| During the process of metal hot forming,the billet and die contact with each other under the conditions of high temperature,high pressure and severe sliding.The surface contact is accompanied by the transfer of heat and force,the migration of material,the evolution of structure as well as the oxidation and damage of material.Some relevant factors are complicated,such as plastic deformation heat of billet,interfacial friction heat,heat transfer properties across interface,oxidation and abrasion of die surface.This is a complex trans-scale and thermo-mechanical coupling problem,covering different interdisciplinary knowledge of mechanics,materials science,tribology and heat transfer.In this paper,based on actual contact surface,the author explored the friction heat,deformation heat and heat transfer ways and investigated the micro-mechanism and heat transfer properties of interface contact heat transfer behavior between the billet and die from the micro perspective.Based on the method of non-equilibrium molecular dynamics?NEMD?and phonon scattering theory,the phonon heat transfer process at the die rough interface in the micro/nano scale was analyzed qualitatively,revealing the mechanism of heat transport at the die contact interface.Meanwhile,the formation and growth process of oxide layer on the working surface of die were simulated using the ReaxFF reaction force field.The simulation results revealed some interesting mechanisms of the formation and growth of interfacial oxides which provided a theoretical basis for further understanding the relationship between interfacial heat transfer and oxidation,damage failure of die surface.Finally,Based on the experimental and simulation results mentioned above,the thermo-mechanical coupling simulation of TC11 titanium alloy gear disk was carried out by using Deform-3D software,and combined with the statistical data of forging production and die failure,the main failure mechanisms of forging die in three easy worn regions were analyzed comprehensively.Aiming at the“root”of failure mechanisms,the composite ceramic coating and the polymetallic heat-resistant coating were then treated on the surface of forging die respectively to improve the heating condition of die surface by reducing the interfacial heat transfer coefficient?IHTC?.It is of great practical significance to study on the project to improve the quality of product and prolong the service life of die.The developed exploratory work and achievements are listed as follows:1)A set of experimental platform based on the real forging environment was developed to study the heat transfer properties of the metal hot forming.Taking those recognized materials which are hard to forming such as GH4169 superalloy and TC11 titanium alloy as the objects of study,the author carried out the upsetting tests respectively on the H13 hot flat die with nitriding heat treatment.During these tests,the multi-channel thermocouples were embedded into the billet and die to collect their temperatures in different time regularly,and the die temperature was properly controlled to calculate the IHTC under the condition of different forging parameters.This experimental platform can also be used for other testing researches of new products and new materials.2)It was the nonlinear estimation method,one of the inverse heat transfer methods,that was adopted to build a mathematic model for the determination of the surface heat flux and IHTC,and latterly a general inverse heat transfer analysis computer program was developed.Through a comparative study on the estimated temperature of numerical simulation and measured temperatures in experiment,the author verified the accuracy and reliability of the inverse heat transfer mathematic model for calculating the IHTC.Based on the inverse heat transfer model,the contact heat transfer behaviors between the billet and die were investigated in detail,especially focusing on the influences of the process parameters on IHTC,such as the effect of billet initial forging temperature,die preheating temperature,holding time,billet deformation rate.Experimental results indicated that the billet heating temperature had a minimal role in IHTC but the other components had a great impact on IHTC.Among them,the billet deformation rate is the most influencing factor on IHTC.Under the die preheating temperature ranging from 150oC to 400oC,it was found that the IHTC was proportional to preheating temperature.A high preheating temperature leading to a high IHTC made the thermal stress on the working surface of die increase rapidly,which was unfavorable to the service life of die.As the holding time increased,the IHTC declined due to billet oxidation,and oxide layer of billet showed an obvious blocking effect on interfacial heat transfer.However,when the holding time was up to a certain time,the blocking effect of oxide layer on interfacial heat transfer was gradually weakened.The influences of deformation rate on IHTC is not significant at the low deformation rate of billet,but when the deformation rate is regulated to over 0.1s-1,the oxide scale is exfoliated in layers under the action of the shearing force,resulting in the direct contact between the billet and die.Meanwhile,a great deal of deformation and friction heat had been generated in short time,which makes the condition of heat transfer to increase greatly,and the IHTC correspondingly did the same way.3)An asymmetric interface contact model was established,and the thermal transport process of phonons in micro/nano scale rough interface was studied by using the molecular dynamics method.The effect of interfacial lattice defect concentration,the average interface temperature and the cross section size of the billet on the phonon heat transfer coefficient were discussed respectively.The simulation results showed that the interfacial lattice defect concentration is an important factor affecting the phonon thermal