Studies Using Simulation to Examine the Behaviour of Sintered Aluminium Preforms during Densification ^†

Abstract

Powder metallurgy process has the advantage of optimized material utilization with near net shape, resulting in reduced costs. The densification behavior of sintered aluminium under different loading conditions plays an important role in assessing the performance of parts. The experimental studies to examine the densification behavior are cost intensive and less flexible. Therefore, simulation studies are conducted and results are compared with the experimentation results. The FEA-based DEFORM-2D tool is used for modeling and simulation purposes. The performance curves of load vs. displacement are plotted. A new material is (Al-cr3c2) made and a flow curve is constructed using the disc compression test findings and using software to meet the experimentally observed flow stress. The relationship between stress and strain is studied under different loading conditions of sintered aluminium preforms during densification. Formability stress and strain in plane stress state condition initially rise with the rise in axial strain and then remain constant throughout the process. With a rise in the height reduction percentage, the relative density rises. The final density is higher because there are more pores in the sample when the aspect ratio is lower and the rate of densification is slower.

1. Introduction

Using compacts of compressed and sintered powder samples, powder forging is the process of creating products in a variety of shapes. On aluminium preforms, cold upsetting tests and various strain tests were conducted to estimate the fracture behavior of porous structured material, and the plastic deformation of permeable materials was predicted using correlation, which involves Poisson’s ratio, relative density, and flow stress [1,2,3]. ki and ni were calculated from the experiments, and it was found that both ni and ki peaked at low packing densities. Researchers have been working on the upset forging technique to densify powder preforms in recent years. Measurements of the (ni) and (ki) for various AR and compositions (ki) in small particles for the uniaxial condition of stress were more than others. The triaxial stress state was consistently found to have a higher hoop stress than the other two stress state scenarios [4,5]. The finite element method was used to study sintered metals’ behavior during forging, focusing on preform shape, density, height–diameter ratio, friction, and porous materials’ deformation under varied friction and aspect ratios. FEA simulations elucidate deformation, fracture strain, and aspect ratio correlation [6,7,8]. Powder metallurgy parts were simulated, analyzing the compacting method, pressure, and temperature’s impact on relative density. The extrusion angle and temperature’s effects on micro structural changes were investigated, finding finer grain structure at lower temperatures. Simulation benefits for material property prediction and product lifecycle optimization we assessed. The impact energy and friction coefficient’s influence on powder metallurgy parts’ green density was assessed, observing density improvement with increased energy and limited effect from friction [9,10,11,12].

2. FEA Simulations on the Densification Process

The experimental investigation focused on aspect ratios (AR) of 1.0, while AR values of 0.75 and 0.5 were analyzed using DEFORM-2D V 8.1. Figure 1 shows the model. Figure 2 depicts the load vs. the displacement from the simulation. Chandrasekhar et al. [11] studied sintered billets’ compaction and deformation characteristics. Disc compression tests determined k and n. Preforms were sintered at 500 °C for an hour with an initial density of 0.88. A preform with a 1:1 aspect ratio was designed for the program’s geometry with a starting density set at 0.88. Compression occurred between hard-surfaced platens, with ram a speed of 0.5 mm/sec and friction between the work piece and dies of 0.3. The displacement, density, and deformation load were recorded, along with the top and bottom bulged dimensions.

3. Discussion

Figure 3 illustrates the axial stress versus axial strain relationship in the simulation and experiments. Stress rises with strain, especially as the preform aspect ratio decreases from 1 to 0.5. The reduced aspect ratio increases the load concentration, closing smaller pores faster, accelerating stress increase compared to higher aspect ratio samples.

Figure 4 depicts stress states and their relation hoop stress σ_θ versus axial strain ε_Z. In both stress conditions, hoop stress σ_θ increases with axial strain, more so in triaxial stress. Simulations and experiments exhibit similar patterns, with slight variations. A reduced aspect ratio augments hoop stress.

Figure 5 depicts the axial strain ε_z and σm relationship, increasing concurrently in triaxial and plane stress conditions across stress states. Hydro static stress σ_m is compressed in plane and tensile in triaxial conditions. The simulation agrees with the experiments, consistent across aspect ratios.

Figure 6 compares the formability stress index (β) and axial strain (ε_Z) under triaxial stress, aligning experimental and simulation data. β increases rapidly with ε_Z, consistently across aspect ratios. Formability stress is comparable up to 0.15 axial strains, and lower ARs signify lower β.

Figure 7 shows the relationship between β and ε_Z under plane stress, initially increasing and then stabilizing. Consistent trends between the simulation and experiment are observed, with aspect ratios insignificantly affecting β.

Figure 8 depicts the relationship between β and axial strain under plane stress. Formability stress initially increases with strain before leveling off, which is consistent across the simulation and the experiment. The aspect ratio minimally affects the formability strain index.

Figure 9 shows the density increasing with higher height reduction percentages. The simulation aligns closely with the experiments, with aspect ratios having minimal effect. Compression tests reveal pore closure, enhancing the density. Higher aspect ratios exhibit slower densification due to increased pore content.

Figure 10 illustrates the relationship between axial strain and relative density for aspect ratios 1, 0.75, and 0.5. The density increases with strain for all ratios. Smaller ratios show a higher density. Compression closes pores, enhancing densification.

Figure 11 illustrates density (ρ) and α relation, indicating an initially rapid increase in α due to limited densification from friction. The subsequent load gradually increases the densification and α, reflecting uniform dimensional changes. The simulation aligns with the experimental values.

Figure 12 shows higher densities at the billet center and die interfaces post-simulation, with a lower density in the bulge area due to reduced pore closure from metal displacement. Figure 12b illustrates the metal flow from the center to the periphery, with a decreased velocity at the die–preform interface due to friction.

4. Conclusions

Hoop stress (σ_θ) rises as the axial stress (σ_Z) rises in both the triaxial and plane stress conditions, and it was discovered that (σ_θ) is higher in the triaxial stress condition. In the triaxial and plane stress states, the hydrostatic stress (σ_m) is tensile and compressive and follows same pattern for all aspect ratios. Triaxial stress state conditions have a higher hydrostatic stress than in the other conditions. Axial strain (ε_Z) causes a rise in the formability stress index. It exhibits the same pattern for all aspect ratios in triaxial stress conditions and is higher for low aspect ratio (AR) specimens compared to high AR specimens. Formability stress and strain in plane stress state conditions initially rise with the rise in saxial strain and then remain constant throughout the process. With a rise in the height reduction percentage, the relative density rises. The final density is higher because there are more pores in the sample when the aspect ratio is lower and the rate of densification is slower.