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        Titanium Nature

        Characteristics of titanium and titanium alloy

        Titanium is a silvery white metal that has many excellent performance. The density of titanium is 4.54g / cm3, 43% lighter than steel, lighter than the prestigious magnesium slightly heavier. The mechanical strength of steel but with almost twice as large than aluminum, magnesium than five times larger. Titanium, high temperature, the melting point of 1942K, higher than gold near 1000K, higher than the steel near 500K. Titanium belongs to the chemical nature of lively metal. And non-metallic effect can O2, N2, H2, S, and halogen when heated. However, at room temperature, titanium dense surface oxide protective layer of very thin films easy to generate, can resist even aqua regia acid role, exhibit strong corrosion resistance. Thus, in general metal becomes riddled with acid, alkali, salt solution and titanium are safe and sound.

        Liquid titanium to dissolve almost all metals, it can form a variety of metals and alloys. Ti added steel obtained titanium steel tough and elastic. Titanium and metal Al, Sb, Be, Cr, Fe, etc. generated between interstitial compound or a metal compound. Aircraft titanium alloy aircraft heavier than other metals also made more passengers in more than 100 people. Submarine made both seawater corrosion resistance, but also anti-deep pressure, the dive depth increased by 80% compared to stainless steel submarine. Meanwhile, the non-magnetic titanium, will not be found in mines, with good anti guardianship role.

        Titanium has a "pro-biotic" sex. In the human body, can resist corrosion and non-toxic secretions, adapt to any sterilization methods. It is widely used in making medical equipment, making artificial hip, knee, shoulder, threatening joints, skull, active heart valves, bone clip. When new muscle annulus package these "titanium bone", these titanium bone began to maintain the body's normal activities. Titanium is widely distributed in the body, normal human in an amount per 70kg body weight does not exceed 15mg, its role is unclear. But titanium can stimulate phagocytic cells, the immune system has been proven to enhance this role.

        Surface treatment technology in titanium and titanium alloys

        Ease of titanium in the air O, H, N, and other elements of the compound embedded Si, Al, Mg and other elements at high temperature reaction, the casting surface layer is formed on the surface contamination, making it excellent physical and chemical properties deteriorate, hardness increases, plasticity, elasticity, fragility. Small density of titanium, so the liquid flow inertia titanium, titanium melting illiquid resulting casting flow rate. Casting mold temperature and the temperature difference (300 ℃) large, fast cooling, casting carried out in a protective atmosphere, titanium casting surface and internal porosity and other defects inevitably occur, a significant impact on the quality of the casting.
        Therefore, surface treatment with titanium castings is more important compared to other dental alloys, titanium due to the unique physical and chemical properties, such as small thermal conductivity, surface hardness, and low elastic modulus, viscous, low conductivity, easily oxidized etc., which is surface-treated titanium brings a great deal of difficulty using conventional surface treatment method is difficult to achieve the desired results. We must use special processing methods and means of operation.
        Post-casting surface treatment is not only to obtain a smooth glossy surface, such as reducing food and plaque accumulation and adhesion to maintain the balance of the patient's normal oral microflora, also increases the aesthetic dentures; more importantly, through these surface treatment and modification process, improve surface properties and suitability of casting, improve denture wear and corrosion resistance to stress and fatigue and other physical and chemical properties.

        First, to remove the surface reaction layer
        Surface reaction layer of titanium castings are the main factors affecting the physical and chemical properties of titanium castings before polishing, must achieve complete removal of surface contamination layer in order to achieve satisfactory polishing effect. By sandblasting pickling method can completely remove the surface reaction layer of titanium.
        1. Sandblasting: sandblasted titanium castings generally used coarse spray better corundum, sandblasting pressure smaller than those of non-precious metals, the general control of 0.45Mpa or less. Because, the injection pressure is too large, the surface of grains of sand titanium intense sparks, temperature can react with the titanium surface, the formation of secondary pollution affecting the surface quality. For 15 to 30 seconds, remove only the sticky sand casting surface, and the surface of the sintered layer and part of the oxide layer can be. The rest of the surface reaction layer structure chemical pickling method should be adopted quickly removed.
        2. Pickling: pickling fast surface reaction layer is completely removed, and no contamination of other surface elements. HF-HCl and HF-HNO3-based system can be used for pickling titanium pickling, but HF-HCl pickling line large hydrogen storage capacity, while the HF-HNO3 pickling lines small amount of hydrogen absorption can be controlled HNO3 reduce the concentration of hydrogen absorption, the surface is bright and treatment, the concentration of HF is generally around 3% to 5%, the concentration of HNO3 is appropriate about 15% to 30%.

