Brazeability of the 6061-T6 aluminum alloy with Al-Si-20Cu-based filler metals

The bond strength of the 6061-T6 aluminum alloy brazed with Al-12Si, Al-9.6Si-20Cu, and Al-7Si-20Cu-2Sn filer metals at a low temperature of 550°C is evaluated. The fractography of these brazements after tensile tests was observed using scanning electron microscopy (SEM). It was found that joints with good integrity can be produced with Al-7Si-20Cu-2Sn filler metal because it can be used in a temperature range of 504 to 526 °C, about 70 °C lower than the traditional Al-12Si filler metal. It was shown that joints of 6061-T6 aluminum alloy as the base metal, when brazed at 550 °C for 60 min using this new filler metal and ward, and after being subjected to a T6 treatment, possessed a high bonding strength of about 121 Mpa.     

Improving the strength of brazed joints with in situ synthesized TiB whiskers

TiB whiskers have been in situ synthesized as reinforcements in 73Cu-27Ti (wt.%) active brazing filler alloy used for the joining of Al₂O₃ and Ti-6Al-4V alloy. The results show that TiB whiskers served as an effective reinforcement phase aid to decrease the residual stress and increase the shear strength of joints. The shear strength of the joint, containing 30 vol.% TiB whiskers was about 239% higher than that of the joint brazed without TiB whiskers.     

Brazeability of a 3003 Aluminum alloy with Al-Si-Cu-based filler metals

Al-Si-Cu-based filler metals have been used successfully for brazing 6061 aluminum alloy as reported in the authors’ previous studies. For application in heat exchangers during manufacturing, the brazeability of 3003 aluminum alloy with these filler metals is herein further evaluated. Experimental results show that even at such a low temperature as 550 °C, the 3003 alloys can be brazed with the Al-Si-Cu fillers and display bonding strengths that are higher than 77 MPa as well. An optimized 3003 joint is attained in the brazements with the innovative Al-7Si-20Cu-2Sn-1Mg filler metal at 575 °C for 30 min, which reveals a bonding strength capping the 3003 Al matrix.     

Microstructure, residual stresses and shear strength of diamond-steel-joints brazed with a Cu-Sn-based active filler alloy

Brazing of diamonds is important in grinding technology. The brazing parameters can strongly influence the grinding tool's performance. In this work a Cu-Sn-based active filler alloy (73.9 Cu-14.4 Sn-10.2 Ti-1.5 Zr, wt.%) was applied to join monocrystalline block-shaped diamonds onto a stainless steel substrate using three different brazing temperatures (880, 930 and 980 °C) and two different dwell times (10 and 30 min), respectively. The characteristics of the joints were investigated by means of scanning electron microscopy and energy dispersive X-ray spectroscopy (microstructure and phase composition), by Raman-spectroscopy (residual stress) as well as by shear testing (bond strength). The microstructural investigations revealed an intermetallic interlayer of type Fe₂Ti at the steel-filler alloy interface, which grew with increasing brazing temperatures and longer dwell durations. The brazing parameters strongly affected the residual stresses in the diamond. Compressive residual stresses with a maximum value of − 350 MPa were found in the samples brazed at 880 and 930 °C, whereas tensile stresses of maximum + 150 MPa were determined in samples joined at 980 °C. The effect of the brazing parameters on the shear strength is very pronounced. The shear strength decreased from (321 ± 107) MPa at 880 °C, 10 min to (78 ± 30) MPa at 980 °C, 30 min.     

Low-melting-point titanium-base brazing alloys—part 1: Characteristics of two-, three-, and four-component filler metals

The melting point, microstructure, phase, and electrochemical behavior of Ti-21Ni-15Cu alloy, together with two-, three-, and four-component low-melting-point titanium-base brazing alloys, are presented in this paper. Five filler metals were selected for the study, in which melting points were measured by differential thermal analysis, phases identified by x-ray diffractometry, and corrosion behaviors tested by potentiodynamic polarization. The experimental results show that the three-component Ti-15Cu-15Ni and the newly developed Ti-21Ni-14Cu alloys exhibit the combination of lower melting point and superior corrosion resistance compared to the two-and four-component titanium alloys, 316L stainless steel, and a Co-Cr-Mo alloy in Hank’s solution at 37 °C. On a short time basis, the presence of Ti₂Ni and Ti₂Cu intermetallics in the Ti-15Cu-15Ni and Ti-21Ni-14Cu alloys should not be preferentially dissolved in galvanic corrosion with respect to the dissimilar Ti-6Al-4V alloy.     

