CHAPTER 2: LITERATURE SURVEY

Transcription

1 7 CHAPTER 2: LITERATURE SURVEY 2.1. Introduction The powder metallurgy processing is one of the oldest and economic routes for producing critical and complex shaped products [1-3]. P/M is one of the most flexible manufacturing processes capable of delivering a wide range of new materials, microstructures and properties. Low alloy P/M steel materials find applications in areas such as structural and automotive parts like clutch, gear, precise engine parts, bearings etc [4-8]. Production of metal powders and converting to useful engineered structures needs a detailed study of the various steps for making the final product. Key steps are powder characterization, mixing, compaction and the subsequent thermal bonding of particles by sintering process. Powder types, characterization, powder fabrication, alloying and mixing for the P/M steels have been analysed by various researchers [9-16]. These studies conclude that low-alloy P/M steels containing Cr, Mo, Si, Ferro Si, Cu and Mn, manufactured from elemental powders are suitable for precision parts of automotive engines and transmissions. Hence, in the present research work admixed elemental metal powders such as Cr, Ni and Mo subjected to single pressing and sintering operation is considered. The phenomenon of compaction, compaction methods, compaction and ejection of the preforms from the die has been analysed. The key stages of powder compaction are rearrangement, localized deformation, homogeneous deformation and finally bulk deformation [16-20]. The sintering process is the most important step in P/M by which

2 8 the required properties such as strength, densification, and dimensional control are attained in the final product. This is due to strong bonding between particles, thereby making it a very useful article in almost all practical applications [21-24]. This chapter presents the survey of earlier literatures pertaining to various aspects of the P/M steels such as plastic deformation, densification, mechanical properties and Artificial Neural network (ANN) approach for the prediction of the plastic deformation and densification behaviour of P/M steels Plastic Deformation and Densification Densification is a precursor to high performance levels in P/M products. Achieving full densification will be possible by several routes and often involves the simultaneous application of stress and temperature to close residual pores. This is achieved through cold and hot forging or extrusion processes. Flow stress is a parameter to select a particular metal forming equipment. Flow stress is a stress, which results in flow of material in one-dimensional stress state. Chemical composition, purity, crystal structure, grain size, heat treatment are some of the influencing factors for flow stress [25]. This flow stress is a major influencing factor for plastic deformation of the material. Due to the plastic deformation, pores inside the material also deform plastically results to densification of material. Earlier experimental data shows that, there is no possibility of attaining 100% densification [32]. Normal and tangential stress exist during plastic deformation [26-28]. Normal stress is acting directly on the punch work piece boundary area, where as tangential stresses result due to the

3 9 relative movement between work piece and the punch. To improve metal forming practices knowledge of the integral parameters such as total plastic deformation, applied force and energy requirements, along with the detailed parameters such as local metal flow, normal and tangential stress distribution over the contact area and the deformation zone is necessary. This is necessary in order to achieve an optimized local parameters distribution such as to attain a homogeneous microstructure and mechanical properties of the finished product, to avoid intensive wear of the preform-tool interface. During cold and hot forging, the plastic deformation and densification characteristics of sintered powder materials are not same compared to conventional materials. Hence, detailed studies are required for the plastic deformation of the sintered P/M steels. Strengthening of sintered P/M low alloy steels can be achieved through densification, alloying and heat treatment [29-32]. Several researchers [29 & 33], have investigated the cold forming of ferrous and nonferrous powder preforms and have reported the merits of the process such as good surface finish, higher accuracy, and superior strength due to work hardening and geometric hardening. Some of the limitations of hot forging are oxidation and decarburization of the surface of billets; excessive die wear, poor surface finishing & dimensional accuracy, and the induction of thermal stresses. As a result, cold forging has been gaining importance in recent times. In the majority of investigations pertaining to cold forming, the focus has been to study the influence of applied stress, strain rates, lubrication, and deformation levels [34-36].

4 10 Further, the quality of the product obtained through Powder Preform Forging (PPF) is very much influenced by the various process parameters such as forging temperature, initial preform density, alloying elements and flow stresses. Chandramouli et al. [37] analysed the influence of material flow constraints during cold forming on the deformation and densification behaviour of hypo-eutectoid P/M steel ring preforms. They concluded that lower geometry ring preforms densify at higher rates under various modes of constrained deformations and under lower flow stresses. Enhancing the carbon content is reported to lead to higher rates of densification under lower flow stress values. Chandramouli et al. [38] studied the deformation, densification and corrosion studies of sintered powder metallurgy plain carbon steel preforms. They concluded that the sintered plain carbon steels containing carbon up to 1% with density near theoretical have potential applications requiring high strength, high hardness, and wear resistance. In the forged conditions, such steels are suitable materials for tools, bearing rollers, fasteners, cams etc Plasticity Theory & Formability Plasticity is the propensity of a material to undergo permanent deformation under applied pressure. The available yield criterion for the conventional materials are not suitable for the powder materials, due to the presence of voids and hence, new yield criterion have been developed [41-49] for porous materials.

