Chen Kangmin

Research on Thermal Wear of Cast Hot Forging Die Steel Modified by Rare Earths

Abstract Thermal wear of cast hot-forging die steel modified by rare earths(RE) was studied and compared with commercially used die steels. The function of RE and the mechanism of thermal wear of cast steel modified by RE were discussed. The results showed that with increasing content of RE, the wear rate of cast steel reduced at first and then increased. By adding 0.05% (mass fraction) RE, the cast hot-forging die steel with optimum thermal wear resistance was obtained, which was better than that of H13 and 3Cr2W8V. The large amount of coarse inclusions, (RE)2O2S, resulted from excessive RE, which obviously deteriorated thermal wear resistance. The mechanism of thermal wear of the modified cast die steel is oxidation wear and oxide fatigue delamination. The wear debris are lumps of Fe2O3 and Fe3O4.

DISLOCATION MECHANISM OF SURFACE MODIFICATION FOR COMMERCIAL PURITY ALUMINUM AND ALUMINUM ALLOY BY LASER SHOCK PROCESSING

Surface modification experiment of the commercial purity aluminum(α-Al) and AlCu -Mg alloyed aviation aluminum alloy 2A02 by laser shock processing(LSP) was implemented.The surface strengthening effect of both the target materials was investigated from dislocation mechanisms of microstructural response by means of TEM method.The results show that the strengthening effect of the two kinds of materials by laser shock processed is significantly different.The strengthening mechanism of a-Al by laser shock can be attributed to the multiplication of a large number of dislocations. With the increase of the impact number of laser shock and the degree of deformation,the new-generated dislocations will pile up and interact with the forest dislocations,and the dislocation lines will gradually evolve into waved-like,or wind into dislocation tangles and dislocation networks. But the hardness curve of the laser shocked(α-Al) will fast and linearly decline due to Bauschinger effect(BE) and stress wave damping.The laser shock strengthening mechanisms of the aging-hardened aluminum alloy 2A02 can be summarized to the enhancement of the matching between the elastic energy of dislocations with the ultra-high energy of laser shock processing due to the higher matrix strength and the dislocation-pinning effect of large number of dispersed precipitates,as well as the complex dislocation networks in between the precipitates constructed by the dislocations induced by laser shock. The matrix strengthened by laser shock processing and the precipitates keep the extra-semi-coherent relationship to coordinate the total deformation,with the number of laser shock increase,dislocation multiplication and the vacancy motion constitutes geometrically necessary boundaries(GNBs),which consists of the sub-grain boundaries to refine the matrix into the nanometer-grains.The strengthening mechanism of surface modification of aluminum alloy by laser shock processing is formed of the internal stress state caused by the combination of the complex dislocation configurations and the Hall-Petch effect of the nanocrystalline grains.

SURFACE LAYER HIGH-ENTROPY STRUCTURE AND ANTI-CORROSION PERFORMANCE OF AERO-ALUMINUM ALLOY INDUCED BY LASER SHOCK PROCESSING

7075 aluminum alloy is an ultra-high strength alloy containing Al, Zn, Mg, Cu and Cr elements,and is widely used in the aviation industry, but it has severe intergranular corrosion characteristics. The high-entropy alloys are composed of more than five major metallic elements and possess excellent corrosion resistance.When laser shock, featuring ultra high energy as well as the thermodynamic and kinetic loading characteristics farfrom-equilibrium states, acts on the surface of alloys with multiple elements, high-entropy alloy surface layer with specific properties may be obtained. In this work, surface modification of 7075-T76 aluminum alloy by laser shock was investigated. The microstructure, formation cause of the amorphous/nano- crystalline composite high- entropy alloy surface layer obtained by laser shock, hardness and corrosion resistance of the laser were analyzed by means of SEM and TEM. The results show that the adiabatic shear thermal effect induced by super high energy, ultra-fast process of laser shock causes surface alloy system to occur entropy increase effect and partitioning. The high mixing entropy contributes to the randomization increase of the alloy system. Thus, the elements in the system spontaneously self- organize in accordance with the law of Boltzmann. The dynamical formation of the nano- crystalline grains coordinates the thermodynamic equilibrium during the process. The strain- hardened layer is composed of amorphous microstructure and nanocrystalline grains, and the total depth of it reaches up to about 100 μm. After 1time laser shock,the depth of the surface high entropy layer is about 20 μm, of which the diameter of the nanocrystalline grains is 6~8 nm. After 3 times laser shock, the thickness of the layer can increase to more than 40 μm, and the diameter of the nanocrystalline grains is 2~3 nm. Meanwhile, the intense ultra high strain-rate induced by the laser shock makes precipitates deform, producing parallelly distribution of deformation twins in order to balance the laser energy. After repeated laser shocks, the hardness of the amorphous/nanocrystalline layer gradually closes to that of the matrix of the alloy because of the disappearing of the support of grain boundaries to the strength, the dislocation strengthening effect in nano-crystalline grains, and the coherent relationship between precipitates and matrix. Due to that the amorphous microstructure can prevent galvanic effect around precipitates, and nano-crystalline has good chemical stability, the nano-crystalline/amorphous composite high-entropy layer on surface of 7075-T76 aluminum alloy induced by laser shock can significantly improve the corrosion resistance, and effectively block the intergranular corrosion of the alloy.

Microstructure Characterization of Titanium Alloy Sheet by Laser Shocking

The TA2 (ASTM F67 Grade 2)Titanium alloy sheet was laser shock deformed by the Nd: YAG laser with the 1054nm output wave length and the 20ns short pulse, with 7 mm laser facular in diameter and tap water as the confinement regime. The experimental results indicated that under the available constraint condition, the obverse and the deformation was sensitive to the strain types. Strain-induced marten site in the compressive-stressed region, and the strain-induced twins in the tensile-stressed region were observed respectively. The super-high energy and super high strain ratio upon laser shock deforming can make the c.p.h. titanium alloy induce a great deal of twin bands, which coordinate the orientations in grains, thus induce auxiliary slip systems operate and make the plastic deformation proceed.