Jonne Näkki

Microstructural and abrasion wear characteristics of laser-clad tool steel coatings

Several different tool steel grades were deposited on mild steel by the laser-cladding process with coaxial powder feeding. With bidirectional scanning pattern, most of the grades were deposited crack-free with hardness up to 1000 HV without additional preheating. In a 3-body abrasion wear study, the laser clad Ralloy® WR6 with significant portion of retained austenite exhibited superior abrasive wear resistance compared with the predominantly martensitic tool steel coatings (M2, M4, H13, HS-23, HS-30) and the reference material, Raex® Ar500 wear resistant steel. The abrasion wear resistance of austenitic–martensitic WR6 tool steel was further enhanced by the external addition of 20% volume percentage of relatively large (45–106 µm) vanadium carbides. In single point scratch tests, predominantly martensitic tool steels outperformed austenitic–martensitic tool steels and wear resistant steel. The differences in wear performances were explained by different wear mechanisms and types of contact between the abrasive and the surface.

Surface pore initiated fatigue failure in laser clad components

A laser clad and machined cylindrical structural steel rod was fatigue tested under four-point bending load. The resulting fracture could be tracked back to a spherical surface pore in the Co-based coating. Due to an oxide inclusion, the pore was not identified by dye penetrant inspection. Two circular buckling strain patterns that were detected beside the pore at the surfaces after fracture confirm local plastic deformation prior to crack initiation. In order to calculate the stress field around the surface pore, linear elastic finite element analysis was carried out. For four-point bending load, a surface pore generally exceeds the maximum stress of a smooth rod as long as the pore is located within an azimuthal angle of ±55°, which was the case for the presented as well as for another pore initiated sample.

High performance corrosion resistant coatings by novel coaxial cold- and hot-wire laser cladding methods

In the last few years, coaxial laser heads have been developed with centric wire feeding equipment, which enables the laser processing of complex-shaped objects in various applications. These newly developed laser heads are being used particularly in laser brazing experiments in the automotive industry. This study presents experimental results of using a coaxial laser head for cold- and hot-wire cladding application. The coaxial wire cladding method has significant improvements compared with the off-axis wire cladding method such as independence of the travel direction, alignment of the wire to the laser beam, and a reduced number of controlling parameters. These features are important to achieve high quality coatings. Cladding tests were conducted on mild steel with a coaxial laser wire welding head using Ni-based Inconel 625 and Thermanit 2509 super duplex stainless steel solid wires in order to determine the properties of the cladding process and the coatings deposited. The corrosion resistance of the ...

Comparison of laser cladding with powder and hot and cold wire techniques

Laser cladding with various methods have been utilized for some years with increasing quantity. Powder has been traditional type of consumable, but some usage of wires has been reported as well. There are also a number of cladding reports with preheated wire deposition. The aim has been to increase the low productivity of laser cladding. The preheating has been produced by various methods, for instance induction, plasma flame and resistance of the wire. This study compares the different deposition techniques with powder, cold wire and resistance heated wire processes. Hot wire cladding increases the deposition rate substantially keeping similar properties of powder and cold wire laser cladding.

Fatigue behavior of laser clad round steel bars

Laser cladding is an overlay welding method to manufacture high performance, fusion bonded metal, and metal matrix composite coatings on metallic substrates with low dilution. Owing to steep thermal gradients, rapid solidification, and possible mismatch in coefficients of thermal expansion between the coating and the substrate, laser cladding induces large tensile residual stresses in coating layer, potentially affecting the service life of clad component under external load-induced stresses. In this study, four-point bending and torsion fatigue tests were conducted on relatively large round laser clad steel bars to determine the effect of laser cladding on fatigue strength. Quenched and tempered 42CrMo4 steel clad with Inconel 625 and S355 structural steel clad with Stellite 21 were subjected to various stress levels for relatively large number of cycles with and without postweld heat treatment (PWHT). The results indicated that Stellite 21 decreased the fatigue life of S355 at all the applied loads, whe...

Effect of minor elements on solidification cracking and dilution of alloy 625 powders in laser cladding

The general specification of nickel-base alloy 625 is quite permissible allowing relatively large variations in chemical composition. In directed energy deposition processes including laser cladding and additive manufacturing, the powder plays a key role in the properties of final products. Therefore, it is important to select the highest-quality powder. Several commercially available alloy 625 powders were characterized and deposited on mild steel by the laser cladding process using a coaxial powder feeding method and a single bead. Clad layer characterization consisted of determination of dilution and quantification of hot cracks. The results showed that powders containing the lowest amount of impurity elements (S, P, B) were the most resistant to hot cracking. Ti and Al were beneficial if impurity element contents were high.