Cu + TiB2 composite filler for brazing Al2O3 and Ti-6Al-4V alloy

Al₂O₃ and Ti-6Al-4V alloy were brazed using Cu + TiB₂ composite filler, which manufactured by mechanical milling of Cu and TiB₂ powders. Typical interface microstructure of joint was Al₂O₃/Ti₄(Cu,Al)₂O/Ti₂Cu + Ti₃Al + Ti₂(Cu,Al)/Ti₂(Cu,Al) + AlCu₂Ti/Ti₂Cu + AlCu₂Ti + Ti₃Al + Ti₂(Cu,Al) + TiB/Ti(s.s) + Ti₂Cu/Ti-6Al-4V alloy. Based on temperature- and time-dependent compositional change, the formation of intermetallics in joint was basically divided into four stages: formation of interfacial Ti₄(Cu,Al)₂O in Al₂O₃ side, formation of Ti₂Cu, Ti₃Al, TiB, Ti₂Cu, and AlCu₂Ti in layers II and IV, formation of Ti₂(Cu,Al) and AlCu₂Ti in layer III, formation of Ti + Ti₂Cu hypereutectoid organization adjacent to Ti-6Al-4V alloy. TiB in situ synthesized in joint not only acted as low thermal expansion coefficient reinforcement to improve the mechanical properties at room temperature, but also as skeleton ceramic of joint to increase high temperature mechanical properties of Al₂O₃/Ti-6Al-4V alloy joint increasing. When the joint containing 30 vol.% TiB brazed at 930 °C and 10 min of holding time, the maximum room temperature shear strength of joint was 96.76 MPa, and the high temperature shear strength of joint was 115.16 MPa at 800 °C.     

Interfacial Microstructure Evolution and Shear Strength of Titanium Sandwich Structures Fabricated by Brazing

The corrugated sandwich structure, consisting of a CP Ti (commercially pure titanium) core between two Ti-6Al-4V face sheets, was brazed using pasty Ti-37.5Zr-15Cu-10Ni as filler alloy, at the temperature of 870°C for 5, 10, 20, and 30 min. The effect of brazing time on the microstructure and elemental distribution of the brazed joints was examined by means of SEM, EDS, and XRD analyses. It was found that various intermetallic phases were formed in the brazed joints, following a brazing time of 5 min, and their contents were decreased by the increment of brazing time, while prolonged brazing time resulted in a fine, acicular Widmanstätten microstructure throughout the entire joint. In addition, shear testing was performed in the brazed corrugated specimens in order to indirectly assess the quality of the joints. The debonding between CP Ti and Ti-6Al-4V was observed in the specimen brazed for 5 min and the fracture of the CP Ti corrugated core occurred after 30 min of brazing time. Additionally, when brazed for 10 min or 20 min, brittle intermetallic compounds in the joints and the grain growth of the base metal were controllable. Therefore, the sandwich structures failed without debonding in the joints or fracture within the base metal, demonstrating a good combination of strength and ductility.     

An investigation on oxidation wear mechanisms of Ti-46Al-7Nb-0.7Cr-0.1Si-0.2Ni intermetallic-based alloys

The results of investigation on oxidation wear mechanism of Ti-46Al-7Nb-0.7Cr-0.1Si-0.2Ni-based intermetallic alloy are presented. Oxidation was carried out in air at temperatures: 900 °C, 925 °C and 975 °C taking into account the micro-geometry of surfaces being heated and oxidized. It was determined that the rise of surface roughness to Ra = 5.8 μm definitely reduces scale of chipping in higher temperature (975 °C). Investigation of the material structure of the specimen and chemical composition of oxidation products was performed. Possibilities of an increase in the heat resistance of the tested alloy by means of application of high roughness surfaces were shown.     

Mechanical and electrochemical characterization of Ti-12Mo-5Zr alloy for biomedical application

We have fabricated a new β metastable titanium alloy that comprised of non-toxic elements Mo and Zr. Ingot with composition of Ti-12Mo-5Zr is prepared by melting pure metals in a vacuum non-consumable arc melting furnace. The alloy is then homogenized and solution treated under different temperature. The alloy is characterized by optical microscopy, X-ray diffraction, tensile tests and found to have an acicular martensitic α″ + β structure and dominant β phase for the 1053 K and 1133 K solution treatment samples, respectively. The elastic modulus of the latter is about 64 GPa, which is much lower than those of pure Ti and Ti-6Al-4V alloy. In addition, it had moderate strength and much higher microhardness as compared with Ti-6Al-4V alloy. The results show better mechanical biocompatibility of this alloy, which will avoid stress shielding and thus prevent bone resorption in orthopedic implants applications. As long-term stability in biological environment is required, we have also evaluated the electrochemical behavior in a simulated body fluid (Hank's solution). Potentiodynamic polarization curves exhibits that the 1133 K solution treatment Ti-12Mo-5Zr sample has better corrosion properties than Ti-6Al-4V and is comparable to the pure titanium. The good corrosion resistance combined with better mechanical biocompatibility makes the Ti-12Mo-5Zr alloy suitable for use as orthopedic implants.     