transport ability.When the lattice defect concentration increases,the phonon mismatch at the interface deteriorates and the interface thermal resistance increases,which leads to the sharp decrease of phonon heat transfer coefficient.The high frequency phonons at high temperature will be decomposed into two or more low frequency phonons during the energy transmission,which increases the propagation probability of phonon at the interface,thus the phonon heat transfer coefficient increases accordingly.The asymmetric contact interface restricts the phonon heat transfer to a certain extent.When the size of die section is fixed and the billet section is increased to near die cross section,the phonon heat transfer coefficient increases continuously.However,the increase of phonon heat transfer coefficient slows down gradually with the increase of interface contact area and tends to a fixed value.Then,the oxidation dynamic characteristics of FeCr alloy and Ni metal at?010?crystal orientation at the average contact temperature of 573K,673K and773K were studied using the ReaxFF reaction force field.The effect laws of die interface temperature on oxidation dynamics was gained after analyzing the total oxygen uptake and film thickness of oxide.The kinetic data were fitted to the island growth model and the growth mechanism of oxide was determined.That is,the growth of interfacial oxides is achieved by the synergistic movement of anions and cations diffusing inward and outward.The growth pattern of oxide is changed from the initial island growth mode?<75 ps?to the final layer by layer growth mode?>100 ps?.The increase of interface average temperature and interfacial lattice defect concentration are beneficial to the growth of interfacial oxide,and the increase of interfacial average temperature promotes the growth of oxide more greatly.4)Based on the transient heat transfer theory,the measured IHTC h was taken as the boundary condition,and the deformation heat and the interface friction heat during the hot forming of billet were considered,then the engineering calculation formula of temperature field of hot forging die surface was derived,and the instantaneous surface temperature peak value of hot forging die in continuous operation was analyzed based on the formula.In addition,the stress field?thermal stress field and mechanical stress field?of forging die was analyzed based on the existing theoretical model,and the formula of the peak value of thermal stress on the surface of forging die was gained.On this basis,the influencing factors of thermal stress were discussed.5)Taking the die forging process of TC11 titanium alloy gear disk produced by an enterprise as an example,the thermo-mechanical coupling simulation was carried out by using the Deform-3D software.Combined with the statistical data of forging production and die failure,the main failure mechanisms of three easy worn regions in forging die surface were analyzed synthetically.In region 1,the contact time between the billet and die is long.With the increase of the heat transfer at the interface,the surface of die cavity is softened,and the adhesive wear occurs in this area under the action of high mechanical compression stress.In region 3,the debris caused by oxide particles and thermal mechanical fatigue cracks is stuck on the surface of forging die,and accompanied by the hot forging process,the debris flows intensively on the surface of die cavity,which directly leads to the abrasive wear on the forging die surface.In region 2,however,its working condition is not as severe as region 3 and 1,the mechanical cracks occur on the surface of forging die due to the existence of thermal stress on die surface and the coupling action of mechanical compressive stress,yet the mechanical cracks did not cause obvious loss of die material.All of these failure mechanisms in die surface are directly or indirectly related to IHTC.The high IHTC value makes the working surface of forging die form a large peak temperature and thermal stress,which is the"root cause"of various damage and failure induced by forging die.For this reason,the surface of H13 forging dies were treated with composite ceramic coating and multi-metal heat-resistant layer respectively according to the designing idea of functionally gradient die material.This is for the purpose of prolonging the service life of forging die by reducing the IHTC and improving the heating condition of die surface.The test results showed that the surface treatment process of composite ceramic coating and heat resistant polymetallic coating are the effective measures to reduce IHTC and to improve the surface heating condition of forging die,which can be used in different forging events.Compared with the surface of forging die with conventional nitriding treatment,both the composite ceramic and polymetallic heat-resistant coatings had a stronger effect on the interfacial heat transfer,in which the blocking effect of the ceramic coating on IHTC in low deformation rate working hours was greater than that of the polymetallic heat-resistant coating due to its lower thermal conductivity.However,the composite ceramic coating was prone to spalling when the billet deformation rate was over 0.1s-1,which will lead to a weaken of heat blocking effect.The polymetallic heat-resistant coating,by contrast,had a strong bonding strength with H13 matrix steel.It was not easy to peel off under high deformation rate and still maintained a strong heat resistance to the interface heat transfer.The research work listed above provided more accurate and reliable experimental data for readers to study the precision plastic forming science and offered a theoretical basis to research the design of functional gradient die material.Meanwhile,it laid the foundation of knowledge to prolong the die service life and to improve the product precision. |
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