Ti-6Al-4V lattice block structure panels were fabricated using an aerospace-quality investment casting process. Testing in compression, bending, and impact show that high strength, ductility, and energy absorption are achieved for both individual struts and full panels, despite the intricacies involved with casting fine struts (1.6 or 3.2 mm in diameter) from a highly reactive, poor-fluidity liquid titanium alloy. The panel stress-strain curve calculated by finite-element modeling correlates well with experimental results, indicating that the occasional defects, which are common to aerospace grade castings and may be present in the struts and nodes, have little detrimental effect on the overall panel compressive properties.

Lattice block structures (LBS) -also called lattice-truss structures, truss-core sandwiches, and cellular lattices -have been fabricated from alloys of aluminum, [1][2][3] copper [2] and iron. [4] Three methods for fabrication of titanium LBS have been reported so far in the literature, to our knowledge. In a first method, struts consisting of a thick slurry of Ti-64 powders in an organic binder are layered into a 0/90 degree pattern forming the LBS which is sintered after binder removal. [5] In a second, related method, [6,7] selective electron beam melting is used to melt titanium and Ti-64 powders under high vacuum layer by layer, resulting in a structure with relative density of 30 % [6,7] characterized by struts, less than 1 mm in diameter, arranged in various architectures. [6,7] Finally, we recently showed that LBS panels with 1.6 and 3.2 mm diameter struts could be investment-cast with the alloy Ti-64, and we studied the mechanical properties of struts and panels at ambient temperature. [8] Such investmentcast titanium panels combine the advantages of high strength derived from the periodic LBS architecture, high mechanical performance inherent to titanium alloys, and low cost from casting. The present paper describes the mechanical properties, at ambient and elevated temperatures, of investment-cast Ti-6Al-2Sn-4Zr-2Mo (Ti-6242) LBS panels. This alloy was chosen because it exhibits improved stiffness and strength at ambient temperature as compared to Ti-64, as well as much improved microstructural stability and mechanical strength up to 565°C, [9] while remaining castable.The Ti-6242 LBS panels were investment-cast in vacuum using the lost-wax approach, following a technique described previously, [2,8] After casting, panels were processed according to standard aerospace-grade titanium casting procedures (AMS 4985B). First, hot isostatic pressing (HIP) was performed at 900°C for 2 hrs under a pressure of 103 MPa, a treatment commonly used to close casting porosity. [11] This was followed by chemical milling to remove the a-case, NAD-CAP-approved nondestructive inspection (visual, radiographic, penetrant), casting weld repair as necessary, and a mill-anneal heat-treatment carried out at 730 ± 15°C for 2 hrs, terminated by furnace cooling, and then final inspections and light etching. Figure 1 shows a ∼ 100 × 100 × 25 mm 3 panel consisting of a core with 3.2 mm diameter struts in a pyramidal arrangement and two faces which consist of a square external frame (with approximate 3.8 × 6.4 mm 2 cross-section) filled by a COMMUNICATIONS ADVANCED ENGINEERING MATERIALS 2008, 10, No. 10 Fig. 1. Photographs of a ∼ 100 × 100 × 25 mm 3 LBS Ti-6242 panel. The white square highlights a 3 × 3 sub-panel used for high temperature compression tests. (a) Top view; (b) Perspective view.

Samples of unalloyed titanium and Ti-6Al-4V with a cast, coarse-grain structure were subjected to simultaneous mechanical loading and thermal cycling about their transformation range to assess their capability for transformation superplasticity. Under uniaxial tensile loading, high elongations to failure (511 pct for titanium, and 265 pct for Ti-6Al-4V) and an average strainrate sensitivity exponent of unity are observed. Samples previously deformed superplastically to a strain of 100 pct show no significant degradation in room-temperature mechanical properties as compared to the undeformed state. Biaxial dome bulging tests confirm that transformation superplasticity is activated under thermal cycling and faster than creep deformation. The cast, coarse-grained titanium and Ti-6Al-4V have similar transformation-superplasticity characteristics as wrought or powder-metallurgy materials with finer grains. This may enable superplastic forming of titanium objects directly after the casting step, thus bypassing the complicated and costly thermomechanical processing steps needed to achieve fine-grain superplasticity.