Hongjun Zhang

Fatigue crack propagation in laser alloyed ductile cast iron surface

Due to a limited dimension of the focused laser beam, multiple passes are required to treat large surface areas, which results in overlapping of laser tracks. This process increases the cracking susceptibility and is a major barrier for the wide application of the laser treatment. To avoid the formation of cracks in multiple overlapping laser tracks, a novel technique referred to as the laser dispersed alloying (LDA) has been developed. The technique consists of creating a dispersed pattern of laser alloying on the surface to be processed. This treatment enhances the fatigue resistance of the surface layer. In the present study, the microstructure, the residual stress, and the fatigue crack growth rate (FCGR) for the ductile cast iron surface formed with the LDA technique were investigated. It was found that the fatigue resistance of the ductile cast iron is markedly increased after the LDA process, due to the compressive stresses and the fine dendritic structure formed in the laser alloying zone, the excellent bond strength between the laser alloying zone and the substrate, and the martensite shell surrounding graphite nodule in the heat affected zone near the boundary of the laser alloying zone.

Influence of adding strong-carbide-formation elements multiply on particle-reinforced Fe-matrix composite layer produced by laser cladding

In the research hotspot of particle reinforced metal-matrix composite layer produced by laser cladding, in-situ reinforced particles obtained by adding strong-carbide-formation elements into cladding power have been attracting more attention for their unique advantage. The research has demonstrated that when adding strong-carbide-formation elements-Ti into the cladding powder of the Fe-C-Si-B separately, by optimizing the composition, better cladding coating with the characters of better strength and toughness, higher wear resistance and free of cracks. When the microstructure of cladding coating is hypoeutectic microstructure, its comprehensive performance is best. The research discovered that, compositely adding the strong-carbide-formation elements like Ti+V, Ti+Zr or V+Zr into the cladding coating is able to improve its comprehensive capability. All the cladding coatings obtained are hypoeutectic microstructure. The cladding coatings have a great deal of particulates, and its average microhardness reaches HV0.2700-1400. The research also discovered that the cladding coating obtained is of less cracking after adding the Ti+Zr.

Laser producing particulate reinforced Fe-based MMC coatings and industrial applications

This paper presents the research achievements on Fe-based metal matrix composite coatings (MMC) on steel and iron substrates by laser alloying and laser cladding and their industrial applications. The high carbon-equivalent design ensures the good formability and high hardness while eliminates hot cracking during rapid solidification. The jointly adding of strong-carbide-formation-elements promotes the precipitation of large quantity of hard particles. The typical microstructure of the MMC coating characterizes about 4000∼5000/mm2 carbide particles (1∼3 micron in diameter) homogeneously precipitated on iron-base matrix. The average hardness of the coating reaches up to Hv0.2 950∼1050. Tribological test proved that this particulate reinforced Fe-based MMC coating posses excellent wear resistance and couple ability with various different frictional counterparts as well as remarkable decrease of the friction coefficient. Its grain-abrasion resistance is better than that of the plasma sprayed Mo coating and its adhesive-abrasion resistance better than the chromium coating. This MMC coating also demonstrated unique corrosion resistance and contact-fatigue resistance. This particulate reinforced MMC coatings have been successfully applied in rock arms, cam shafts, piston rings of engine, plunger pistons and kerf of cutting knives.This paper presents the research achievements on Fe-based metal matrix composite coatings (MMC) on steel and iron substrates by laser alloying and laser cladding and their industrial applications. The high carbon-equivalent design ensures the good formability and high hardness while eliminates hot cracking during rapid solidification. The jointly adding of strong-carbide-formation-elements promotes the precipitation of large quantity of hard particles. The typical microstructure of the MMC coating characterizes about 4000∼5000/mm2 carbide particles (1∼3 micron in diameter) homogeneously precipitated on iron-base matrix. The average hardness of the coating reaches up to Hv0.2 950∼1050. Tribological test proved that this particulate reinforced Fe-based MMC coating posses excellent wear resistance and couple ability with various different frictional counterparts as well as remarkable decrease of the friction coefficient. Its grain-abrasion resistance is better than that of the plasma sprayed Mo coating and i...