        Second, the process of casting defects
        Internal shrinkage porosity and internal defects: can wait for hot isostatic pressing (hot isostatic pressing) removed, but the accuracy of dentures will have an impact, preferably after testing by X-ray, in addition to the exposed surface grinding pore with a laser welding. Surface porosity defects directly with local laser welding repair.

        Third, grinding and polishing
        1. Mechanical polishing: high chemical reactivity of titanium, low thermal conductivity, viscosity large, mechanical polishing grinding ratio is low, and easily react with abrasives, abrasive materials should not be used for grinding and polishing of titanium, preferably super good thermal conductivity hard abrasives, such as diamond, cubic boron nitride, polishing line speed is generally 900 ~ 1800m / min. is appropriate, otherwise, prone to the titanium surface grinding burns and microcracks.
        2. Ultrasonic grinding: by ultrasonic vibrations that are abrasive grinding and polishing surface and polished surface between the relative movement to achieve grinding and polishing purposes. The advantage is that less than a conventional rotary grinding tool ditch, nest and narrow parts become easy, but the larger castings grinding effect is still not satisfactory.
        3. Electrochemical Mechanical polishing: abrasive use of conductive, electrolyte and applying a voltage between the grinding wheel and the grinding surface, the joint action by mechanical and electrochemical polishing, to reduce the surface roughness to improve the surface gloss. The electrolyte is 0.9NaCl, voltage 5v, speed is 3000rpm / min., This method can only be milled plane, polishing complex denture frameworks is still in the research stage.
        4. Barrel polishing: use of centrifugal force grinding barrel revolution and rotation generated by the barrel to make dentures and abrasive friction relative movement serve to reduce the surface roughness of grinding purposes. Grinding automation, high efficiency, but only reduce the surface roughness can not improve the surface gloss, poor precision grinding, rough grinding and deburring can be used with dentures fine before polishing.
        5. Chemical polishing: chemical polishing is polished to achieve the purpose of leveling the metal oxidation-reduction reaction by chemical medium. The advantage is that the hardness of the metal and chemical polishing, polishing the area regardless of the shape of the structure, where the contact parts are polished with the polishing liquid, do not need special equipment complex, simple, more suitable for polishing complex structure of titanium denture stent. But chemical polishing difficult to control the process parameters required without affecting the accuracy of the denture denture situation can have a good polishing effect. Preferably titanium chemical polishing liquid HF and HNO3 is formulated by a certain percentage, HF is the reducing agent, can dissolve titanium metal, played the role of leveling, concentration <10%, HNO3 from oxidation, preventing excessive and titanium dissolved hydrogen absorption , and can produce light effects. Titanium slurry require high concentration, low temperature, short polishing time (1 ~ 2min.).
        6. Electropolishing: also known as polishing or electrochemical anodic dissolution polishing, due to the low conductivity of titanium oxide, strong performance, the use of water as an acidic electrolyte HF-H3PO4, HF-H2SO based electrolyte hardly polished titanium, after applying an external voltage, anodic oxidation of titanium immediately, leaving the anodic dissolution impossible. But the use of anhydrous chloride electrolyte at a low voltage, titanium has good polishing effect, a small mirror-polished specimens obtained, but for complex prosthesis still can not achieve the purpose of polished, perhaps by changing the shape of the cathode and cathode additional the method can solve this problem requires further study.

        Fourth, the surface modification of titanium
        1. Nitriding: using plasma nitriding, ion plating, ion implantation and laser nitriding and other chemical heat treatment technology in titanium denture surface golden TiN Permeation Layer, thereby improving the wear resistance of titanium, corrosion resistance and fatigue. But the technology is complex and expensive equipment for the surface modification of titanium denture is difficult to achieve clinical practical use.
        2. Anodizing: titanium anodizing technology easier, in some oxidizing medium, plus the applied voltage, the titanium anode can form a thick oxide film, thereby improving the corrosion resistance and abrasion resistance and weather resistance. Anodizing electrolyte commonly used H2SO4, H3PO4 aqueous and organic acids.
        3. Atmospheric oxidation: titanium in high temperature atmosphere can form a thick solid anhydrous oxide film on the titanium general corrosion, crevice corrosion are effective, method is relatively simple.

