Aluminum alloys such as F357 are used for structural applications in AM for their low shrinkage and narrow temperature range for solidification. These materials could offer a viable lower-priced alternative to titanium, especially on larger parts. Powder bed fusion 3D printing of F357, however, can present challenges such as the presence of dissolved gases (hydrogen or argon) leading to blistering and poor surface quality. In a new study, researchers from Quintus Technologies, SLM Solutions and the University of Arizona have shown that the use of a novel Uniform Rapid Quenching (URQ) HIP technology can lead to F357 parts with no observable defects, ultimate and yield strength exceeding MMPDS cast properties and little geometric distortion of part geometries.
In many industrial manufacturing processes, Hot isostatic pressing (HIP) is used to reduce the porosity of metal parts. The process is generally used to densify powders or cast and sintered parts in a furnace at high pressure (100-200 MPa) and at temperatures from 900 to 1250°C for example for steels and superalloys. The gas pressure acts uniformly in all directions to provide isotropic properties and 100% densification.
F357 HIP processing is typically performed at temperatures above 500°C and at 75-150 MPa argon atmosphere pressure. In addition to the benefit of increased density of the aluminum alloy, there is a decrease in porosity and an overall increase of fatigue, toughness, and ductility. A standard T6 temper heat treatment consisting of a solution heat treatment and artificial age is applied after the HIP process so that desired mechanical properties can be achieved, but when a separate 540°C solution heat treatment is performed at atmospheric pressure there is a risk of the pores closed in the HIP cycle being opened during T6 and growing in size.
The enlargement of pores due to the T6 tempering can be a result of the high pressure that remains in the pores after HIP due to the low diffusivity of hydrogen or argon in the aluminum lattice and the lower resistance of alloy deformation at high temperatures. To avoid blistering or TIP, Quintus Technologies, a company that specializes in the design of high-pressure processing equipment, has developed an alternative to having two separate processes of standard HIP cycle and solution heat treatment. The novel Uniform Rapid Quenching (URQ) HIP furnace allows for HPHT (High-Pressure Heat Treatment) by a combined solution heat treatment and HIP process. This would entail a HIP under pressure as a solution treatment and then rapidly quenched—all performed in the HIP vessel. With the URQ system, the combination of performing a HIP and T6 solution heat treatment could result in proper hardening precipitate formation during the subsequent age while reducing the chances of any pores re-opening.
A case study was carried out by the University of Arizona, SLM Solutions, and Quintus Technologies to evaluate the application of HPHT in a HIP URQ furnace on a PBF-LB high-strength aluminum alloy F357. The study evaluated the density, microstructure, mechanical properties, and distortion of this novel HIPing approach. Experimental Design Materials were printed by SLM Solutions using an SLM 280 PBF-LB 3D printer with gas atomized F357 powder.
The results showed a robust post-processing method offering excellent tensile properties while avoiding thermally induced porosity and distortion. The study demonstrated that processing aluminum F357 using the Quintus URQ process is a promising approach for creating high-quality AM aluminum components. The HPHT process provides benefits such as tensile strength and ductility comparable to that of standard HIP+T6 material; low geometric distortion of components, even with complicated geometries; avoidance of blistering and lower overall process cycle time. Additional work is ongoing to assess the fatigue properties of URQ F357 and to further explore optimizing the balance of strength, ductility and manufacturability in these components.
The full White Paper is available here.