NiTi-Al interface strength in ultrasonic additive manufacturing composites

Ultrasonic Additive Manufacturing (UAM) is a new rapid prototyping process for creating metal-matrix composites at or near room temperature. The low process temperatures enable composite materials that have tailored CTEs through utilizing recovery stresses generated by highly prestrained Shape Memory Alloy (SMA) fibers embedded within the matrix. The strength of the fiber-matrix interface, which is the limiting factor in UAM composites, has not been characterized. In this study, we characterize the shear strength of the fiber-matrix interface and study the bonding between the fiber and matrix in composites fabricated with prestrained NiTi embedded in an Al 3003-H18 matrix. In heating the composite, stresses develop due to the blocked behavior of NiTi and the difference in CTE of the matrix and fiber. Differential scanning calorimetry is used to observe composite failure temperatures; an average interface shear strength of 7.28 MPa is determined using constitutive models of the NiTi element and Al matrix. The constitutive models describe the thermally-induced strain of the composite, showing an effective CTE of zero at 135 °C. The models show that by increasing the embedded fiber length, interface failure temperatures can be increased so that zero CTE behaviors can be utilized without irreversibly changing the NiTi prestrain. Results from energy dispersive X-ray spectroscopy indicate that the bonding between the fiber and interface is mechanical in nature with no evidence to support chemical or metallurgical bonding.