資源目錄里展示的全都有，所見即所得。下載后全都有,請(qǐng)放心下載。原稿可自行編輯修改=【QQ：401339828 或11970985 有疑問可加】 Laser weld: microstructure and corrosion study of Ag–Pd–Au–Cu alloy of the dental application M.L. Santos*, H.A. Acciari, L.C.O. Vercik, A.C. Guastaldi Instituto de Qu?′mica de Araraquara-UNESP, C.P. 355, 14800-900 Araraquara, Sa?o Paulo, Brazil Received 10 June 2002; accepted 20 June 2002 Abstract The laser welding process was introduced into dentistry by the end of the 1980s, resulting on a great impulse to that area with the development of cheaper and smaller equipment, using simpler technique. This allowed greater use of that process on the confection of prostheses compared to the brazing process since the heat source for that process is a concentrated light beam of high power, which minimizes distortion problems on the prosthetic pieces. Ag–Pd–Au–Cu alloy used on the confection of dental implant prostheses was observed before and after subjection to the laser welding process. The microstructure was analyzed with the use of optic microscopy and the corrosion resistance was studied by the traditional electrochemical techniques and by electrochemical impedance, under environmental conditions simulating the aggressiveness found in the mouth cavity. A structural change was detected on the weld area, which presented a refined microstructure deriving from the high-speed cooling. The base metal out of the weld area presented a fusion coarse microstructure. The electrochemical essays showed differences on the potentiodynamic polarization behavior in both weld and metal base areas, indicating superior corrosion resistance in the weld area. The impedance spectra were characterized by capacitive distorted components, presenting linear impedance in the low frequencies area. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Ag–Pd–Au–Cu; Laser; Corrosion; Dental alloys 1. Introduction In search for alternative metal alloys for odontological purposes, some researchers have applied the AgPd alloy to substitute the gold alloys, trying to reduce costs and to improve mechanical properties and corrosion resistance [1–4]. Due to some difficulty in obtaining adaptation in prosthetic pieces, mainly the larger ones such as metallic structures molten into one piece, called cast monoblocks, the use of welding is necessary since this technique accepts the work with segments of the prosthesis, which makes possible a balanced force distribution and the best suitable adaptation, occurring in an accurate passive way [4,5]. The process of laser welding produces a coherent, monochromatic, concentrated light beam of high power, and it has been applied to substitute the brazing in odontological prostheses welding. The laser welding process was introduced into dentistry by the end of the 1980s, resulting on agreat impulse to the area with the development of cheaper and smaller equipment due to its advantages and wide application, which made possible to use welding in a wide variety of metals and prostheticpieces [6]. The use of electrochemical techniques in the corrosion study is important for the understanding of its performance, biocompatibility and biofunctionality, when clinically applied, for these are constantly exposed to aggressive environments. This research observes Ag–Pd–Au–Cu alloy microstructure behavior and the material’s resistance to corrosion under environmental conditions simulating the aggressiveness found in the mouth cavity, when used on dental implant prostheses before and after subjected to the laser welding process. 2. Experimental Table 1 presents the mineral composition of the studied material, using Wave Dispersive Spectroscopy—WDS. The cylindrical test specimens, with 0.27-cm diameter and 1.0-cm length, have been subjected to the welding process on butt joints [7]. The welding machine, Dentaurum DL 20002S, used for the laser welding, uses a crystal NdYAG as source of laser, and the beam power was approximately 6.08 kW in 14 ms, originating a welding energy of approximately 85.12 J. The test specimens were manually placed in the chamber, with shield atmosphere of argon, and spots of lap welding, in approximately 2/3 of the surfaces, were applied in the whole section of the joint, with 60% of beam penetration. A precise disc model 15 HC DIAMOND was used to obtain the test specimens of Ag–Pd–Au–Cu alloy with area comprehending only the welding area, and an ISOMET 1000-BUEHLER machine was used to separate the base metal from the welding area after the laser process. The exposed geometric areas of the welding cord and of the base metal were 0.057 cm2. The metallographic analysis of the exposed surface of the base metal and the welding area was done with optic microscopy, after polish with emery cloth from 180 to 1000 mesh, alumina with granulation 1 and 0.3 Am and nitro-muriatic acid application [8]. The work electrodes were prepared from the test specimens used on the metallographic analysis. Measures of open circuit potential versus time were used in the electrochemical essays, as well as potentiodynamic polarization and electrochemical impedance. An