Microstructure Evolution and Mechanical Properties of Titanium/Alumina Brazed Joints for Medical Implants

A Novel Approach for Powder Bed Fusion of Ceramics Using Two Laser Systems

The one-step AM process is considered the goal many researchers seek in the field of Additive Manufacturing (AM) of high-technology ceramics. Among the several AM techniques, only Powder Bed Fusion (PBF) can directly print high-technology ceramics using one step. However, the PBF technique faces numerous challenges to efficiently be employed in the PBF of ceramics. These challenges include the formation of cracks, generated thermal stress, effective laser-powder interaction, and low acquired relative density. This study developed a new preheating mechanism for ceramic materials using two laser systems to surpass beyond these challenges and successfully print ceramics with a single-step AM method. One laser is used to preheat the powder particles before the second laser is utilised to complete the melting/sintering process. Both lasers travel along the same scanning path. There is a slight delay (0.0001 s) between the preheating laser and the melting/sintering laser to guarantee that the melting/sintering laser scans a properly preheated powder. To further facilitate testing of the preheating system, a numerical model has been developed to simulate the preheating and melting process and to acquire proper process parameters. The developed numerical model was shown to determine the correct process parameters without needing costly and time-consuming experiments. Alumina samples (10 × 10 × 6 mm³) were successfully printed using alumina powder as feedstock. The surface of the samples was nearly defect-free. The samples' relative densities exceeded 80%, the highest reported relative density for alumina produced by a single-step AM method. This discovery can significantly accelerate the transition to a one-step AM process of ceramics.

Multi-Layer Simulation of the Powder Bed Selective Laser Processing of Alumina for Residual Stress and Distortion Evaluation

A numerical model was developed to simulate the real process of alumina powder bed selective laser processing (PBSLP) to thoroughly investigate the residual stress and distortion experienced in printed parts when multi-layer scanning with a CO₂ laser source is considered. The model contains a user-defined function (UDF) for the laser source, temperature-dependent material properties, scanning strategies, and build orientations, and it is solved using ANSYS 2020R2. In addition, the model’s validation was confirmed with experimental results. The results revealed that a high scanning speed (up to 1200 mm/s) and low laser power are effective for the PBSLP of alumina, owing to alumina’s high absorptivity for CO₂ lasers, and a high manufacturing rate can be achieved. During the multi-layer printing simulation, the accumulated heat inside the part increased gradually with an increased number of printed layers. Additionally, the calculated residual stress exceeded the yield limit for all the studied build orientations due to the printed part’s high-temperature difference. When preheating was applied, the residual stress decreased by 23% and the distortion decreased by 54%. For the successful PBSLP of ceramics, commercial printers cannot be used effectively. A particular printer equipped with a temperature controller and a preheating system is required for ceramics.

Powder Bed Selective Laser Processing of Alumina: Scanning Strategies Investigation

Powder Bed Selective Laser Processing (PBSLP) is a promising technique for the additive manufacturing of alumina. For the method’s success, PBSLP process parameters such as laser power, scanning speed, hatching distance, and scanning strategies need to be investigated. This paper focuses on studying the scanning strategies’ effects on the PBSLP of alumina numerically and experimentally. Scanning strategies such as linear with different orientation, concentric, and islands were investigated. A numerical model was developed in which the PBSLP parameters, scanning strategy effects, and interpreting the experimental results could be observed. The numerical model proved its ability to reach the proper process parameters instead of using experimental trails which are time and cost consuming. For relative density, the island strategy succeeded to print alumina samples with a high relative density reaching 87.8%. However, there are round passages formed inside the samples that remain a barrier for the island strategy to be effectively used in PBSLP of alumina. Both linear and concentric strategies achieved a relative density of 75% and 67%, respectively. Considering the top surface roughness, samples printed with linear strategies gave low top surface roughness compared to the island and concentric strategies. Linear-45° is considered the effective strategy among the studied strategies as it achieved good relative density and low roughness at top and side surfaces. For PBSLP of alumina, new scanning strategies should be considered, and this study presents a new scanning strategy that is mainly based on space filling mathematical curves and should be studied in future work.

Investigation of the Joining Technology of FRP/AZ31B Magnesium Alloy by Welding and Riveting Hybrid Bonding Method

A novel joining technology was applied to join Fiber Reinforced Polymer (FRP) and AZ31B Mg alloy, which combined the laser-arc welding source and riveting joining methods. The design idea of the stepped rivet was proposed. The weld morphology, mechanical properties, microstructures of welds under two different rivet structures were investigated. FRP and AZ31B Mg could be joined successfully by the new hybrid joining method when it used two different structural rivets. The maximum tensile shear load of the joint under stepped rivet of small size was only 800 N, while that of the joint under stepped rivet of the larger size could reach 1419 N, nearly 90% of that of FRP. There was no reaction between the FRP plate and AZ31B rivet. While the magnesium elements and aluminum elements diffused and reacted with other elements in the FRP plate/AZ31B plate interface.