Investigation of material and mechanical properties of laser beam welded (LBW) laser powder bed fusion (LPBF) and wrought titanium alloy samples
Creators: name: Tamaddon, Ali
Publisher: name: RMIT University
Year: 2024
Subjects: classificationCode: 401408
subject: Manufacturing processes and technologies (excl. textiles)
classificationCode: 401607
subject: Metals and alloy materials

description: Metals are one of the primary class of materials used for additive manufacturing. Minimal material wastage and freedom in creating complex geometries have enabled this technology to reap such benefits with more expensive metallic alloys, namely titanium alloys. Titanium alloys are significantly more expensive than aluminium and steel alloys but have excellent corrosion resistance and weight to strength ratios, making them appealing to highly corrosive applications such as aerospace and medical devices. Despite the widespread adoption of additive manufacturing technologies and increase in additive manufacturing machine size and speed of fabrication, the limitations are still significantly considerable for the purpose of mass production of parts at large scale. Laser powder bed fusion (LPBF), which is the most promising of all metal additive manufacturing technologies has chamber sizes confined to approximately one cubic meter or less; and has part build rates at 180 cm3/h or less. This is in stark contrast to traditional manufacturing methods, which can produce large volumes at high rates.

This research investigates a means of negating such shortcomings and reaping the benefits of both LPBF additive manufacturing and traditional manufacturing methods by joining such parts together using laser welding in the autogenous mode (no fillers required).

Based on the literature review conducted in this field and the gaps identified, a two-stage methodology was utilised, as per following.

At the first stage of the research, 20x20x4mm pilot samples were made using wrought Ti6Al4V titanium alloys and LPBF fabricated Ti6Al4V titanium alloys with the same geometry and dimensions, then welded together using disc laser beam at various power levels and speed, which were selected using existing literature. The welded joints were examined for porosity, geometry of weld zones, microhardness and microstructure. Energy density and rate of cooling resulting from the various welding parameters were the driving factors of the welded joint results. The results of examinations were analysed, and laser beam power level of 2200 Watts and 1200 mm/ min were the most promising parameters.

At the second stage of the research more samples were fabricated, in a similar fashion to the first stage, but had geometries and dimensions as per ASTM E8 and E466 standards. All samples in the second stage were welded using the most promising weld parameters found during the first stage and subjected to tensile and fatigue tests. Each sample was checked for porosity before and after each test and the results produced a profile of tensile and fatigue characteristics of such assemblies.

It is demonstrated that the welding of LPBF to wrought samples, and LPBF to LPBF is a viable solution to overcoming the limitations in build volume and production speed in LPBF technologies. The governing factors in the success of such welds are laser energy density (cooling rates) and LPBF base metal initial build quality (porosities).