High temperature performance of laser deposition GH105 layers on nickel base super alloy blade

Laser deposition is considered a promising rejuvenation approach for nickel base super alloy blades for various kinds of erosion, abrasion or cracking resulted from high temperature, high pressure and complicated vibration environments. The induced residual stress/its impact on the fatigue property and the high temperature performance are two main research issues for successful rejuvenation of blades. We have previously reported our research on residual stress measurement, using laser shock peening, electronic shock peening and magnetic process to change the residual tensile stress into a compress stress and such significantly improves the fatigue properties.This paper focuses on the high temperature performance of laser deposition GH105 layers. Laser deposition of a nickel base super alloy (GH105) powder was performed via a powder feeding technique on nickel base supper alloy blade by a high power CO2 laser. The research indicates that the cladding layers of GH105 under optimized processing parameters have dense microstructure, ultra-fine grains, good surface quality and good metallurgical bond with the substrate. The microstructure consists of γ-Ni matrix, a few carbide precipitates and γ’-phase. The micro-hardness of the cladding layer reaches HV0.2 422, while the blade substrate is HV0.2 343. The elevated temperature tensile strength of the cladding layer at 850℃ is 530MPa, higher than that of the cold-drawn rod, which exceedes the requirement of the blade substrate. The ball-on-flat wear test at 500℃ indicates that the relative abrasive wear property of the laser deposited layer is better than that of the substrate. As the result, the GH105 laser deposition layer meets the high temperature performance requirements for blade rejuvenation.Laser deposition is considered a promising rejuvenation approach for nickel base super alloy blades for various kinds of erosion, abrasion or cracking resulted from high temperature, high pressure and complicated vibration environments. The induced residual stress/its impact on the fatigue property and the high temperature performance are two main research issues for successful rejuvenation of blades. We have previously reported our research on residual stress measurement, using laser shock peening, electronic shock peening and magnetic process to change the residual tensile stress into a compress stress and such significantly improves the fatigue properties.This paper focuses on the high temperature performance of laser deposition GH105 layers. Laser deposition of a nickel base super alloy (GH105) powder was performed via a powder feeding technique on nickel base supper alloy blade by a high power CO2 laser. The research indicates that the cladding layers of GH105 under optimized processing parameters ha...

Nano WC powder cold surface enhancing of aluminium alloy 5A06 via laser shock peening

Aluminum alloy 5A06 has many superior properties such as high strength, good corrosion resistance and welding performance. It is widely used in aerospace, ship craft and transportation areas. Unfortunately, poor surface hardness and wear resistance and shorter life time limit its applications in industries. Many hardfacing processes such as heat treatment, plasma spray, micro-arc oxidation, laser cladding and laser alloying can increase the wear resistance of the 5A06 alloy. However, almost all of these processes are thermal process which may induce lighter element melting loss, porosity and cracking. Nano-materials have unique acoustic, optic, electric, magnetic and thermodynamic characteristics because of their quantum size and surface effect. It can play an important role in hardfacing of the light metal alloys. However, nano powders will completely melted into the melt pool due to their relative low melt points, this will lose many of their unique properties.We have recently developed a novel process: nano powder cold enhancing of light metal surface via laser shock peening process. This is a cold process, by which the nano powders are squeezed into the surface of the light metal alloys by the very high pressure (up to Giga or even tens of Giga Pascal) induced by the laser shock peening process.This paper reports our research work one nano WC powder cold surface enhancing of 5A06 alloy by using a 50J Q switch Nd:YAG laser system for shock peening, focusing on the influences of the processing conditions such as surface roughness of the aluminium sample, the powder layer thickness, the absorbing layer, the constrain layer and the single pulse energy on the enhanced nano powder percentage and nanopowder dispersion degree in the enhanced nano WC coatings. The paper will also report the hardness, tribology property, wear resistance of the as-enhanced layers. The results confirm that nano powder cold surface enhancing vis laser shock peening can dramatically improve the surface performance of the aluminium alloy by the combined strengthening mechanism of laser shock peening, nano-particles and nano-particle intensified shock peening.Aluminum alloy 5A06 has many superior properties such as high strength, good corrosion resistance and welding performance. It is widely used in aerospace, ship craft and transportation areas. Unfortunately, poor surface hardness and wear resistance and shorter life time limit its applications in industries. Many hardfacing processes such as heat treatment, plasma spray, micro-arc oxidation, laser cladding and laser alloying can increase the wear resistance of the 5A06 alloy. However, almost all of these processes are thermal process which may induce lighter element melting loss, porosity and cracking. Nano-materials have unique acoustic, optic, electric, magnetic and thermodynamic characteristics because of their quantum size and surface effect. It can play an important role in hardfacing of the light metal alloys. However, nano powders will completely melted into the melt pool due to their relative low melt points, this will lose many of their unique properties.We have recently developed a novel process:...