electrochemical cell containing NaCl 0.15 mol l _ 1 (0.9%) airy solution with three electrodes was also used, with the saturated calomel electrode (SCE) as reference system and a graffiti cylinder as auxiliary electrode. Electrochemical measures of corrosion were done with a potentiometer Solartron SI1287. Potentiodynamic polarization curves were observed at 0.001 V s _ 1 immediately. Impedance measures were done with the analyzer of frequency response, Solartron 1255, connected to an electrochemical interface, Solartron 1287, and an amplitude of 10 mV was applied to a frequency channel that varied from 100 kHz to 6 MHz,obtaining five points for each frequency decade, controlled by the software Zplot [9]. The software Zveiw [10] was responsible for the adjustments 3. Results and discussion Fig. 1 presents a coarse biphasic fusion microstructure in the base metal area. Fig. 2 illustrates a refined dendritical microstructure in the laser weld area, deriving from the high speedy cooling imposed by the laser weld because of a located fusion process, followed by a quick cooling during the welding, which does not allow the microstructure to return to its initial biphasic structure. Fig. 3 shows the open circuit potential versus time curves for the base metal and laser weld areas of the Ag–Pd–Au–Cu alloy. The stabilization of the potential was observed 3 h after immersion for both areas, and the laser weld presented a stabilization potential 50 mV higher. Some AgPd alloy researchers have observed that, usually, an alloy open circuit potential increases with the increase of the noble metals concentration[1]. show minute details the observation Show minute details the observation to include to scan to give or get an electric shock the mirror and golds generally mutually analytical wait. Scanning to give or get an electric shock the mirror analysis is to will break the oral test piece to place in scan to give or get an electric shock the mirror observation Its crystal structure, split the characteristic,deep, pore, hot influence area Size etc..The gold is mutually analytical to try the piece to the welding to make into the gold first to whet mutually Slice, through the sour eclipse, clean, blow the stem to postpose is mutually the microscope in the gold next,Observe its area organization structure, the is deep and melts the area and hot influence areas Size etc..Chai etc. the is in the gold of the piece of mutually analytically detection, Connect the set displays the brittleness open wound shape of split the , but matched control then is . The tenacity open wound of the cup form, the cone form phenomenon., this is from the tiny view last reflection The tenacity of the piece of lower:Discover the laser while study the laser welding The hot influence area grain of the piece of is smaller, appearing only one horse surname body structure, But deal with contact for the welding of the 3 mms at the diameter all the not yet finished whole is deeply The polarization curves on Fig. 4 present differences on the anodic behavior, with the occurrence of an area corresponding to the first transpassive region close to + 0.07 V (SCE) on the laser weld. The numbers obtained for the corrosion potentials, Ecor, indicate that the laser weld area presents higher corrosion resistance.The impedance responses originated in the open circuit potential, obtained in the steady state for the base metal area, present the occurrence of one distorted semicircle at high frequencies (Fig. 5). The equivalent electrical circuit model better adjustable to the characteristics of the resulting spectrum is composed of a parallel association of RTC and CPE, which represents the electrochemical behavior of the interface in the high frequencies area, include-including only one charge transfer process. In the low frequencies areas, the spectrum is controlled by the occurrence of a straight line, and a new Rp and CPEp composition was used to represent the formation of a permeable nature interface since this dispersion, observed during the frequency variation, may have been originated from the formation of pits on the surface, thus confirmed by the optic microscopy analysis after the corrosion essays (not showed) and by the significant decrease on the polarization resistance number from around 10 kV cm2 to 100 V cm2 (Table 2). According to the correspondent impedance diagrams obtained with the Bode format, fair concordance between the experimental and calculated numbers is observed. The use of one CPE to substitute the double electrical layer is due to a correction of the distortions caused by the uniformity in the current distribution caused by the geometry of the electrodic surface [11–13]. A diffusion process represented by a straight line on the complex plane and by the distortion degree, ac0.5 (Table 2), is observed at low frequencies. The laser welded joint area presented two distorted semicircles within the studied frequency channel (Fig. 6). For that reason, an equivalent electrical circuit model with two series RC terms was proposed. On the laser weld, Rp2 was considerably higher than Rp1 (Table 3), with the occurrence of a passive nature layer, formed from the corrosion products themselves. Fig. 6 also presents the corresponding Bode formats. According to the variation of the phase angle versus frequency, the first maximum point is observed close to 20j. Lemaitre et al. [14] have observed that a phase angle of 22.5j, half of 45j (free diffusion of the species in solution), may indicate diffusion due to some specific type of pore, which can be taken into account in this case because of this alloy’s permeable nature. 