The oxidation of two ternary Fe-Cu-10 at.% Al alloys in 1 atm of pure O2 at 800-900 °C

The oxidation of two ternary Fe-Cu-Al alloys containing 10 at.% Al (Fe-65Cu-10Al and Fe-30Cu-10Al) has been studied at 800-900 °C under 1 atm O₂. Under all conditions both alloys show an initial faster stage during which Fe-65Cu-10Al corrodes more rapidly at 800 °C than at 900 °C, while Fe-30Cu-10Al follows nearly identical kinetics at both temperatures. As oxidation proceeds, a continuous alumina layer is eventually established on the surface of the two alloys, thus decreasing significantly their oxidation rates. Altogether, the Fe-rich alloy Fe-30Cu-10Al oxidizes slightly faster than the Cu-rich alloy Fe-65Cu-10Al at both temperatures. The possible reasons for the decrease in the critical Al content needed to form external alumina scales for the Cu-rich alloy in comparison with binary Cu-Al alloys are examined.     

TLP bonding of Ti-6Al-4V and Mg-AZ31 alloys using pure Ni electro-deposited coats

Transient liquid phase (TLP) bonding of Mg-AZ31 and Ti-6Al-4V alloys was performed using pure thin Ni electro-deposited coat interlayer (12 μm). The effect of bonding temperature, time and pressure on microstructural developments and subsequent mechanical properties across joint interface was studied at a temperature range from 500 to 540 °C, bonding time from 1 to 60 min and bonding pressure from 0 to 0.8 MPa. The mechanisms of bond formation varied across the joint region, with solid-state diffusion dominant at the Ti-6Al-4V interface and eutectic diffusion at the Mg-AZ31 interface. Joint microstructure was examined by scanning electron microscopy (SEM), and energy dispersive spectroscopy (EDS). X-ray diffraction (XRD) was used to detect the formation of intermetallic phases at the fracture surface. The maximum joint shear strength of 61 MPa was obtained at a temperature of 520 °C, 20 min and at a bonding pressure of 0.2 MPa. This joint strength was three times the bond strength reported for joints made using adhesives and represents 50% of the Mg-AZ31 alloy shear strength.     

High-temperature oxidation behavior of Nb-Si-Cr alloys with Hf additions

The oxidation behavior of two alloys from the Nb-Si-Cr system containing hafnium has been investigated under isothermal and cyclic conditions. Nb-20Si-20Cr-(5,10)Hf alloys (composition in atomic percent) were exposed to air for 24 and 168 h over a range of temperatures from 700 °C to 1400 °C. A gravimetric method was used to determine the oxidation kinetics; weight gain per unit area as a function of temperature or time. Computed isothermal sections of the quaternary Nb-Si-Cr-Hf phase diagrams were used for alloy selection. XRD, SEM and EDS were used to characterize the phases present in the oxidation products and the alloys. Oxidation experiments revealed extremely good oxidation resistance at 700 °C and 800 °C and above 1200 °C under isothermal conditions for both alloys. Partial pesting was observed when the samples were exposed to 800 °C. Complete oxide formation was observed above 1000 °C for 5Hf and above 900 °C for 10Hf up to 1200 °C. Beneficial effects have been observed with the addition of 10Hf to the alloy compared to 5Hf at 700 °C, 1200 °C and 1300 °C resulting in a reduction of weight gain per unit area.     

