Jan_AMP_Digital

3 1

A D V A N C E D M A T E R I A L S & P R O C E S S E S | J A N U A R Y 2 0 1 6

(a)

Fig. 2 — EDS composition profile at the interface for LRM solid-state weld joint.

grindability, thus providing design options and an assessment of joint strength uniformity toward the wire central axis. All tested samples passed the U-bend preconditioning test, which indicates excellent joint bending prop- erties. Tensile tests yielded an average joint strength of approximately 80% of the tensile strength of Nitinol wire. All tensile samples failed at the stainless steel to Nitinol interface, leading to the hypothesis that a relatively small HAZ and presence of fine grain struc- ture contribute to the high strength of the joint. With small sample sizes, 95% of the confidence interval indicates no loss of stiffness as the core diameter is reduced via grinding as seen in Fig. 3. Full guidewires were built using the proprietary solid-state welded joints located approximately 40 cm from the distal tip. The grind profile and joint location aligned with leading competitive guidewires and enabled comparative bench testing. Figure 4 shows the lateral stiffness results, and the inset of Fig. 4 shows the LRM solid-state weld joint in comparison to the hypotube joint design. It is evi- dent that the LRM joint is significantly shorter than the 3-cm long hypotube joint. The solid-state weld also shows a smooth and even bending transition from stainless steel to Nitinol, while the

EXPERIMENTAL RESULTS

(b)

Results show that the new solid- state welding process produces a clean and defined transition between the stainless steel and Nitinol and that the interface is free of defects and/or po- rosity. Figure 1 shows SEM images of a joint cross-section between 0.018-in. stainless steel and 0.020-in. Nitinol. Figure 1(a) shows the interface after polishing while Fig. 1(b) shows the in- terface after etching the stainless steel side. The heat-affected zone (HAZ) is approximately 0.012-in. (~300 µm) long, and is distinguished from the drawn wire elongated grain structure by the presence of a fine, uniform grain struc- ture. A fine grain size is an inherent solid-state processing advantage com- pared to fusion welding, which is char- acterized by the presence of a cast den- dritic structure and large grains in the HAZ [2,7] . Figure 2 shows the EDS analysis at the interface of the solid-state weld joint, collected within 10 µm on either side of the joint. The data shows that chemical intermixing of Nitinol and stainless steel extends approximately 1 µmon either side of the joint interface. LRM engineers ground welded 0.018-in. stainless steel and 0.020-in. Nitinol wires to different diameters, post joining in order to determine joint

Fig. 1 — Post polishing SEM image of joint shows its seamless nature (a); SEM image of joint after etching shows presence of small grains in HAZ (b).

strength and joint quality evaluations closer to the core central axis. The wire, including the joint area, was ground to diameters of 0.014, 0.010, and 0.008 in., then tensile tested to evaluate change in joint strength throughout the cross- sectional area. After initial joint strength and quality assessments, full guidewires were assembled using cores joined with the LRM solid-state weld process from 0.014-in. stainless steel and 0.014-in. Nitinol. The core wire distal grind pro- file for this study mimicked the stiffness profile of a commercially available bi- metal guidewire, enabling performance comparisons between the two designs. The competing design wire consisted of Nitinol and stainless steel joined via a Nitinol hypotube and adhesive. The two designs were tested side-by-side com- paring lateral stiffness, tensile strength, and simulated clinical performance in a 2D plate model emulating a tortuous vessel. After tensile testing, the fracture surface was analyzed using a tabletop SEM (Hitachi TM 300).