| Graphite is an ideal substitute for the hanrmfu l element Pb in lead brass due to its excellent chip breaking performance and lubricity,and has the advantages of green environmental protection and abundant reserves.However,the poor wettability between grapthite and cooper will reduces the mechanical properties of Cu40Zn.This paper adds the alloying element Ti to the graphite brass to improve the wettability of the Gr and Cu40Zn matrix.On the basis of ensuring the machinability of graphite brass,the purpose of improving its mechanical properties is achieved.Three series of alloy samples,Cu40Zn-Gr,Cu40Zn-Ti,Cu40Zn-Gr-Ti,were prepared by powder metallurgy using Spark Plasma Sintering and hot extrusion.Micro structure analysis,mechanical properties and machinability tests were carried out to study the effects of Gr content,Ti content and dual phase ratio on the performance of lead-free brass,and the enhancement mechanism and cutting mechanism were elaborated in detail.Gr and Cu40Zn matrix neither wet nor react with each other,there is no new phase formed when Gr is added into Cu40Zn.Gr still exists in the matrix in the form of elemental substanc.e after sintering.The yield strength,tensile strength,elongation and hardness of the material decrease with the increase of Gr content.There are a lot of cracks on the fracture surface of tensile specimens,and there are obvious tearing edges near the Gr particles.The reason is that the addition of Gr to Cu40Zn is simply mechanical locked,and cracks nucleate and grow preferentially in the vicinity of interface of Gr and Cu40Zn during tension until failure occurs.With the increase of Gr content,the cutting force of the material shows a downward trend.The chip breaking effect and lubrication effect generated by Gr can significantly improve the cutting per-formance of the material.The results of Cu40Zn-Ti study show that the strength and hardness of Cu40Zn increase gradually with the increase of alloy element Ti.The addition of Ti will react with Cu in the matrix to form CU2Ti4O compound.CU2Ti4O is distributed on the matrix Cu40Zn,and the strength of the material is enhanced by dispersion strengthening together with nano-sized Ti precipitated from Cu during cooling.On the other hand,Ti atoms are slightly dissolved in Cu atoms at room temperature.The lattice distortion is caused by the different radius of Ti atoms,which increases the resistance of dislocation movement and increasing the strength of the materials.In addition,the existence of CU2Ti4O dispersion strengthening phase in the garin boundary makes the Cu40Zn grain size smaller and plays a significant role in fine grain strengthening.After extrusion at 600 °C,both alpha and beta phases are distributed along the extrusion direction.CU2Ti4O compounds are distributed along the grain boundary.After extrusion,they distribute dispersively between the plastic deformation zones.The cutting force of Cu40Zn-Ti increases with the increase of Ti content.When graphite powders and titanium powders were added into Cu40Zn,the presence of Gr inhibits the Cu2Ti4O generation.The CU2Ti4O compound decreased with the increase of Gr content.TiC is formed on the surface of Gr particles,and the existence of TiC improves the wettability of Gr and matrix.When the content of Gr is constant,the strength and hardness of Cu40Zn-Gr-Ti materials increase with the increase of Ti content,and the elongation decreases slightly.Through mechanism analysis,the strengthening mechanism of alloying element Ti includes dispersion strengthening and solid solution strengthening.The tensile fracture has both dimples and tearing edges,which belongs to ductile fracture.Compared with Cu40Zn-Gr,the cutting force of Cu40Zn-Gr-Ti material first decreases and then increases.Cu40Zn-0.SGr-1.5Ti has good comprehensive properties.Its yield strength,tensile strength and hardness are 326 MPa,525 MPa and 145,respectively.The cutting force is basically consistent with Cu40Zn-0.5 brass. |
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