Effects of hot isostatic pressing temperature on casting shrinkage densification and microstructure of Ti6Al4V alloy

Abstract

T i6Al4V alloy has the properties of small density, high strength and good corrosion resistance. However, Ti6Al4V alloy castings are prone to shrinkage porosity due to its poor fluidity, leading to the poor performance of components. The defects must be reduced to a minimum level, especially in the field of aerospace and other high-quality-requirement industry. Hot isostatic pressing (HIP) has been proved to be a very effective way to eliminate or reduce shrinkage. A workpiece is heated and put under uniform pressure by inert gas, and the shrinkage is eliminated without changing the shape by the mechanism of plastic deformation, creep and diffusion bonding [1][2] . For decades, remarkable work has been done on the effect of HIP temperature on alloy's densification and microstructure. Gang Wang [2] et al. have studied the influence of HIP temperature on the microstructure of 2A12 powder compact. It was found that the microstructure Abstract:The Ti6Al4V alloy castings were produced by the investment casting process, and the hot isostatic pressing (HIP) was used to remove shrinkage from castings. The processing pressure and holding time for HIP were 150 MPa and 20 min, respectively. Four different HIP temperatures were tested, including 750 ºC, 850 ºC, 920 ºC and 950 ºC. To evaluate the effects of temperature on densification and microstructure of Ti6Al4V alloy treated by HIP, non-destructive testing and metallographic observation was performed. The experimental results show that the shrinkage was completely closed at 920 ºC and 950 ºC. The densification of Ti6Al4V alloy increased as the HIP temperature increased below 920 ºC. The lamellae were more uniform, the thickness of lamellae was obviously broadened and the structure was coarsen. Besides, the Norton creep equation was used to simulate the effect of different temperatures on the densification of Ti6Al4V alloy during HIP. The simulation results were in good agreement with the experimental results. It was also found that 920 ºC is a suitable temperature for HIP for Ti6Al4V alloy.Key words: Ti6Al4V alloy; hot isostatic pressing; temperature; casting shrinkage; densification; microstructure of raw powders changed from a dendrite structure to an equiaxed structure from room temperature to 600 ºC. Lei Xu [3] also investigated the effects of temperature and pressure on microstructure of powder compacts during HIP. Through the comparison of the samples HIPped at 800 ºC and 910 ºC, it was found that more stable and homogeneous microstructure can be obtained at higher temperature. Chung-Hung Tam [4] et al. have studied the effects of three different HIP temperatures 1,323 K, 1,373 K and 1,423 K on the densification of Cr-Si targets, and it was found 1,373 K was a suitable HIP temperature for the three Cr-Si targets. Yu Zhou [5] investigated the effects of HIP temperature on the densification of Nibased superalloy. It was found that HIP temperature played a dominant role in the cavity healing process. With gradually increasing HIP temperature, the cavity healing behavi...

Closure Behavior of the Artificial Gas Pore inside the As-Cast Ti6Al4V Alloy during HIP: Constitutive Modeling and Numerical Simulation

“…The fraction of recrystallized grains around the pore increased significantly, and the recrystallized grains obviously grew up with the increase in HIP dwell time compared with HIP-2. When compared with shrinkage cavities and shrinkage porosities [14], the gas pores cannot be completely closed after a standard HIP cycle (HIP-3).…”

“…To our knowledge, there are only a few publications focusing on the simulation of the void closure during HIP. Xu et al [14] investigated the effect of different HIP temperatures on void closure and found that the void can be closed when the HIP temperature is above 920 • C. Epishin et al [13] studied the pore annihilation in a single-crystal nickel-based superalloy during HIP by experiment and simulation. Two mechanisms for pore closure were revealed: the void was closed by plastic deformation and diffusion.…”

Influence of hot isostatic pressing on microstructure, properties and deformability of selective laser melting TC4 alloy

“…Hot isostatic pressing (HIP) can compact the internal defects through high pressure, thus improving the overall mechanical properties of the components. Xu et al [16] studied the effects of HIP temperature on as-cast TC4, the results showed that the shrinkage was reduced as the HIP temperature increased, and even completely closed above 920 ºC. Jiang et al [17] found that after HIP, the porosity defects in direct laser deposition Ti60 alloy disappeared, and the elongation increased from 6.5% to 12.0%, while maintaining tensile strength over 1,000 MPa.…”