Prediction of forming characteristics of titanium alloy self-locking nuts

Titanium alloy is an important class of aerospace material due to its high specific strength, excellent anti-corrosion and anti-oxidation. In this paper, a three-dimensional thermo-mechanical coupled simulation was carried out to predict the formation characteristics of TC4 titanium alloy self-locking nut during the upset forging process. The stability of the upset forging was analyzed, and the influences of initial temperature and deformation velocity on the formation quality were investigated. The results show that if length-diameter ratio of the sample less than 3.27, the upset forging formation tends to be stable, and here, the length-diameter ratio of 2.89 was selected. Additionally, the forming quality of TC4 self-locking nut improves with the increase of initial temperature and decreases with the increase of the velocity of the upper die. The analysis results can provide a theoretical guidance for the upset forging formation of TC4 titanium alloy nuts.

Forging of Mg-Al-Zn Magnesium Alloys on Screw Press and Forging Hammer

Magnesium alloys are highly strain rate sensitive and exhibit good workability in a narrow forging temperature range. Consequently, parts made of these materials are usually forged with low-speed hydraulic presses, using specially designed tool heating systems in order to ensure near-isothermal conditions. This study investigates whether popular magnesium alloys such as Mg-Al-Zn can be forged in forging machines equipped with high-speed forming tools. Experimental upset forging tests on AZ31B, AZ61A and AZ80A specimens were conducted, using a screw press with a ram speed of 0.5 m/s and a die forging hammer with a ram speed at stroke of about 5 m/s. Test specimens were preheated to 350 °C, 410 °C and 450 °C. After the upset forging process, they were air- or water-cooled and then examined for their workability, hardness and grain size. To validate the results, a forging process for a producing handle was designed and modelled by the finite element method. Distributions of strain, temperature and fracture criterion were analysed, and energy and force parameters of the forging process were calculated. After that, experimental tests were performed on AZ31B and AZ61A specimens in order to determine mechanical properties of forged parts and examine their micro- and macrostructure. Results have demonstrated that AZ80A is not suitable for forging with either the screw press or the die forging hammer, that AZ61A can be press- and hammer-forged but to a limited extent, and that AZ31B can be subjected to forging in both forging machines analysed in the study.

Rotary Friction Welding of Molybdenum without Upset Forging

A large instantaneous axial forging load is required to be applied for the final stage of rotary friction welding (RFW), which is usually conducive to obtaining clean, compact, and high-quality joints. However, for slender fuel claddings made of molybdenum (Mo) with low stiffness, the instantaneous axial forging load cannot be applied at the final stage of welding. This study carried out RFW tests without upset forging on Mo in the atmospheric environment and investigated the effects of welding time on joint morphology, axial shortening, microstructures, microhardness, tensile strength, and tensile fracture morphology. It found that the excessive and abrupt burning and a lot of smoke were generated around the weld zone during welding and spiral flashes were observed after welding. Under welding pressure of 80 MPa and spindle speed of 2000 r/min, the minimum average grain size and maximum tensile strength can be obtained in 4 s when the welding time is between 2–5 s. Scanning electron microscope (SEM) results show that there were morphologies of a large number of intergranular fractures and a small number of transgranular fractures in the fracture. The above results demonstrated that it is feasible to use RFW without upset forging to seal the last weld spot on upper end plugs of fuel claddings made of Mo in high-pressure inert gas, which would not only obtain reliable welding quality but also seal high-pressure inert gas in cladding tubes. The research results have a practical guiding significance of manufacturing accident-tolerant Mo nuclear fuel cladding.