WC Nano powder cold planting via laser shock peening onto aluminium/magnesium alloy surfaces

We have developed a novel process for hardfacing aluminium/magnesium alloys: nano powder cold planting via laser shock peening (NPCP/LSP). The pre-coated nano powders are planted into the near surface layer by the GPa pressure induced by a laser shock peening process. This paper reports our recent work on WC NPCP/LSP onto aluminium/magnesium alloys, focusing on the combination of nano powders with the substrate, the interface bond between the nano particles and the matrix, and also the significant improvement on the surface hardness, wear resistance, and tribologocal properties.

Laser deposition of Ti6Al4V-316L composition gradient structure: Challenge on intermetallics

Functional gradient materials (FGM), with their composition gradually varied in different positions, have attracted much attention for research and development during the years. Normally there are two types of functional gradient materials: one without the formation of intermetallics which is easier for fabrication due to less scientific and technological challenge; the other one with the formation of intermetallics in some part of the gradient changed composition range such as Ti-Al, Ti-Fe, Ni-Al gradient materials, which still exists big challenge on intermetallic, resulting in high brittleness and unavailability.Laser deposition is a competitive candidate for FGM due to its capability on accurate composition and precise dimension control. According to an aeronautical application requirement to gradually change the composition from Ti6Al4V to 316L, this paper reports on laser deposition of Ti6Al4V to 316L gradient composition structure. A big challenge is the formation of the TiFe intermetallic especially near the composition of Ti50Fe50, which results in very high cracking tendency. Many efforts have been made to optimize the Ti-Fe composition ratio, the deposition process and the deposition parameters, not a single Ti6Al4V-316L composition gradient structure was successful due to the formation of the very brittle TiFe and TiFe2 intermetallics. This indicates that it is almost impossible to direct deposit Ti6Al4V to 316L without cracking and separation. We used Inconel 625 powder as an interlayer between the Ti6Al4V and the 316L, thus a sound Ti6Al4V-316L composition gradient structure was successfully achieved by laser deposition. The brittle TiFe and TiFe2 intermetallics were replaced by a series of phase evolutions: α+β → β+Ti2Ni→Ti2Ni+TiNi→TiNi→TiNi+TiNi3→Ni3Fe→ Ni2Fe3→γFe. The brittleness was dramatically decreased or diluted and crack-free composition gradient structure was available. The gradient structure was further analyzed on hardness, microstructural evolution, the Young’s modulus and Poisson’s ratio.Functional gradient materials (FGM), with their composition gradually varied in different positions, have attracted much attention for research and development during the years. Normally there are two types of functional gradient materials: one without the formation of intermetallics which is easier for fabrication due to less scientific and technological challenge; the other one with the formation of intermetallics in some part of the gradient changed composition range such as Ti-Al, Ti-Fe, Ni-Al gradient materials, which still exists big challenge on intermetallic, resulting in high brittleness and unavailability.Laser deposition is a competitive candidate for FGM due to its capability on accurate composition and precise dimension control. According to an aeronautical application requirement to gradually change the composition from Ti6Al4V to 316L, this paper reports on laser deposition of Ti6Al4V to 316L gradient composition structure. A big challenge is the formation of the TiFe intermetallic especial...

Effects of rare earth oxide on the nucleation of carbide particles in laser cladded MMCp coating

Particulate reinforced metal matrix composite (MMCp) has excellent properties such as good wear resistance, corrosion resistance and high temperature properties. Various volume fractions of ceramic particles are usually deposited onto the metal surface to form MMC with improved performances by laser cladding. Recent literatures show that laser melting powders containing carbide-formation elements and carbon, is favorable for the formation of in situ synthesized carbide particles in MMC.Researches approve that the rare earth oxide (RExOy) plays a very important role in the precipitating of the in situ synthesized carbide particles especially as a heterogeneous nucleus of the carbides, which will greatly increase the distributing density of the enhanced particles. RExOy is commonly used to increase the laser absorptivity of the coating powder, to decrease the activity and improve the solubility of the alloy elements in the laser melt. At present, many researches have been made on the refinement of crystal grain by adding RExOy. However, nearly few have been reported on the accelerating of nucleation of the reinforcement particles as heterogeneous nuclei, especially in the field of laser producing MMCp.By the observation and analysis of SEM, EDS and TEM, it is found that in the precipitating of the carbide particles, some of the RExOy are partially dissolved, while the other ones change into rare earth carbides. However, both of them act as heterogeneous nuclei for the forming of particles. A clear interface between the nucleus and periphery of the particle can be observed by SEM after etching due to their differences in composition and structure. After deeply etched, the nuclei may be completely eroded and then left holes.Particulate reinforced metal matrix composite (MMCp) has excellent properties such as good wear resistance, corrosion resistance and high temperature properties. Various volume fractions of ceramic particles are usually deposited onto the metal surface to form MMC with improved performances by laser cladding. Recent literatures show that laser melting powders containing carbide-formation elements and carbon, is favorable for the formation of in situ synthesized carbide particles in MMC.Researches approve that the rare earth oxide (RExOy) plays a very important role in the precipitating of the in situ synthesized carbide particles especially as a heterogeneous nucleus of the carbides, which will greatly increase the distributing density of the enhanced particles. RExOy is commonly used to increase the laser absorptivity of the coating powder, to decrease the activity and improve the solubility of the alloy elements in the laser melt. At present, many researches have been made on the refinement of crystal g...