High temperature scaling of Ni-xSi-10 at.% Al alloys in 1 atm of pure O2

The oxidation of Ni-xSi-10Al alloys (with x = 0, 2, 4 and 6 at.%), has been studied at 900 and 1000 °C in 1 atm of pure O₂ to examine the effect of different silicon additions on the behavior of ternary Ni-Si-10Al alloys. The kinetic curves of Ni-10Al are approximately parabolic at both 900 and 1000 °C. Conversely, the kinetics of the ternary alloys at both temperatures correspond generally to a rate decrease faster than predicted by the parabolic rate law, except for the oxidation of Ni-6Si-10Al at 1000 °C, which exhibits a single nearly-parabolic stage. Oxidation of the binary alloy formed at both temperatures an internal oxidation zone beneath a layer of NiO. Oxidation of Ni-2Si-10Al at both temperatures and of the other two alloys at 900 °C formed initially a zone of internal oxidation of Al + Si. However, a layer of alumina forming at the front of internal oxidation after some time blocked the internal oxidation and produced a gradual conversion of the metal matrix of this region into NiO, with a simultaneous decrease of the oxidation rate. Conversely, the oxidation of Ni-4Si-10Al and Ni-6Si-10Al at 1000 °C did not produce an internal oxidation, but formed an alumina layer directly on the alloy surface after an initial stage when also Ni was oxidized. Therefore, silicon exerts the third-element effect by reducing the critical Al content needed for the transition from its internal to its external oxidation with respect to the corresponding Ni-Al alloy. This result is interpreted by means of an extension to ternary alloys of Wagner’s criterion for the same transition in binary alloys based on the attainment of a critical volume fraction of internal oxide.     

Effects of cooling air temperature on cryogenic machining of Ti-6Al-4V alloy

This paper presents experimental investigations on influence of different coolant strategies such as dry, wet, minimum quantity lubrication (MQL) and MQL with cooling air on performance in milling of the Ti-6Al-4V alloy with uncoated cemented carbide inserts. Cutting force, tool wear, surface roughness and chip morphology are experimentally studied to compare the effects of different cooling air temperatures. The results showed that minimum quantity lubrication (MQL) with cooling air significantly reduces cutting force, tool wear and surface roughness. Unfortunately, MQL (without cooling air) condition cannot produce evident effect on cutting performance, and flaking wear on the flank surface of the insert has been found under this condition. Four different cooling air temperatures are used to investigate the effects of cooling air temperature on the machinability characteristics of Ti-6Al-4V alloy. Based on the experimental results, MQL with cooling air of −15 °C provides more favourable effects compared to other cooling air temperatures (0 °C, −30 °C,−45 °C). Short chips are produced under MQL with cooling air conditions due to the high velocity of cooling air enhances the chip brittleness for easy chip breaking, and the effective penetration of lubricant to the chip-tool interface results in lower friction. However, due to the dramatic increase in chip hardness at lower temperature, MQL with cooling air environments cannot promote chip curl to some extent.     

Rolling of Plates and Sheets from As-Cast Ti-6Al-4V-0.1B

Trace boron addition (~0.1 wt.%) to conventional titanium alloys reduces the as-cast prior-beta grain size by an order of magnitude to about 200 μm, a grain size typically observed after ingot breakdown. In this study, the feasibility of producing plate and sheet by hot rolling of as-cast Ti-6Al-4V-0.1B (wt.%) was evaluated. Starting from an initial thickness of 25 mm, as-cast Ti-6Al-4V-0.1B was successfully rolled to 2 mm sheet in a multistep rolling process. As-cast Ti-6Al-4V (without boron addition) rolled under similar conditions exhibited severe cracking. Tensile properties of the sheets and plates made from the boron-containing alloy met or exceeded AMS 4911 specifications for Ti-6Al-4V plates and sheets produced by conventional processing route. The process of making plate and sheet stock from cast titanium alloy ingots, without recourse to expensive ingot breakdown, can significantly reduce the number of expensive and time-consuming processing steps for making titanium alloy components, thereby enhancing the affordability and expanding the range of titanium applications.     

Titanium alloys after surface gas nitriding

Experimental studies using differential scanning calorimetry (DSC) for nitriding of four titanium-alloys near α Ti-8Al-1Mo-1V, near α Ti-6Al-2Sn-4Zr-2Mo, α + β Ti-6Al-4V and near β Ti-10V-2Fe-3Al at different temperatures and for different periods of time are presented. The X-ray diffraction (XRD) technique was used in order to study the phase transformations that occur during gas nitriding. As a result of the nitrogen interaction, a nitrided layer was formed that consists of titanium nitrides, followed by an interstitial solution of nitrogen in the hcp α titanium phase. The microstructural changes of these alloys in relation to the alloy composition and processing parameters were studied. It was found that the microstructure of alloys nitrided at temperatures below their β transus temperatures for various periods of time is uniform and homogeneous. With the increase of the temperature above their β transus temperatures the microstructure changes to irregular. Microindentation hardness testing using a Knoop indenter was conducted on the nitrided titanium alloys to analyse their hardness evolution in relation to the nitriding processing parameters and alloy composition. It was found that the microhardness increases with the increase of the temperature and time of nitriding. The surface morphology of the Ti-6Al-2Sn-4Zr-2Mo alloy in relation to the nitriding processing parameters was analysed.     

