Characterisation and Shear Testing SLM Manufactured Ti-6Al-4V Hyper-Pins for Composite to Metal Joining Application

Abstract

The ability to produce efficient and durable joints between high strength titanium alloys and composites is becoming increasingly important in airframe manufacturing.  Hyper-joints containing mechanical locking features on the metal components surface, and designed to penetrate into the composite plies, are known to be an effective method of increasing load transfer and improving the strength and toughness of composite-metal adhesive joints. However, questions remain as to the best method for manufacturing the hyper-pin features and how the manufacturing process affects their performance.

In this study, hyper-pin arrays have been produced on Ti-6Al-4V test pieces by additive manufacturing using Selective Laser melting (SLM). This method allows great flexibility in producing different hyper-pin geometries, but little is currently known about the microstructure and mechanical properties of the pins.  Results are reported on a novel method for testing the shear strength and properties of individual hyper-pins manufactured via this process. Interpretation of the shear test was assisted by finite element modelling. The flexibility of SLM and modelling allowed the effect of the blend radius, pin diameter, loading location and heat treatment on the load bearing capacity of the hyper-joint features to be determined. Detailed microstructural analysis was also performed on the hyper-pins features allowing direct correlation to their fracture behaviour. Failure typically occurred at the root of the pin where the expected stress concentration was enhanced by surface roughness, inherent to the SLM manufacturing process. The performance of the pins was also affected by their microstructure, which was mainly martensitic after SLM, but could be varied to different tempered states depending on the post manufacture heat treatment.

Key words: Ti-6Al-4V, SLM- ALM, Microstructure, Shear Testing, Tomography and FE Modelling.