Effects of copper infiltrant amount and infiltration method on mechanical properties of sintered steel

In this paper, the effects of copper infiltrant amount and copper infiltration method on the mechanical properties of Fe-Cu-C-based sintered steel were investigated. The results indicate that the density, tensile strength, impact energy, and hardness of the sintered steel increase with higher copper infiltration. The optimal mechanical properties were achieved when the copper infiltrant amount reached 15%, resulting in a surface hardness of HRC31, an impact energy of 31.34 J, and an ultimate tensile strength of 759 MPa. Furthermore, different copper infiltration methods affected the material’s mechanical properties. When the copper infiltrant amount was 12.5%, the impact energy of the sintered steel sample was 15.41% higher using the bottom copper infiltration process (30.19 J) compared to the top copper infiltration process (26.16 J). These findings provide valuable insights for enhancing the mechanical properties of iron-based powder metallurgy sintered steel with copper infiltration.


Introduction
Iron-based powder metallurgy products, also known as sintered steel products, constitute the predominant category within the powder metallurgy industry.These products are characterized by their high density, strength, and precision, making them ideal for various applications and serving as a key focus for further advancements [1][2] .Nevertheless, the presence of pores in general iron-based powder metallurgy parts leads to lower densities compared to those achieved through traditional casting and forging methods.Consequently, the material exhibits significantly diminished dynamic mechanical properties, limiting its widespread utilization [3][4][5] .
Presently, the production of high-density powder metallurgy sintered steel parts commonly employs the copper infiltration process, a widely utilized technique for metal treatment in various industries.Through modifications in the material's chemical composition, crystal structure, and grain boundary properties, the copper infiltration process substantially enhances the density, strength, wear resistance, thermal conductivity, and fatigue resistance of sintered steel [6][7][8][9] .Over the past few decades, the steel industry has extensively implemented copper infiltration to fulfil the requirements of numerous technical applications.As science and technology continue to progress, research on the copper infiltration technique's impact on the microstructure and mechanical characteristics of copperinfiltrated sintered steel has advanced significantly.
Although copper infiltration is commonly employed in manufacturing, the precise effect of optimal process conditions on copper infiltration remains uncertain.In industrial settings, the standard copper infiltration rate hovers around 10%.Campos [10] conducted experiments using 5%, 10%, and 15% copper infiltrant amounts.The results revealed that positioning the copper infiltrant at the top of the test samples and achieving a 15% copper infiltrant amount resulted in the material's optimal mechanical properties.Nevertheless, this method may not be cost-effective in an industrial context.Conversely, Dyachkova [11] positioned the copper infiltrant at the base of the test samples, implying that it facilitated the removal of gas from the pores.However, the full extent of the effect of the copper infiltration method was not explicitly documented.To date, there is no unanimous agreement on the ideal copper infiltration method and amount as two pivotal process parameters.A thorough investigation into the effects of copper infiltrant amounts and methods on mechanical performance allows for optimising sintering process parameters, leading to improved production efficiency and control [12][13][14][15][16] .
The primary objective of this study is to understand how different copper infiltrant amounts and methods affect the mechanical performance of sintered steel following copper infiltration, to achieve optimal results in terms of uniformity and depth control.Identifying the ideal copper infiltration rate and method has the potential to reduce energy consumption, minimize raw material wastage, and improve the overall quality consistency of sintered steel.Moreover, this enhances our comprehension of copper infiltration mechanisms and their connection to material performance, offering valuable insights and references for producing and applying copper-infiltrated sintered steel.

Experimental methods
The reduced iron powder, electrolytic copper powder, and graphite were utilized to make the ironbased briquettes that were used in this experiment.To create the correct composition, the powders were weighed and blended in the proper ratios.As carbon, electrolytic copper powder with a particle size less than 50 μm, graphite with a particle size less than 40 μm, and atomized iron powder with a particle size less than 200 μm were employed.Figure 1   The powder metallurgy sintered steel composition selected for this experiment was FC0208 premixed powder, which was then pressed into a 55 mm*10 mm*10 mm billet with a density of 6.8 g/cm 3 .The copper infiltrant was also pressed into the billet and placed on the surface of the iron substrate.The copper infiltration sintering was carried out in a mesh belt furnace, using a nitrogen-hydrogen mixture as the sintering atmosphere, at a temperature of 1120 °C for 60 minutes.
A thin layer of residue remains on the surface of the fusion-infiltrated composite, which needs to be roughly ground and then finely ground and polished for subsequent testing and characterization.After some of the samples were polished and cleaned of excess copper residue, each sample was tested.The density of the material was detected by Archimedes' principle, the physical phase of the material was characterized by X-ray diffraction, the hardness of the material was detected by HR-150 Rockwell (a) (b) (c) hardness tester, the unnotched impact toughness was tested by pendulum impact toughness tester, and the microstructure of the sample was observed and analyzed by scanning electron microscope coupled with energy-diffraction spectrometer (EDS).

