James G. Harris

Effect of laser beam on machining of titanium alloys

Laser assisted machining of titanium alloys has been investigated in comparison with conventional machining. The effects of laser beam on the cutting forces, chip formation, machined surface and tool vibration have been examined.It is found that the cutting forces were reduced with the assistance of laser beam. The reduction of cutting forces was primarily dependent on the cutting speed, tool-beam distance and laser lens-workpiece distance. With increasing cutting speed, the chip formation changed from the sharp segmented chip through continuous chip to segmented chip which is the typical chip formation in conventional machining of titanium alloys.Laser assisted machining of titanium alloys has been investigated in comparison with conventional machining. The effects of laser beam on the cutting forces, chip formation, machined surface and tool vibration have been examined.It is found that the cutting forces were reduced with the assistance of laser beam. The reduction of cutting forces was primarily dependent on the cutting speed, tool-beam distance and laser lens-workpiece distance. With increasing cutting speed, the chip formation changed from the sharp segmented chip through continuous chip to segmented chip which is the typical chip formation in conventional machining of titanium alloys.

Investigation of capillary waves excited by a Nd : YAG laser using pyrometry

A mechanism for excitation of capillary waves on a weldpool formed by a laser is shown to be due to coupling of the weldpool height with the laser beam profile. This leads to an additional low frequency mode, which has been observed and has a frequency of about an order of magnitude lower than the classically expected capillary waves. This mechanism for wave excitation on the weldpool can only happen if the focal point of the laser beam is located above the surface of the weldpool; otherwise the waves are damped.

Effect of increment and single-track geometry on the formation of multi-track laser cladding

The variation of track geometry during multi-track laser cladding of stellite 6 on mild steel starting with different geometry profiles and levels of dilution in the single-track clad was examined. In transverse cross-section of the multi-track clad, the total area in each track includes the areas of melted powder (clad area), remelted previous track (remelted area) and melted substrate. Both clad area and total area increase with track number and finally reach constant values, but the increase of total area is much greater than that of clad area. The remelted area of previous track increases with the level of dilution of the single-track clad and reaches its maximum value when the dilution of single-track clad is over 20%. The percentage of the maximum remelted area of the previous track equals the percentage of the track overlap. The inter-track porosity will appear when the difference of the total area and the remelted area of the previous track is closer to or smaller than the clad area because there is not enough laser energy to melt the powder captured by the melt pool.The variation of track geometry during multi-track laser cladding of stellite 6 on mild steel starting with different geometry profiles and levels of dilution in the single-track clad was examined. In transverse cross-section of the multi-track clad, the total area in each track includes the areas of melted powder (clad area), remelted previous track (remelted area) and melted substrate. Both clad area and total area increase with track number and finally reach constant values, but the increase of total area is much greater than that of clad area. The remelted area of previous track increases with the level of dilution of the single-track clad and reaches its maximum value when the dilution of single-track clad is over 20%. The percentage of the maximum remelted area of the previous track equals the percentage of the track overlap. The inter-track porosity will appear when the difference of the total area and the remelted area of the previous track is closer to or smaller than the clad area because there ...

Laser assisted machining of commercially pure titanium

Laser assisted machining (LAM) has been performed on commercially pure titanium bar. Cutting forces have been measured with a 3-component force sensor. Both the scale and fluctuation of cutting forces were dramatically reduced, and the machined surface was smoother after laser assisted turning. The effects of laser power, laser spot size, tool-beam distance and cutting speed on the reduction of cutting forces have been investigated and the change in chip formation during LAM is reported. The reduction of cutting forces primarily depends on the tool-beam distance and cutting speed. With increasing cutting speed, the segmented chips become sharper in the conventional machining compared to the change observed in LAM. Deformation twins were observed in the machined sub-surface under both conventional cutting and LAM, however, the twins in conventionally machined sub-surface were finer and more dense.Laser assisted machining (LAM) has been performed on commercially pure titanium bar. Cutting forces have been measured with a 3-component force sensor. Both the scale and fluctuation of cutting forces were dramatically reduced, and the machined surface was smoother after laser assisted turning. The effects of laser power, laser spot size, tool-beam distance and cutting speed on the reduction of cutting forces have been investigated and the change in chip formation during LAM is reported. The reduction of cutting forces primarily depends on the tool-beam distance and cutting speed. With increasing cutting speed, the segmented chips become sharper in the conventional machining compared to the change observed in LAM. Deformation twins were observed in the machined sub-surface under both conventional cutting and LAM, however, the twins in conventionally machined sub-surface were finer and more dense.

Laser cladding repair of lp steam turbine blades

The reliable, efficient and cost effective running of large steam powered electric generators is essential for the maintenance of our modern society. Erosive wear of blades as a result of droplet formation within the turbines is both endemic and costly. It results in the need for complex and expensive off-line repairs as well as unwanted downtime of power generation equipment. Furthermore, if the results of erosion are ignored there can be inefficient operation, reduced blade life and in extreme cases catastrophic failure can occur within the turbine.This paper presents the results of research into successful repairs to the outer leading edge of low pressure steam turbine blades. A high power fibre delivered diode laser was installed on site at the power station to laser clad a Stellite 6 coating to the eroded areas of the blades and so effect a repair. This required the construction of a compact co-axial laser cladding head capable of fitting within the restricted space that exists between the blades that were still attached to the rotor shaft. The blades themselves are constructed of martensitic stainless steel which is highly susceptible to cracking and metallurgical change when subjected to heating and cooling [1]. The cladding process has resulted in a crack free blade with minimal distortion.The reliable, efficient and cost effective running of large steam powered electric generators is essential for the maintenance of our modern society. Erosive wear of blades as a result of droplet formation within the turbines is both endemic and costly. It results in the need for complex and expensive off-line repairs as well as unwanted downtime of power generation equipment. Furthermore, if the results of erosion are ignored there can be inefficient operation, reduced blade life and in extreme cases catastrophic failure can occur within the turbine.This paper presents the results of research into successful repairs to the outer leading edge of low pressure steam turbine blades. A high power fibre delivered diode laser was installed on site at the power station to laser clad a Stellite 6 coating to the eroded areas of the blades and so effect a repair. This required the construction of a compact co-axial laser cladding head capable of fitting within the restricted space that exists between the blades tha...