        Fifth, coloring
        In order to increase the beauty of titanium denture prevent titanium denture under natural conditions to oxidation discoloration, can be surface nitriding treatment, atmospheric oxidation and anodic oxidation surface coloring treatment, the surface formed yellow or golden yellow, titanium to improve denture beauty. Anodic oxidation method by using titanium oxide film interference effects of light, natural hair color, by changing the voltage in the titanium surface grooves colorful color formation.

        Six other surface treatments
        1: Surface roughening: In order to improve the bonding properties of titanium and veneer resin must be roughened titanium surface treatment to improve its bond area. Coarse sand is often used in clinical treatment, but blasting can cause pollution of the titanium surface of alumina, we use oxalic acid etching methods, get good coarsening effect, 1h etched surface roughness (Ra) up to 1.50 ± 0.30μm, etching 2h Ra was 2.99 ± 0.57μm, than a single blast of Ra (1.42 ± 0.14μm) more than doubled to improve its bond strength increased by 30%.
        2: Surface treatment high temperature oxidation resistance: In order to prevent rapid oxidation of titanium at a high temperature, the surface of the titanium in the titanium compound and silicon titanium aluminum compound to prevent titanium oxide at temperatures above 700 ℃. Such surface treatment on high temperature oxidation of titanium is very effective, perhaps titanium surface coating of such compounds, titanium porcelain bonding advantageous still further study.

         
        Titanium and titanium alloy welding performance

        First, titanium and titanium classification and characteristics
        Domestic titanium industry has TA1, TA2, TA3 three, the difference is that the content of nitrogen containing hydrogen impurities, these impurities make commercially pure titanium strengthening, but significantly reduced ductility. Despite the strength of commercially pure titanium is not high, but good ductility and toughness, especially those with good low temperature impact toughness; also has good corrosion resistance. Therefore, this material is used for the chemical industry, oil industry, etc., in fact, more than 350 ℃ for the following operating conditions. According to the organization at room temperature titanium alloy in the annealed condition, the titanium alloy can be divided into three types: α-type titanium alloy, (α + β) titanium alloy and β-type titanium alloy. α-type titanium alloy, the application of more TA4, TA5, TA6 type alloy and Ti-AI TA7, TA8 type Ti + AI + Sn alloys. In this alloy at room temperature, its strength can reach 931N / mm2, and at a high temperature (500 ℃ or less) stable performance, good weldability. β type titanium alloy in less amount of applications, its use should be further expanded.
         