Laser producing Ni-matrix composite reinforced by in-situ synthesized particles

Nickel base superalloy has been widely used in key parts in aerospace field for its excellent heat resistance and wear resistance, but some problems always occur in laser cladding layer produced directly with different commerce nickel base superalloy powder as cladding powder, such as lower hardness or bad formation accompanying with higher hardness.The concept on producing Fe-matrix in-situ synthesized particles reinforced composite coatings by means of energy fluctuation and concentration fluctuation in melt by jointly adding of multi strong-carbide-formation elements and carbon is used in laser producing Ni-matrix composite. By adding strong-carbide-formation elements Ti, Zr, W and C into Inconel 625 powder, Ni-matrix composite reinforced by in-situ synthesized particles can be produced by laser cladding or laser alloying, without pores and cracks. The microstructure of the composite characterizes the dispersive and homogeneous precipitation of about 104/mm2 particles in micron size distributed homogeneously on γ-Ni dendritic. The average micohardness of the composite is above HV0.2400. EDAS analysis indicates that there are some proportions of strong-carbide-formation elements Ti, Zr, Nb, Mo, W and C in the particles, so they can be called as complex carbide.The influence of C content and W content in powder joined is specially discussed about.Nickel base superalloy has been widely used in key parts in aerospace field for its excellent heat resistance and wear resistance, but some problems always occur in laser cladding layer produced directly with different commerce nickel base superalloy powder as cladding powder, such as lower hardness or bad formation accompanying with higher hardness.The concept on producing Fe-matrix in-situ synthesized particles reinforced composite coatings by means of energy fluctuation and concentration fluctuation in melt by jointly adding of multi strong-carbide-formation elements and carbon is used in laser producing Ni-matrix composite. By adding strong-carbide-formation elements Ti, Zr, W and C into Inconel 625 powder, Ni-matrix composite reinforced by in-situ synthesized particles can be produced by laser cladding or laser alloying, without pores and cracks. The microstructure of the composite characterizes the dispersive and homogeneous precipitation of about 104/mm2 particles in micron size distributed homogen...

Formation of micro-nano structure by laser deposition and dealloying of copper alloy layer on macro-component surface

Nano-alloys or nano-structure often display novel physical, chemical and mechanical properties. It is of great scientific and engineering meanings to deposit micro-nano structure on macro-component surface to locally achieve nano-properties for potential applications. Laser deposition offers a promising approach to deposit a designed structure with designed composition on any location of a component.This paper focuses on the formation of micro-nano porous structure by laser deposition and dealloying of copper alloy layer on macro-component surface. A MnxCu1-x powder mixture was laser deposited on steel substrates to obtain layers with exact composition by laser processing optimization and original power composition optimization. These layers were then dealloyed in electrolyte by both free corrosion and electrochemical etching of manganese component. Bicontinuous porous copper layer with porosity in size of about 700 nm was formed after electrochemical etching, while no porous copper layer could be obtained in free corrosion. Dilute ratio, chemical composition and microstructure of the alloy layers were investigated using SEM, X-EDS and XRD. The results indicate that porous copper layer could be obtained by integrating laser deposition with low dilute ratio and electrochemical dealloying.Nano-alloys or nano-structure often display novel physical, chemical and mechanical properties. It is of great scientific and engineering meanings to deposit micro-nano structure on macro-component surface to locally achieve nano-properties for potential applications. Laser deposition offers a promising approach to deposit a designed structure with designed composition on any location of a component.This paper focuses on the formation of micro-nano porous structure by laser deposition and dealloying of copper alloy layer on macro-component surface. A MnxCu1-x powder mixture was laser deposited on steel substrates to obtain layers with exact composition by laser processing optimization and original power composition optimization. These layers were then dealloyed in electrolyte by both free corrosion and electrochemical etching of manganese component. Bicontinuous porous copper layer with porosity in size of about 700 nm was formed after electrochemical etching, while no porous copper layer could be obtaine...