetrical hardness profile across the dissimilar joint is observed with significantly higher hardness values in the WZ, and no soft zone is present in the dissimilar joint. The LFWed dissimilar joint exhibits essentially symmetrical hysteresis loops and an equivalent fatigue life to the base metals, which increases with decreasing strain amplitude. While cyclic stabilization appears at lower strain amplitudes up to 0.6% for the joint, cyclic softening basically occurs at higher strain amplitudes. In the joint fatigued at a high strain amplitude of 1.2%, a short initial cyclic hardening occurs, corresponding to the presence of twinning and the resistance to the dislocation movement. Fatigue failure of the dissimilar joint occurs on the TC4 side and is far from the weld line, suggesting that a highly durable and sound dissimilar joint is achieved via the present solid-state LFW. Fatigue crack initiation occurs from the specimen surface or near-surface defect, and crack propagation is mainly characterized by fatigue striations which are perpendicular to the crack propagation direction, in conjunction with some secondary cracks.

Effects of heat treatment on microstructure and microhardness of linear friction welded dissimilar Ti alloys

A detailed investigation for the influence of post weld heat treatment (PWHT) on the microstructure of TC4 and TC17 dissimilar joints was analyzed. The fully transformed microstructure in the as-welded zone indicated that the peak temperature exceeded the β-transus temperature at the weld interface during linear friction welding. TC4 side was mainly composed of martensite α′ phase with random distribution and it was single β for that of TC17. In the thermomechanically affected zones of TC4 and TC17, the structure undergoes severe plastic deformation and re-orientation, yet without altering the phase fractions. After PWHT, in the weld zone of TC4 alloy, the phase transformation α′→ α+β occurred and the acicular α was coarsened, which resulted in a decrease in hardness. In the weld zone of TC17 alloy, fine α phase precipitated at the grain boundary and within β grains, which resulted in a sharp increase in hardness.

Brazing of carbon fiber reinforced SiC composite and TC4 using Ag–Cu–Ti active brazing alloy

Carbon fiber reinforced SiC (Cf/SiC) was successfully joined to TC4 with Ag–Cu–Ti alloy powder by brazing. Microstructures of the brazed joints were investigated by scanning electron microscope (SEM), energy dispersive spectrometer (EDS) and X-ray diffraction (XRD). The mechanical properties of the brazed joints were measured by mechanical testing machine. The results show that the brazed joint mainly consist of TiC, Ti3SiC2, Ti5Si3, Ag, TiCu, Ti3Cu4 and Ti2Cu reaction products. TiC+Ti3SiC2/Ti5Si3+Ti2Cu reaction layers are formed near Cf/SiC composite side while Ti3Cu4/TiCu/Ti2Cu/Ti2Cu+Ti reaction layers are formed near TC4 side. The thickness of reaction layers of the brazed joint increases with the brazing temperature or holding time increasing. The maximum shear strengths of the brazed joints at room temperature and 500°C with brazing temperature 900°C and holding time 5min are 102 and 51MPa, respectively.