Comparison of linear and spinning laser beam technologies for welding of thermoplastics to timber

The focus is given to the fundamentals of the spinning laser beam technology for welding of wood fiber composites to thermoplastics.Spinning laser beam processing excites increasing process efficiency by decreasing the viscosity for various materials. Hence, increasing the process speed is possible without increasing laser power (→ 170%, cal.).The results are presented by using infrared radiation visualization, shear stress measurements and optical analysis of the weld-seam in comparison to the conventional (linear) laser beam.The focus is given to the fundamentals of the spinning laser beam technology for welding of wood fiber composites to thermoplastics.Spinning laser beam processing excites increasing process efficiency by decreasing the viscosity for various materials. Hence, increasing the process speed is possible without increasing laser power (→ 170%, cal.).The results are presented by using infrared radiation visualization, shear stress measurements and optical analysis of the weld-seam in comparison to the conventional (linear) laser beam.

Effects of strain rate on the properties of laser welded high pressure die cast AM50 and AM60 alloys

The influence of strain rate on the properties of laser welded AM50 and AM60 magnesium alloys has been investigated. Butt welding without filler material was conducted on 3mm thick high pressure die cast (HPDC) plates using a high power Nd:YAG laser. All butt welds were produced at 2kW nominal laser power, 0.6mm diameter laser spot size and plate feed rate of 2.5m/min. Tensile testing of the parent HPDC alloys and welded samples was performed at three different strain rates ranging from 0.0016s-1 to 2s-1. For the welded samples, both longitudinal and transverse tensile tests were performed where the loading axis was parallel and perpendicular to the weld bead respectively. Results are presented and the effects of strain rate on the properties of the welded joints are discussed in this paper.

Monitoring and control of laser cladding and welding

Cladding and welding using a laser can be fast and precise processes. Quality control is usually achieved by “open loop” adjustment of the laser process to the particular application. Each application requires the development of a new procedure. This paper looks at the conditions under which closed loop control of laser processes is practical. If on-line, closed loop control is not possible then quality control can only be achieved by on-line monitoring and then accepting or rejecting the work after processing.Cladding and welding using a laser can be fast and precise processes. Quality control is usually achieved by “open loop” adjustment of the laser process to the particular application. Each application requires the development of a new procedure. This paper looks at the conditions under which closed loop control of laser processes is practical. If on-line, closed loop control is not possible then quality control can only be achieved by on-line monitoring and then accepting or rejecting the work after processing.

Nd: YAG laser reactive fusion cutting of thick mild steel by novel techniques

Reported here are the results of experiments investigating the cutting of thick (>20 mm) mild steel plate using various novel cutting techniques with a 2.5 kW, fibre coupled, Nd:YAG laser. Cutting methods used were conventional reactive fusion cutting, spinning beam cutting, wobbling beam cutting, low pressure oxygen dominated cutting (LoPOx) and high pressure oxygen dominated cutting (HiPOx). Comparison of these cutting methods shows that despite the changes to the beam delivery, the cut mechanism itself remains unchanged. The process of cutting thick steel plate with the Nd:YAG is primarily a balance between incident laser energy, assist gas conditions and most importantly the ability to remove the dross from the bottom of the cut. This paper shows and compares the results of each cut method, as well as providing insight into laser reactive fusion cutting of thick steels.

Cutting 50 mm thick mild steel plate with a Nd:YAG laser

We report results of experiments investigating the feasibility of cutting thick (> 15 mm) mild steel plate with a fibre coupled Nd:YAG laser. The experiments were performed with a continuous wave 2.5 kW Nd:YAG laser delivered to the workpiece through a 0.6mm diameter silica core optical fibre. The mild steel samples range in thickness from 10 to 50 mm. The effects of a range of operating parameters such as focal spot and cutting nozzle position relative to steel surface, assist gas pressure, power and process speed, on cut surface quality are presented and discussed. Results to date show that it is possible to cut up to 50 mm thick mild steel plate at speeds up to 200 mm/min with as low as 500 W of Nd:YAG laser power. The cut surface is smooth and there is no dross. These results show promise for the application of Nd:YAG laser technology for the cutting of thick steel plate.We report results of experiments investigating the feasibility of cutting thick (> 15 mm) mild steel plate with a fibre coupled Nd:YAG laser. The experiments were performed with a continuous wave 2.5 kW Nd:YAG laser delivered to the workpiece through a 0.6mm diameter silica core optical fibre. The mild steel samples range in thickness from 10 to 50 mm. The effects of a range of operating parameters such as focal spot and cutting nozzle position relative to steel surface, assist gas pressure, power and process speed, on cut surface quality are presented and discussed. Results to date show that it is possible to cut up to 50 mm thick mild steel plate at speeds up to 200 mm/min with as low as 500 W of Nd:YAG laser power. The cut surface is smooth and there is no dross. These results show promise for the application of Nd:YAG laser technology for the cutting of thick steel plate.