Effect of argon shielding on strength of cast Fe-Pt magnetic alloy laser-welded to Co-Cr alloy

 

Naoki Baba¹, Ikuya Watanabe², Yasuhiro Tanaka¹

Kunihiro Hisatsune¹, Mitsuru Atsuta¹

 

1 Department of Developmental and Reconstructive Medicine

Nagasaki University Graduate School of Biomedical Sciences, Nagasaki, Japan

 

2 Department of Biomaterials Science, Baylor College of Dentistry

Texas A&M University System Health Science Center, Dallas, TX, USA

 

Introduction

Iron-Platinum (Fe-Pt) alloys have soft and hard magnetic properties and excellent corrosion resistance compared to the stainless steal which is usually used for a york cap of commercially available dental magnetic alloy. It is clinically beneficial to use Fe-Pt magnetic alloys because they can be cast into desirable shape and thickness using dental casting machines. When using Fe-Pt alloys for clinical dentistry, it is sometimes required to connect theses alloys with other metal frameworks such as Co-Cr alloy, gold alloy and etc. However, there was a little information available about the joint properties of Fe-Pd alloy connected the other alloys. This study investigated the joint properties of cast Fe-Pt magnetic alloy laser-welded to Co-Cr alloy and the effect of argon (Ar) shielding on the properties of joint strengths.

 

Materials & Methods

Cast plates (0.5 x 3.0 x 10 mm) were prepared with a custom made Fe-Pt alloy (Fe-36at%Pt) and a commercial Co-Cr alloy (Vitallium, Austenal). After the cast Fe-Pt plates were sealed in an evacuated silica tube, they were heat-treated,at 1325 $B!k(BC for 45 minutes, then quenched in ice water. They were butted against cast Co-Cr plates and then bilaterally laser-welded using Nd:YAG laser (Neolaser L, Girrbach Dental Systems, Germany) at 200V (voltage), 10ms (pulse duration) and 1.0mm (spot diameter). Control (non-welded: 20 mm length) and homogeneously welded specimens were also prepared for each alloy. Five laser shots were applied on each side of the specimens. Laser welding was performed with and without Ar shielding. Tensile testing (n=5) was conducted at a crosshead speed of 1 mm/min and a gauge length of 10 mm. Fracture force (Ff: N) and elongation (%) were recorded. After tensile testing, the fractured surfaces were examined using SEM (JSM-6300, JEOL, Peabody, MA, USA).

 

Results

 

Fig. 1. Fracture force of the specimens

 

Fig. 2. Elongation of the specimens

 

Fig. 3. SEM photographs of entire fracture surfaces. A: Control Co-Cr alloy specimen, B: Control Fe-Pt alloy specimen; C: Laser-welded Co-Cr specimen with Ar; D: Laser-welded Co-Cr specimen without Ar; E: Laser-welded Fe-Pt specimen with Ar; F: Laser-welded Fe-Pt specimen without Ar; G: Fe-Pt laser-welded to Co-Cr specimen with Ar; H: Fe-Pt laser-welded to Co-Cr specimen without Ar.

 

Fig. 4. SEM micrographs of fracture surfaces. A: Control Co-Cr alloy specimen; B: Control Fe-Pt alloy specimen (near surface of specimen); C: Control Fe-Pt alloy specimen (interior region of specimen); D: Laser-welded Co-Cr specimen without Ar; E: Laser-welded Fe-Pt specimen with Ar; F: Fe-Pt laser-welded to Co-Cr specimen with Ar; G: Fe-Pt laser-welded to Co-Cr specimen without Ar; H: Fe-Pt laser-welded to Co-Cr specimen without Ar.

 

Discussion

Fracture surface of laser-welded Co-Cr without Ar shielding showed deeper penetration depth than laser-welded Co-Cr alloy with Ar shielding and welded area covered almost whole of cross section area (Figs. 3, C and D). Therefore laser-welded Co-Cr alloy without Ar shielding has higher failure force than laser-welded Co-Cr alloy with Ar shielding and equivalent value for control Co-Cr alloy.

Control Fe-Pt alloy exhibited higher Ff value followed by control Co-Cr alloy and laser-welded Co-Cr alloy without Ar shielding and the highest elongation. Fracture surface of control Fe-Pt alloy showed equiaxial dimple fracture near surface region (Fig. 3, B and 4, B). It indicated ductile fracture. Despite center region of plate has more brittle fracture and large crack (Figs. 3, B and 4, C). It might be caused by different cooling rate during the casting. Solidification of alloy started from surface region toward to the center of plate because of lower temperature of mold compared to the molten alloy. Center region of molten alloy solidified finally then micro-shrinkage is concentrated and crack was made. Moreover several porosities were observed at near surface region. These casting defects cause reduction of Fe-Pt alloy strength.

Some cracks which might be created during the laser-welding were observed on fracture surface of both laser-welded Fe-Pt alloys (Figs 3, E and F). After welding, the solidification of the molten Fe-Pt alloy occurs quickly. This rapid solidification causes a constriction of the welded region at the same time. Thus, a concentration of stress occurs at the laser-welded region, and the cracks are created. These cracks might reduce the joint strength of laser-welded Fe-Pt alloy. Both laser-welded Fe-Pt alloys displayed more brittle fracture surface with river pattern (Fig. 4, E) compared to the fracture surface of the control specimens due to the difference in the solidification modes between the laser-welded and the control specimens. The laser-welded specimens quickly solidified compared to the control specimens. It might affect a microstructure and order of Fe and Pt atom, and cause reduction of joint strength.

Fe-Pt welded Co-Cr showed a mixture of brittle and ductile fractures (Figs. 3, G and H). Some region has equiaxial dimple fracture like the control Fe-Pt alloy (Fig. 4, F). Brittle fracture regions were similar to the laser-welded Co-Cr alloy and control Fe-Pt alloy. This fracture surfaces reflect that the Ff value of the welded Fe-Pt/Co-Cr fell into the middle of homogeneously welded Fe-Pt (lowest) and Co-Cr (highest) alloys. Several welding defects, such as cracks, solidification shrinkage on the surface and porosities which caused incorporation of surrounding gas, were observed on fracture surface.

During the welding, atoms which consist of both Fe-Pt and Co-Cr alloy were diffused in the weld pool. This diffusion may cause formation of intermetallic compounds (CoPt, CoFe and etc.) in welded region. Theses intermetallic compounds are thought to affect joint strength however further research needs this point.

Argon shielding did not affect joint strength of laser-welded Fe-Pt alloy and Fe-Pt alloy welded to Co-Cr alloy since penetration depth is sufficient to cover the cross section of sample under both shielding condition. Even though welding area covered whole cross section of laser-welded Fe-Pt alloy with and without Ar, they have only 12~14% Ff value of control Fe-Pt alloy compared to the Co-Cr alloy.  It may cause different microstructure and the difference how much heat influences the mechanical properties during the welding.

Joint strength of Fe-Pt alloy welded to Co-Cr alloy is lower than control Fe-Pt and Co-Cr alloy. Hence, it is necessary to have a sufficient welding area around Fe-Pt alloy to function when Fe-Pt alloy is required to connect with Co-Cr alloy in clinical situation.