| Because of the good physical and mechanical properties,Cu alloys have attracted major attentions in many application domains,especially the large-scale integrated circuits,and have become the hot studied advanced materials all over the world.Lead frame is the base component of the integrated circuit(IC),and the development of IC toward large-scale and high power requires the lead frame having more legs and smaller distance between these multi-legs.In order to meet these requirements,the materials used for preparing lead frame are required to have not only strength higher than 600 MPa and electrical conductivity(EC)higher than 80%IACS,but also a good sheet formability along with high surface quality.Cu-Ni-Si alloy is one of the typical precipitation strengthening alloys,which has many potential advantages,such as moderate strength,high EC,good plasticity,environmental friendliness,and so on,and thus considered as idea candidate for lead-frame materials.The Cu-Ni-Si alloy prepared by traditional method is easy to form a network of coarse Ni-Si intermetallic compound(Ni31Si12),and the large Ni-Si phase is difficult to be sole-dissolved into the Cu matrix during solid solution treatment.As a result,the quantity and distribution of precipitated phase after aging treatment are limited,and the electrical conductivity and mechanical properties of the alloy cannot be guaranteed.In this thesis,Cu-Ni-Si alloys were firstly prepared by powder metallurgy method from Cu,Ni and Si elemental powder,the influence of hot-pressed sintering parameters on density,microstructure and phase composition were studied,and then the formation subsequence of-the Ni-Si intermetallics was discussed.Subsequently,Cu-Ni-Si alloys with different Ni and Si contents as well as different Ni/Si mass ratios were prepared,and an optimal chemical composition was confirmed from the viewpoints of both strength and EC.Thirdly,solution and aging treatments were used to regulate the amount,size,and distribution of Ni-Si intermetallics in the Cu-4.93 Ni-1.07 Si alloy,and the precipitation thermodynamic behavior of δ-Ni2Si phase was discussed.Finally,hot deformation behavior of the Cu-4.93 Ni-1.07 Si alloy was investigated by using the thermal simulation experiments,and a hot processing diagram was established and verified.The main conclusions can be drawn as follows:(1)During the hot-pressed sintering process of Cu-Ni-Si alloy,some amount of Ni and Si preferentially react at the grain boundary to form Ni31Si12 phase,while the other part dissolves into the Cu matrix by solid-state diffusion during the heating and holding stages.During the subsequent cooling process,Ni atoms and Si atoms in the α-Cu(Ni,Si)solid solution precipitate as δ-Ni2Si phase.Compared with that in the alloys sintering at lower temperature(700℃),the amount of Ni31Si12 phase is significant lower but that of δ-Ni2Si phase is higher in the Cu-Ni-Si alloys prepared by sintering at high temperature(950℃).However,the high sintering temperature also causes high grain grow speed,since the decrease activity energy for grain growth with increase of temperature.(2)There is always a mutually contradictory law between the strength and electrical conductivity of Cu-Ni-Si with a fixed Ni/Si mass ratio,and a high hardness occurs in the alloys with higher than 10 wt.%(Ni+Si)content,but a high EC occurs in that with lower than 2 wt.%(Ni+Si)content.At the condition of constant Ni+Si content,the hardness and EC varies with the change of Ni/Si mass ratio since the change of the amount of δ-Ni2Si phase.When the Ni/Si mass ratio is 4.6:1,the amount of δ-Ni2Si phase precipitated is the largest,and then the combination performance of the alloy is the best.(3)Avrami phase transformation kinetics equation for Cu-4.93 Ni-1.07 Si alloys during aging process is established based on the experimental studying on the relationship among EC and aging parameters,and then the function of EC equations was also obtained since there is a close dependence of EC on the amount of precipitations.The established equations were demonstrated to predict the EC of Cu-Ni-Si alloys after aging at different temperature and time length well.(4)According to the research results on sintering craft,chemical composition and heat treatments,the best combination properties,viz.hardness 248.3 HV,yield strength 601.6 MPa,tensile strength 667.5 MPa,the EC 42.2%IACS,is obtained.The corresponding chemical composition is Cu-4.93 Ni-1.07 Si,sintering temperature and time length are 950℃and 0.5 h,solution temperature and time length are 850℃ and 2 h,aging temperature and time length are 450℃ and 16 h,respectively.(5)Constitutive equation of Cu-4.93 Ni-1.07 Si alloy considering strain effect is established based on Arrhhenius constitutive model and the thermal simulation experiments at the temperature range 800℃～950℃ and strain rate 0.01 s-1～1 s-1,and then the hot working diagrams were established by using the dynamic material model and Prasad instability criterion.The hot processing window is heating temperature 840℃～960℃ and the strain rate was 0.01 s-1～0.1 s-1,which was verified to be effective for Cu-4.93 Ni-1.07 Si alloy. |
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