        Second, the welding of titanium and titanium alloys
        Welding properties of titanium and titanium alloy, has many notable features, these welding characteristics due to physical and chemical properties of titanium and titanium alloy decisions.
        1. Effect of impurity gases and pollution on weldability
        At room temperature, titanium and titanium alloys are relatively stable. But the test table, in the welding process, the liquid metal droplet and the pool has a strong absorption of hydrogen, oxygen, nitrogen role, and in the solid state, these gases have their effect occurred. With increasing temperature, titanium and titanium alloys absorb hydrogen, oxygen, nitrogen capacity also will be increased significantly, about 250 ℃ begins to absorb hydrogen, from 400 ℃ begins to absorb oxygen, nitrogen from 600 ℃ began to absorb these gases after being absorbed, it will directly cause embrittlement of welded joints, it is extremely important factor affecting the quality of welding.
        (1) Effect of hydrogen is hydrogen gas impurities affect the mechanical properties of titanium for the most serious factor. Weld hydrogen content on the weld performance changes impact the most significant, mainly due to the increase of the hydrogen bomb containing a seam with the amount of weld precipitated flake or needle TiH2 increased. TiH2 strength is very low, so the flake or needle-guard action to notch HiH2 example, significantly reduce combined impact performance; weld hydrogen content changes reduce the role of improving the strength and plasticity is not very significant when. 
        (2) the influence of oxygen in the titanium oxide phase of α and β want in both a higher degree of melting, and can form a crystalline solid deep wound serious distortions gap phase, the right to use titanium, thereby increasing the hardness of titanium and titanium alloy and strength, so that the plasticity is significantly reduced. In order to ensure the welding collusion performance, in addition to prevent weld metal and weld heat affected zone made by the main oxidation during the soldering process, but should also restrict the basic metals and welding oxygen content.
        (3) The effect of nitrogen at temperatures above 700 ℃, nitrogen and titanium occurred drama role in the formation of brittle titanium nitride (riN) and nitrogen is formed with the titanium lattice distortion degree move the gap caused by solid solution than is the amount of oxygen due to the consequences more serious, therefore, to improve nitrogen industrial pure titanium weld tensile strength, hardness, lower plasticity properties of the weld is more significant than oxygen.
        (4) the impact of carbon is carbon-titanium and titanium alloy common impurities, experiments show that when the carbon content was 0.13%, the result of carbon deep in α titanium weld strength limit some increase, some plastic down, but less oxygen strong effect of nitrogen. But when further increase the carbon content of the weld, the weld has appeared mesh TiC, their number increases with the carbon content increased and the weld plastic sharp decline in the welding prone to stress cracks. Therefore, the carbon content of titanium and titanium alloy base material is not more than 0.1%, weld carbon content does not exceed the base metal carbon content.
        2. The welded joints crack problem
        Titanium and titanium alloy welding, weld joints produce hot cracking possibility is very small, because the impurities in titanium and titanium S, P, C and so rarely, low melting point by the S, P less prone to the formation of eutectic in the grain boundaries, coupled with effective crystallization temperature range narrow, when titanium and titanium alloy solidification shrinkage of small, does not produce weld metal hot cracking. Titanium and titanium alloy welding time, heat-affected zone cold cracking can occur, wherein after welding cracks in a few hours or even longer called delayed cracking. The study showed that the proliferation of such cracks and welding process related to the hydrogen bomb. Hydrogen welding process to a lower temperature heat affected zone of high temperature diffusion deep pool, increase the hydrogen content of the region so that the amount of precipitation increases TiH2, increase the heat affected zone embrittlement, due to additional hydride precipitation volume expansion stress caused by larger organizations , plus a hydrogen atom diffusion and aggregation to the high stress areas of the region, resulting in the formation of cracks. This delay prevents cracks approach, mainly to reduce the welded joint source of hydrogen, when the invoice is also carried out deep stayed empty fire containment process.
        3. weld porosity problems
        Titanium and titanium alloy welding, porosity is often encounter problems. The root causes of the formation of pores is the result of the impact of hydrogen. Weld metal to form pores primarily affects the fatigue strength of the joint. Prevent blowholes and process measures are: (1) protect the neon gas to pure purity should not be less than 99.99% (2), the elimination of weld surface, oxide on the wire surface oil and other organic matter. (3), impose on the bath good gas protection, control flow and velocity well argon, to prevent turbulence, impact protection. (4), the correct choice of welding parameters, increasing the residence time of a deep pool at the right to use bubbles escape, which can effectively reduce pores.
           