4. Conclusions The weld area presented refined microstructure, deriving from the high speedy cooling, while the base metal out of the weld area showed a fusion coarse microstructure. The Ag–Pd–Au–Cu alloy presented high corrosion resistance both for the base metal and for the laser weld areas. AgCl probably forms the passiveness films occurring in both circumstances. In general, all the areas studied presented linear impedance response at low frequencies, including a non-uniform diffusion. The Ag–Pd–Au–Cu laser alloy impedance responses were adjusted by an equivalent electrical circuit model involving two series RC terms with Rp2{Rp1 and the Rp numbers varied from 10 to 103 V cm2. On the base metal area of the Ag–Pd–Au–Cu alloy, the impedance responses at low frequencies were interpreted from a model that considers the occurrence of a pore layer. The dentistry material laser welding method and other welding methodses compare In recent years, have the scholar to the of welding method carry on the research,Combine to carry on various welding method comparison.The Roggensaek waits the old comparison Laser and etc.s ion of the , as a result discover two kinds of methods Tired strength indiscrimination of anti- connect behind;But etc. the ion piece deal with contact Carry in the extreme limit under compare to appear early tired, show minute details the degree of hardness high in laser Have obviously of hot the change responds the area with heat, the laser hot change Small Two kinds of welding methods are all in keeping with to weld the metal alloy, but laser Connect better than etc. ion .Manieone etc. compared the laser and Welding result of the red and outside copper method to , discover the of the laser welding Connect the area metals combines the interface even.Has no tiny bore, the gold learns the analysis mutually Imply the chemical element of .But the metals interface boundary of the red and outside copper is obvious, Weld the area implies the and coppers.Both of hot reaction area all show minute details the knot. The variety, show minute details the degree of hardness to all go up.The laser welding area onlies have the dollar of Vegetable, more more ideal than red outside welding method.Neo etc. deleted to compare the laser The and the pole sloth air protections are two kinds of weld the method to the of anti-Pull the influence of stretch the machine function, discover that the latter piece is through hot processing Accept defeat the strength and flexible mold quantities behind obvious high in other set, laser The piece accepts defeat the strength and a strength of the extreme limit anti-s to lower obviously, both of the elongation quantity is obvious all low in female material.There is scholar use 4 kinds of welding method Welding pure , namely laser , electron beam , usage Ag- Ti Copper and the electricity s of anticipate.Show the microstructural analysis watch to the piece of Clear, the laser welding area structure is a very complete and female material same Wait the stalk grain, the hot reaction area is small;The welding area the grain of the copper enlarge Erupt to living the appearance change:The electron beam grain enlarge and appearance change. The degree is is all big in the copper , the grain of that is subjected to the influence locates the whole Area, the hot reaction area is big;A structure of that gives or get an electric shock the is subjected to the breakage,Become the crystal structure of the thin layer needle form, make the tired strength of the anti- of the piece of Lower, the anti- pulls to stretch the ability to build up. More the result can see a laser Welding of structure near to most the female material after the hot reaction area least. Was along with the good property of the recognize by the large mouth cavity worker Know, the will replace other metals material of the existing mouth cavity repair gradually Anticipate. From weld the research of method to the and metal alloy of in the last few years. The comparison that various welding method welds the result to see, the of the laser Connect the quantity best, is clinical currently up more satisfied welding square Method, have the good and applied foreground. 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Guastaldi, Mater. Lett. 36 (1998) 148–151. [12] C.M.A. Brett, H.A. Acciari, A.C. Guastaldi, Mater. Science Forum, in press. [13] H.A. Acciari, A.C. Guastaldi, C.M.A. Brett, Electrochim. Acta 46 (2001) 3887– 3893. [14] L. Lemaitre, M. Moors, A.P. Van Peteghem, J. Oral Rehabil.