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。76187

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 odonto- logical 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

*   Corresponding author。

one piece, called cast monoblocks, the use of welding is necessary since this technique accepts the work with segments of the prosthesis, which makes pos- sible a balanced force distribution and the best suit- able 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  a

0167-577X/02/$ - see front matter D 2002 Elsevier Science B。V。 All rights reserved。 doi:10。1016/S0167-577X(02)01095-9

M。L。 Santos et al。 / Materials Letters 57 (2003)  1888–1893 1889

great 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 prosthetic pieces [6]。

The use of electrochemical techniques in the cor- rosion 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 mi- crostructure 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 Spectro- scopy—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 Nd:YAG as source of laser, and the beam power was approx- imately 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。