Figure. 2 XRD patterns of sintered steels with different copper infiltrant amounts
Figure 2 shows the XRD pattern of the matrix infiltrated with different amounts of copper infiltrant, which shows sharp diffraction peaks.It is clear that the copper phase without copper infiltration only has iron diffraction peaks, whereas after copper infiltration, copper diffraction peaks appear and iron diffraction peaks are shifted to the left.This is because, during copper infiltration, copper atoms interact with iron atoms in the steel matrix, which causes the crystals to grow larger.The copper atoms and iron atoms in the steel matrix solidify as copper infiltration rises, increasing the relative intensity of the diffraction peaks.

Figure. 3 Microscopic morphologies of materials with different copper infiltrant amounts
Figure 3 displays the SEM images of samples with varying copper infiltrant amounts.The samples exhibit pearlite, ferrite, copper phases, and pores.As the amount of copper infiltration increases, the pore content gradually decreases.At a copper infiltrant amount of 10%, a more scattered distribution of copper is observed, with a higher presence of pores dispersed along grain boundaries and intergranular voids.With a further increase in copper infiltration (12.5%), larger pores significantly decrease, grain size becomes smaller, the structure densifies, and regional grain boundary uniformity improves.Upon further increasing the copper infiltration (15%), the copper phase adopts a reticulate and continuous distribution, giving rise to localized copper accumulation or aggregation.

Effect of copper infiltrant amount on mechanical properties
The highest copper infiltrant amount was set at 15% due to the matrix's porosity being approximately 14%. Figure 4 shows the samples' variations in density versus hardness, impact work, and tensile strength at various copper infiltration levels.As copper infiltration increased, sintered steel's density gradually climbed, reaching values of 7.32 g/cm 3 , 7.50 g/cm 3 , and 7.69 g/cm 3 , respectively.Second, as the amount of copper infiltration increased, the tensile strength of the sintered steel increased as well, rising from 691 MPa to 743 MPa to 759 MPa, showing that the increased copper infiltration improved the densification of the sintered steel and had a positive effect on its strength.This is because copper strengthens the connections at the grain boundaries, which increases the material's structural integrity.Additionally, it was found that when copper infiltration grew, the sintered steel's impact toughness and hardness also increased.The impact energy was 23.41 J, 30.19 J, and 31.41J, and the corresponding hardness values were HRC20, HRC27, and HRC31.This suggests that the addition of copper not only increases the material's impact resistance but also increases its hardness, which in turn improves the material's durability and wear resistance.When copper infiltrates into the sintered steel matrix, the copper phase is a reinforcing phase that can prevent the slippage of grain boundaries and the separation of particles, improving the hardness of the material.The mechanical properties of powder metallurgy steel are primarily determined by the microstructure.An appropriate amount of copper infiltration can enhance the structure of grain boundaries and boost the material's toughness, whereas an excessive amount of copper infiltration can result in localized copper phase accumulation and reduced grain boundary continuity, which only marginally increases the toughness of sintered steel.This enhancement is attributed to the reinforced load-bearing structure, reduced stress concentrations related to open porosity, and an elevation in the microhardness of the steel matrix [17] .

Fracture analysis of different copper infiltrant amounts
Figure 5 illustrates the fracture morphology of sintered steel infiltrated with various levels of copper.As observed, the fractures in sintered steel infiltrated with copper display particle accumulation interspersed with rounded intergranular closed voids.The fracture mechanism for this infiltration also involves the partial fracture of the pearlitic grains.Porosity decreases as copper infiltration increases, and the impact fracture eventually transitions from a sintered neck fracture to an iron particle disintegration fracture.From Figure 5(d)'s EDS elemental distribution image at 15% copper infiltration, it can be observed that the distribution of the copper phase is non-uniform, forming a network surrounding the iron particles and reducing their direct contact.This arrangement is beneficial for enhancing the toughness of sintered steel.