Wear and corrosion resistant laser coatings for hydraulic piston rods

Hydraulic piston rods on oil and gas drilling platforms, hydro-power stations, chemical plants, and underground mines are exposed to severe tribo-corrosive conditions under static and dynamic mechanical loads. Piston rods made of carbon, quenched and tempered (QT) and stainless steels are frequently surface coated with methods such as thermal spraying, hard chrome plating, and overlay welding. Unfortunately, several premature failures have been reported particularly in marine applications due to insufficient coating properties. Laser cladding has recently drawn lot of attention in this field due to high coating quality and significant improvements in productivity. In this study, several potential Fe-, Ni-, and Co-based alloys were laser clad on carbon and QT steels. Their corrosion and mechanical performances were explored in long-term salt spray, immersion, hardness, abrasive wear, and four-point bending fatigue tests. Most of the laser coatings outperformed hard chrome in corrosion properties, but hardness values were somewhat lower. In single point abrasion tests (scratch tests), the hardest laser coatings, however, outperformed hard chrome due to brittle nature of hard chrome layers. Postmachining induced significant superficial hardness increases in laser coatings, which was the main reason for good wear performance. Fatigue performance was strongly dependent on material pairs, presence of cladding defects, and applied loads.

A comparison of hot cracking sensibility of some alloy 625 powders on laser cladding

Some properties of laser claddings made with several commercially available alloy 625 powders were investigated. Nickel based alloy 625, generally known also as Inconel 625, is widely used also on laser claddings due to its good corrosion properties in various environments. The general specification of alloy 625 is quite permissible and it has been discovered that there can be relatively large variation on properties of alloy 625 powders depending on powder manufacturer and powder batches. And these variations on powder lead to variations on properties of subsequent cladding. The emphasis on this study is especially on the variation of the tendency to hot cracking on laser cladding.Some properties of laser claddings made with several commercially available alloy 625 powders were investigated. Nickel based alloy 625, generally known also as Inconel 625, is widely used also on laser claddings due to its good corrosion properties in various environments. The general specification of alloy 625 is quite permissible and it has been discovered that there can be relatively large variation on properties of alloy 625 powders depending on powder manufacturer and powder batches. And these variations on powder lead to variations on properties of subsequent cladding. The emphasis on this study is especially on the variation of the tendency to hot cracking on laser cladding.

Corrosion resistant laser coatings for hydraulic piston rods

Hydraulic piston rods on oil & gas drilling platforms, hydro-power stations, chemical plants and underground mines are exposed to sever e tribo-corrosive conditions under static and dynamic mechanical loads. Piston rods made of carbon, quenched & tempered and stainless steels are frequently surface coated with methods such as thermal spraying, hard chrome plating and overlay welding. Unfortunately, several premature failures have been reported particularly in marine applications due to insufficient coating properties. Laser cladding has recently drawn lot of attention in this field thanks to high coating quality and improvements in productivity. In this study, several potential Fe-, Ni-and Co-based alloys were laser clad on carbon and quenched & tempered steels. Their corrosion and mechanical performances were studied in long-term salt spray, immersion, hardness, abrasive wear and 4-point bending fatigue tests. Most of the laser coatings outperformed hard chrome in corrosion properties but hardness values were somewhat lower. Fatigue performance was strongly dependent on coating material.Hydraulic piston rods on oil & gas drilling platforms, hydro-power stations, chemical plants and underground mines are exposed to sever e tribo-corrosive conditions under static and dynamic mechanical loads. Piston rods made of carbon, quenched & tempered and stainless steels are frequently surface coated with methods such as thermal spraying, hard chrome plating and overlay welding. Unfortunately, several premature failures have been reported particularly in marine applications due to insufficient coating properties. Laser cladding has recently drawn lot of attention in this field thanks to high coating quality and improvements in productivity. In this study, several potential Fe-, Ni-and Co-based alloys were laser clad on carbon and quenched & tempered steels. Their corrosion and mechanical performances were studied in long-term salt spray, immersion, hardness, abrasive wear and 4-point bending fatigue tests. Most of the laser coatings outperformed hard chrome in corrosion properties but hardness values...

Hot-wire cladding process studies

Hot-wire laser cladding is a coating method with excellent properties and high deposition rate. Process is rather simple and fundamental relationships are studied experimentally and theoretically. Basic parameters like laser power, current, voltage, travel speed and spot size are studied. Solid wires are easily melted by the melt pool generated by laser beam, but there are some difficulties concerning cored wires. With solid wires the coatings are very homogeneous but hot-wire cladding with cored wires can lead to inhomogeneous structure. Methods to improve cored wire coating structures are adjustments of the wire feed direction, wire tip placement, wire stick out and hybrid cladding method. Connection of process parameters and cladding results are presented.Hot-wire laser cladding is a coating method with excellent properties and high deposition rate. Process is rather simple and fundamental relationships are studied experimentally and theoretically. Basic parameters like laser power, current, voltage, travel speed and spot size are studied. Solid wires are easily melted by the melt pool generated by laser beam, but there are some difficulties concerning cored wires. With solid wires the coatings are very homogeneous but hot-wire cladding with cored wires can lead to inhomogeneous structure. Methods to improve cored wire coating structures are adjustments of the wire feed direction, wire tip placement, wire stick out and hybrid cladding method. Connection of process parameters and cladding results are presented.