        Third, titanium tungsten plate welding manual welding test
        Titanium and titanium alloy welding is the most widely used in the production of tungsten plate welding, vacuum argon welding applications are very common. TIG arc under the protection of the cooling effect of a stream of argon arc heat is more concentrated, high current density, small heat-affected zone, higher welding quality.
        1. titanium and titanium alloy welding, when the temperature is higher than 500 ℃ ~ 700 ℃, easily absorb air gas, hydrogen and nitrogen, seriously affecting the quality of welding. Therefore, the welding of titanium and titanium alloy, and high-temperature portion of the pool full letter (400 ℃ ~ 650 ℃ or more) of the weld area must be strictly protected, for titanium and titanium alloys must take special protective measures during welding, which uses spray larger size torch to expand gas area, when the nozzle is insufficient to protect the hot metal and the weld seam zone near, it must be accompanied by argon protection cover tow. And near the weld seam zone is the protective effect of the color standard wings. Silver represents the best protection, yellow slight oxidation is generally permitted. Surface color should be consistent with (1) the provisions of the table.
        Taking into account the practical, efficient use of the project, we first prepared a simple drag hood. As shown in (a), argon from the air inlet into the distribution pipe through the hole distribution directly into the protected area. With this drag cover welding protective effect is not very good, dark blue bead. According to the analysis airflow directly into the protected area from the distribution pipe. Airflow is not very uniform, smooth, high-temperature bead is not well protected from oxidation. Therefore, we further improved drag shield structure, as shown in (b), after argon gas from the inlet port into the distribution pipe by dragging back at the top of the hood; through the porous plate, perforated plate acts as the main screen and distribution of gas, so that argon flow more smoothly, better protection welding bead silvery or yellow river. Drag cover charge length L is 40 ~ 100mm made of brass. Titanium and titanium alloy welding, welding should also pay attention to the north channel, taking into account the distortion, we use a fixed slotted copper plate method argon protection, in order to weld the back row to adequate protection, but also in Distillers plus a perforated brass, brass argon through holes uniformly into the protected area, the protective effect is good, back bead with silver white. Select manual tungsten argon arc welding process and plate parameters (1) preparation before welding weldment and wire surface quality has a great influence on the mechanical properties of welded joints must be strictly clean up. Iron and titanium wire can be mechanical cleaning and chemical cleaning methods.1) mechanical cleaning or pickling welded by demanding quality difficulties weldments, can be used fine sandpaper or steel wire brush to clean, but it is preferable to use titanium carbide yellow scraper to remove the oxide film. 2) chemical cleaning before welding can be performed on the test piece and the wire pickling acid lotion available HF5% + HNO335% of melt water. Rinse with water after pickling, drying immediately after welding. Or wipe with a titanium plate and groove on both sides of acetone, ethanol, carbon tetrachloride, methanol, etc. (within each 50mm), wire surface, the fixture in contact with the titanium part. Choose titanium (2) welding equipment and welding titanium tungsten plate gold should be used with decreased external characteristics, high-frequency DC arc welding power supply and gas delivery delay time less than 15 seconds, to avoid welding suffered oxidation, pollution. (3) Select the purity argon welding materials shall not be less than 99.99%, the dew point below -40 ℃, mass fraction of total impurities & amp; lt; 0.001%. When the argon gas cylinder pressure drops 0.981MPa, should stop using, to prevent the impact of quality of welded joints. In principle, the basic metal component should choose the same titanium wire, sometimes in order to hold high ductility weld metal can also be used with lower intensity than the basic metal wire. (4) groove in the form of choice principles to minimize the number of layers of welding and weld metal. With the increase in the number of layers of welding, weld accumulated intake is set to increase, as well as affect the performance of welded joints, but also due to the welding of titanium and titanium alloy weld pool size is large, so the specimen billing V-70 ~ 80 ° bevel .(5) Specimen Group and tack welding to reduce welding deformation before welding tack welding, general welding positioning spacing 100 ~ 150mm, a length of 10 ~ 15 mm. Tack welding wire used in the welding process parameters and conditions should be the same gas shielded welding and welding joints. Gap 0 ~ 2mm, blunt edge 0 ~ 1.0mm. (6) welding parameters are shown in Table (2) shown by our comparison of the performance of welded joints under different process, worked out a more suitable welding procedure specification.
        Process (1), the welding current is 150A, 170A, 180A, Click welding parameters, welding joint surface, showing a dark blue, plain gold, piecing oxidation more serious, does not meet the technical requirements, this process is not desirable.
        Process (2), the welding current is relatively reduced to 120A, 150A, 160A, Click welding parameters, weld surface showing purple gold, yellow, X-ray detection defect-free, but the mechanical properties of bending test failed, indicating that the welded joint plastic significantly reduced, not meet the technical requirements, this process also undesirable.
        Process (3), the welding current is 95A, 115A, 120A, Click welding parameters, weld surface was white, pale yellow, X-ray detection defect-free, but the mechanical properties of bending test pass, tensile strength meet the requirements, the welding joint properties meet the technical requirements, this process is more appropriate. When the welding of titanium and titanium alloy, the material has a coarse crystal orientation, directly it affects the mechanical properties of welded joints. Therefore, the selection of welding parameters to take into account not only the weld metal and oxide form pores, grain coarsening factors should also be considered, it should try to use a smaller heat input welding process (1), (2), since the welding specifications larger factors, resulting in oxidation of the joint than the process (3) severe. Metallographic and microscopic experimental results show that the joint degree than grain coarsening process (3) serious. So the poor mechanical properties of welded joints.
        Select the gas flow in order to achieve a good protective effect prevail, too much traffic is not easy to form a stable laminar flow, and increase the cooling rate of the weld, the weld surface layer appears more α phase, as well as cause microcracks. Insufficient drag argon flow hood, seam showing different oxidation color; while the amount of traffic, it will interfere with the main nozzle flow effect. Argon gas flow rate on the back of the weld can not be too large, otherwise it will affect the first layer to the front seam gas protection. (7) of titanium and titanium alloys GTAW operating essentials 1) manual arc welding, between the wire and the weldment should be kept to a minimum angle (10 ~ 15 °). Smooth front end of the wire along the pool, even into the bath, the wire ends may not be removed from the argon gas reserve. 2) When welding torch without substantially horizontal swing, swing when needed, the frequency is lower, swings should not be too large, in order to prevent the effects of the protection of argon. 3) When broken arc welding and finishing, to continue through argon, before removal of the torch until the metal weld and heat affected zone when cooled to below 350 ℃. (8) Quality Inspection 1) Appearance check compliance with GB / T13149-91.2) ray deep wounds consistent with JB4730-94.3) Mechanical tests in line with GB / T13149-91.

         

         

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