Figure. 5 Fracture morphology and EDS energy spectrum analysis of different copper infiltrant amounts
The fracture microstructure morphology of sintered steel with varying levels of copper infiltration is presented in Figure 6.It can be observed that at 10% copper infiltration, the fracture surface appears relatively rough, with a relatively flat fracture edge.The copper infiltration provides a certain degree of plasticity, yet the material still exhibits some degree of brittleness under impact loads.With 12.5% copper infiltration, the fracture surface becomes even rougher, the pearlitic structure becomes more pronounced, and distinct tearing characteristics emerge.At 15% copper infiltration, the fracture exhibits increased toughness characteristics, with the copper phase aggregates proving more effective at absorbing impact energy and retarding fracture propagation.This enables the sintered steel to more effectively absorb energy and resist fracture under impact loads.As the material approaches full density, copper is capable of transmitting external loads and promoting the fracture of iron particles [10] .The fracture sequence unfolds as follows: plastic deformation of the copper network, the transfer of loads to the iron particles, significant fracture through cleavage, and ultimately, copper yielding while maintaining ductility.

Influence of copper infiltration method
The copper infiltrant was positioned as shown in Figure 7 and divided into top infiltration and bottom infiltration for the copper infiltration process.

Figure. 7 Copper infiltration method
Figure 8 illustrates the impact energy for various copper infiltrant amounts for top fusion and bottom fusion infiltration.The impact toughness of sintered steel is affected differently by bottom and top copper infiltration.When the bottom copper infiltration is applied, the impact toughness of sintered steel steadily increases under conditions of varying copper infiltration concentration, and the values are 23.41J, 30.19 J, and 31.41J. On the other hand, the trend of the impact toughness is comparatively negligible when the top copper infiltration is applied, with the impact toughness being 25.24 J, 26.16 J, and 30.19J.When the top copper infiltration is used, the impact toughness of sintered steel is also increased when the copper infiltrant amount is variable.

Figure. 8 Impact energy of top and bottom infiltration of materials with different copper infiltrant amounts
The copper infiltrant is positioned above the substrate, and when it melts, a capillary force and gravity are working together, so theoretically, the infiltration is more uniform and the infiltration depth is deeper.However, the residual gas in the substrate's pores will become a barrier to homogenization, and the bottom melting infiltration will help the residual gas in the substrate's pores to be discharged.According to the experimental findings, the bottom copper infiltration has a more pronounced improvement effect on the impact toughness of sintered steel than the top copper infiltration when the amount of copper infiltration is higher than 12.5%.With a copper infiltrant amount of 12.5%, the bottom copper infiltration has a more complete solid solution reaction with the sintered steel matrix, improving the sintered steel's toughness.The two copper infiltration methods have the largest differences in impact work, with the bottom copper infiltration's impact work increasing by 15.41%.

Conclusion
The goal of this study is to determine how copper infiltration method and amount affect the mechanical characteristics of sintered steel.The following conclusions were reached by comparing the mechanical property test results under process conditions: (1) Sintered steel's density, tensile strength, impact toughness, and hardness all exhibit an upward trend with increasing copper infiltration.The best mechanical characteristics are attained at a copper infiltration level of 15%: surface hardness of HRC31, impact toughness of 31.14J, and ultimate tensile strength of 759MPa.
(2) The sintered steel samples with different copper infiltration directions showed significant differences in impact toughness.At a copper infiltrant amount (12.5%), the impact energy of the sintered steel sample with bottom copper infiltration (30.19 J) is 15.41% higher than that with top copper infiltration (26.16 J).
(3) With the increase of copper infiltration, the copper phase exists in the form of a network and continuous distribution, forming a local copper accumulation or aggregation, which helps to absorb the impact energy and delay the expansion of the fracture.
depicts a schematic diagram of the sample substances used.

Figure. 4
Figure. 4 Relationship between density and (a) impact energy, (b) hardness, (c) tensile strength of samples with different copper infiltrant amounts

Figure. 6
Figure.6 Microstructure and morphology of sintered steel fracture with different copper infiltrant amounts