Research on metal flow law and strengthening mechanism of cold extrusion internal thread

In this paper, the method of combining numerical simulation and experiment is used to study the flow law of metal during cold extrusion of internal threads and theoretically explain the strengthening mechanism of cold extrusion internal threads. By establishing a simplified mechanical model and a finite element model, the load change law of the tap during the extrusion process, the flow law of the metal and the surface hardening mechanism are studied. The research results show that during the extrusion process of the internal thread, the extrusion torque and the three-way load both increase first and then decrease and reach the maximum in the stable extrusion stage. When the internal thread is formed, the metal flows along the surface of the extruded prismatic teeth, and the metal flow rate in the crest area is the fastest and the displacement is the largest, followed by the metal flow rate and displacement in the flank area, and the metal flow rate in the tooth base area is the slowest and the displacement is the smallest. Compared with the original material, the hardness of the surface layer of the extruded internal thread is increased by 15% to 28%, and the hardness is proportional to the density of the metal fiber arrangement. The thread tensile test shows that when the tooth height ratio is between 65 and 80%, the tensile failure load of extruded internal threads is about 20% higher than that of cutting internal threads. Under the premise of the same tooth height, cold-extruded internal threads can withstand higher tensile loads than cutting internal threads. The above research results further clarify the plastic strengthening mechanism of the metal in the extrusion process of internal threads and have guiding significance for the performance evaluation of extruded threads.