Vacuum brazing Mo using Ti-Ni-Nb braze alloys

A novel approach of brazing Mo using three clad Ti-Ni-Nb foils, 40Ti-35Ni-25Nb, 50Ti-35Ni-15Nb and 60Ti-15Ni-25Nb in wt.%, has been performed in the experiment. Similar microstructural evolution of the joint is observed for three foils. The joint using 60Ti-25Ni-15Nb foil brazed at 1250 °C for 600 s demonstrates the highest bending strength of 526 MPa. The clad Ti-Ni-Nb foils show potential in brazing Mo for industrial application.     

Interactions between Nb-silicide based alloy and yttria mould during directional solidification

Pure yttria moulds have been prepared for directional solidifications of Nb-22Ti-16Si-6Hf-2Al-2Cr (at. %) alloy with a withdrawal rate of 1 × 10^-4 m s^-1 at 1850 °C and 1900 °C. This study demonstrates that a mild interface reaction occurred between the yttria mould and hafnium, the most reactive element in the alloy. A sequential reaction layer of HfO₂ and Y₂O₃ was formed at metal-mould interface, and its thickness depended on both heating temperature and holding time. HfO₂-Y₂O₃ inclusions were dispersed in the metal matrix, and the majority of them concentrated around the solid-liquid interface rather than in steady state growth region. Some inclusions were merged with each other. The reaction mechanism between Nb-silicide based alloy and yttria mould also has been described.     

A comparative study on titania layers formed on Ti, Ti-6Al-4V and NiTi shape memory alloy through a low temperature oxidation process

For improving the bioactivity and biocompatibility of metals for medical applications, anatase titania layers were synthesized on Ti, Ti-6Al-4V and NiTi shape memory alloy (SMA) using the H₂O₂-oxidation and hot water aging treatment method at 80 °C. The thickness of the titania layers on Ti, Ti-6Al-4V and NiTi SMA was 7.43 ± 0.93 μm, 3.14 ± 0.38 μm and 4.04 ± 0.25 μm, respectively. X-ray diffraction (XRD) and transmission electron microscopic (TEM) analysis indicated that the titania layers formed were poorly crystalline anatase. Fourier transform infrared spectroscopy (FTIR) suggested that abundant Ti-OH functional groups were produced on titania, which could improve bioactivity of the metals. In addition, the titania layer formed on Ti substrate was shown to contain more molecularly chemisorbed water and Ti-OH functional groups than those on Ti-6Al-4V and NiTi SMA. Atomic force microscopic (AFM) results showed that the surface roughness values of metal samples depended on the scanning size and that surface roughness of samples significantly increased after the H₂O₂-oxidation and hot water aging treatment for all three metals. Compared to Ti-6Al-4V and NiTi SMA, the H₂O₂-treated and aged Ti samples exhibited the roughest surface. The wettability of samples was evaluated through water contact angle measurements. After the H₂O₂-oxidation treatment, the three metals exhibited high hydrophilicity. The bonding strength of titania layers on Ti, Ti-6Al-4V and NiTi was also investigated. Potentiodynamic polarization tests indicated that the corrosion resistance of H₂O₂-treated and aged Ti, Ti-6Al-4V and NiTi SMA was significantly improved due to the titania layer formation.     

High-Temperature Deformation Behavior of Ti-6Al-4V Alloy without and with Hydrogenation Content of 0.27 wt.%

Isothermal compression of Ti-6Al-4V alloy without and with hydrogenation content of 0.27 wt.% was carried out on Gleeble-1500D thermal simulation machine at deformation temperature between 760 and 1000 °C and strain rate from 0.001 to 1 s⁻¹. The experimental results show that hydrogenation can decrease the deformation temperature or increase the strain rate of Ti-6Al-4V alloy. The apparent activation energy was determined to be 667 kJ mol⁻¹ for isothermal compression of the Ti-6Al-4V alloy without hydrogenation content of 0.27 wt.% in the α + β phase region (760-960 °C), and this value was about 655 and 199 kJ mol⁻¹ for the alloy with 0.27 wt.% of hydrogenation content in the α + β phase region (760-840 °C) and β phase region (840-960 °C), respectively. Constitutive equation was developed for the high-temperature deformation of Ti-6Al-4V alloy both without and with hydrogenation content of 0.27 wt.%.