5 11 Narayanasamy and Ponalagusamy [47-49] developed a mathematical theory of plasticity for compressible P/M materials. The new yield function developed by Doraivelu et al. [46], taking into account the hydrostatic stress, is considered in their work. Numerical integration has been carried out in order to compute the yield stress. Their research concludes that the ratio of the yield stress of the P/M porous material to the yield stress of the fully dense material is found to increase with increase in densification due to plastic deformation. Fashang Ma and Kikuo Kishimoto [50] introduced a non-associated flow rule to characterize the yield and subsequent plastic deformation of porous solids by FEM analysis on the axisymmetric unit cell. The post-yield response of the unit cell is in good agreement with the prediction of the non-associated constitutive model. Justino et al [51] developed a flexible constitutive model for the elastic-plastic analysis of porous sintered materials using ABAQUS general-purpose finite element program. Narayanasamy and Pandey [52 & 53] analysed the phenomenon of barreling in solid cylinders during cold upset forming. They derived the empirical relationships between the measured radius of curvature of the barrel and other variables such as the hydrostatic stress and the stress-ratio parameter. Vujovic and Shabaik [54] developed a new workability criterion for ductile metals. According to them the forming limit curves are important aids in determining the extent of deformation of the material during forming process. In their studies, they proposed a forming limit criterion for bulk metal working processes, based on the magnitude of the

6 12 hydrostatic component and the effective stress of the state of stress by means of three simple tests, namely, tension, compression and torsion tests. Further, the formability limit curve of a material is reported to provide its formability potential as a function of the stress state. This information is essential for the design of the forming processes and for optimum utilization of materials and processes. Abdel-Rahman and El-Sheikh [55] investigated the effect of the initial relative density on the forming limit of P/M compacts in upsetting. A workability factor describing the effect of the mean stress and the effective stress-which is a function of the relative compact density, has been formulated by them. Further, the effect of the relative density is discussed using Khun & Downey theory and Whang & Kobayashi theory for P/M characterization in plastic deformation and fracture. Narayanasamy et al. [56-58] used the workability studies introduced by Vujovic and Shabaik, Abdel-Rahman and El-Sheikh and others and simplified the calculation of various stresses and strains Mechanical Properties In common P/M ferrous materials and components produced by the conventional powder compacting and sintering process, there is a considerable amount of remaining pores which, as the susceptible notches producing stress concentration, have a strong effect on the mechanical properties, especially on toughness. Therefore, a detailed analysis has to be performed on P/M materials for their mechanical properties such as tensile, impact strength and hardness.

7 13 Danninger et al. [59] have presented a review of the existing literature on the relationship between mechanical properties and microstructure of P/M alloys. Danninger et al. [60 & 61] have reported that the tensile, impact and fatigue properties of sintered steels are sensitive to the sintering intensity and are linked to the macroscopic plastic deformation such as fracture elongation and impact energy. They have concluded that both compacting pressure and sintering parameters influence the mechanical properties of sintered steels. Mechanical properties of P/M steels having alloying elements such as Cr, Ni, elemental Silicon or Ferro Silicon, Mo etc., have been studied [62-82]. These studies conclude that the addition of Cr improves the mechanical properties of sintered steel through solution hardening and carbide precipitation. Addition of Cr is understood to increase the hardenability and nitridability. Further, addition of copper and nickel improve the density of Mo alloyed sintered steels, and as a consequence, their mechanical properties. Introduction of the alloying element Ni helps to retain austenite even at high relative densities and the strength of the sintered alloy could replicate the behaviour of wrought steels [79]. Further, it is also reported that small amounts of retained austenite are advantageous and the oxide inclusions in the steel did not affect the strength. Among the metallic additives Mo has been reported to lead to the fastest homogenization rate followed by Cu and Ni. Youseffi et al. [78 & 80] studied the sintering, microstructure and mechanical properties of P/M Mn, Mo steels and reported that the sintered microstructures are sensitively

8 14 dependent on the cooling rate after sintering. According to the findings of the above mentioned work, Mo has been identified as having beneficial effect on the mechanical properties of the steels studied. Further, it was suggested that these alloys might be considered along with Cr and fine Si at higher sintering temperatures for further study on mechanical properties. The influence of microstructure on mechanical properties of Mo alloyed P/M steels were studied by Candela et al. [83]. They have reported that prior to 1990 s research work on low-alloy steels was limited to low Mo content (less than 0.5%) combined with high sintering temperatures above C. The low Mo addition also demands extensive repressing and resintering cycles. They conclude that the addition of Mo promotes a bainitic microstructure and leads to enhancement of mechanical properties. An attempt has been made in the present research work to employ marginally higher Mo addition to elemental metal powder mixes coupled with lower sintering temperature in order to characterize the tensile properties of the sintered and forged low alloy steels without resorting to extensive procedures such as repressing and resintering cycles. The tensile properties of sintered and forged low alloy Fe-C-Ni-Mo-Cr steels are found to be comparable to those of conventional wrought alloy steels of similar compositions, containing Mo, Ni & Cr [84].

9 Artificial Neural Network The plastic deformation, densification and formability of P/M preforms depend on the preform size, fractional density, porosity, pore geometry etc. Therefore, a non-linear response in plastic deformation is observed during deformation due to the applied axial pressure. To predict these non-linear characteristics of the P/M preforms, a Knowledge Based System (KBS) namely, ANN modeling has been considered. ANN tool has been used for predicting the formability of sintered aluminium alloys [85]. The mechanical properties of sintered steels have been predicted using BPN ANN technique [86-87] Limitations of the existing research Based on the above literature survey, the following limitations in existing P/M technology have been identified. Most of the investigations are confined to influence of different alloying elements such as Cu, Ni, Mn, Si, Ferro Si, and Mo with C on densification. Only a few studies are conducted with Cr alloyed steels and low Mo alloyed steels. Majority of research work were performed with high sintering temperatures namely, above 1120 o C. It is observed that addition of Mo to steels will be limited to % and any increase in Mo contents needs complex resintering and repressing cycles [39]. Most of the research pertains to the prealloyed powders, even though the cost of prealloyed powders is high.

10 16 In view of the limitations in existing technologies of P/M processing, cited above, an attempt is made in the present work to implement lower sintering, to utilize elemental powders and to correlate the structure with mechanical properties of sintered and forged low alloy P/M steels.