Environmentally-Friendly Laser Hardening Method with Low Power Laser
Sachiko OGAWA*, Toshiki HIROGAKI** and Eiichi AOYAMA***
(Received May 24, 2010)
Recent emphasis on creating a sustainable society has motivated eco-friendly manufacturing. The focus of this study is on an environmentally-friendly laser hardening process for small parts oriented to hybrid machining process. Through-thickness hardening of thin material with low power laser was carried out and the energy efficiency and environmental impact of the process were calculated. It was found that through-thickness hardening was obtained at laser scanning speeds less than 200 mm/min for 0.5 mm sheet thickness at low power laser. Further, it was demonstrated that the diode laser used in this study was more energy-efficient than other low-power lasers and therefore offers a low environmental impact method for the hardening of small parts.
laser hardening, power consumption, low power, low environmental impact, small-sized parts
Considerable emphasis is being placed today on sustainable development due to the environmental issues the earth currently faces. Rising demand for a pollution-free environment has motivated environment-friendly product design and manufacturing 1). Conventionally, manufacturing was based on only a two-dimensional viewpoint, namely performance and cost. However, environmental efficiency must also be considered in today’s manufacturing enterprise.
Consequently, a new concept for sustainable manufacturing is proposed by adding the green/sustainability axis as the third axis (Fig. 1). Especially, activity aimed at minimizing environmental load is of great interest in the manufacturing field as represented by the 3R’s, Reduce, Reuse, and Recycle.
Generally, “Reduce” principle is most effective in decreasing the environmental load by making the product size smaller and thereby reducing the consumption of energy and resources 2). In this study, which is based on the “Reduce” concept, we draw attention to the idea that small parts should be machined by machines of small size and low power. Specifically, on-machine laser heat treatment 3) for hybrid manufacturing is
the focus of this study. In conventional heat treatment, a furnace is used for the hardening of carbon steel, however, it is inefficient for hardening of small parts. Therefore, the target of this study was laser hardening of small parts via a low power laser, which consumes less energy and thereby offers an eco-friendly process for hardening of small parts.
Through-thickness laser hardening was attempted for small parts. Further, energy efficiency was calculated with various hardening methods and their environmental impact was compared.
* Department of Mechanical Engineering, Doshisha University Graduate School, Kyoto
Telephone: +81-774-65-6445, E-mail: eth1302@mail4.doshisha.ac.jp, sachiko.ogw@gmail.com
**Department of Mechanical Engineering, Doshisha University, Kyoto Telephone/Fax: +81-774-65-6503, E-mail: thirogak@mail.doshisha.ac.jp
***Department of Mechanical Engineering, Doshisha University, Kyoto
Telephone: +81-774-65-6506, Fax: +81-774-6829, E-mail: eaoyama@mail.doshisha.ac.jp
Fig. 1. New approach for sustainable development.
Cost Conventional performance
Green/Sustainability
High
Low
High
High
Traditional machine tools based on a traditional strategy (performance/cost)
New concept Machine tools
A balance is important.
Cost Conventional performance
Green/Sustainability
High
Low
High
High
Traditional machine tools based on a traditional strategy (performance/cost)
New concept Machine tools
A balance is important.
Traditional machines
machines
Traditional machines base on a traditional strategy
(performance/cost) New concept
machines
A balance is important High
High High
Low
Cost Conventional performance
Green/Sustainability
High
Low
High
High
Traditional machine tools based on a traditional strategy (performance/cost)
New concept Machine tools
A balance is important.
Cost Conventional performance
Green/Sustainability
High
Low
High
High
Traditional machine tools based on a traditional strategy (performance/cost)
New concept Machine tools
A balance is important.
Traditional machines
machines
Traditional machines base on a traditional strategy
(performance/cost) New concept
machines
A balance is important High
High High
Low
Regarding to evaluation of saving energy by using low power laser, our approach is based on a new idea that includes the life cycle of the secondary tool. This idea comes from the following two points. (1) Laser is a secondary tool and this will generate the environmental load in its own life cycle. However, this idea is not included in general life cycle concept. (2) The
“Reduce” approach is effective to reduce the environmental impact. However, downsized and low-powered laser is less in practical use. (3) It will be possible to have on-machine hybrid manufacturing system by using compact fiber laser. This will also contribute to cut the waste process and reduce the environmental load.
Regarding to above point (1), there are many reports that cover the life cycle assessment (LCA) of the primary tool.
However, there have been few reports that deal with the secondary tools such as machine tool and laser, despite those are necessary to manufacture primary tool. It is important to consider the cycle of the secondary tool for total sustainable manufacturing system 4). Figure 2 shows the industrial cycle considering the secondary tool. The lower area in Fig. 2 shows the conventional cycle only considering the primary tool. The upper (bold-printed) area in Fig. 2 shows the cycle of the secondary tool. There occurs the environmental load during the chain process of the cycle of the secondary tool and therefore it is necessary to consider this environmental load to evaluate the entire life cycle. In this study, the main focus is placed on the usage phase of the secondary tool, which is relevant to the manufacturing phase of the primary tool.
Regarding to above point (2) and (3), miniaturization of primary tool is getting increased as represented by the mobile communication terminal equipment. However, downsizing of secondary tool is less-advanced. The use of downsized and low-powered laser will be effctive for low environmental impact. This laser heat treatment method will also cut the transport phase between manufacturing processes and reduce the energy usage compared to the conventional use of furnace for hardening.
In this study, laser hardening was carried out and the efficiency was compared with various laser types and also other heat treatment methods. Figures 3-5 show the three kinds of laser picked up in the study: a diode laser (Hamamatsu Photonics, LD-HEATER L10060), an Ytterbium-doped near-IR fiber laser (IPG Photonics, YLM-30), and a CO2 laser (SYNRAD, 57-1). Figure 6 shows an electric furnace (DENKEN, KDF-S70, inside of furnace is 120 mm×90 mm×220 mm, 1.4 kW). This furnace was used to implement the conventional hardening method and compared with laser hardening. Table 1 shows the basic specifications of the lasers.
The workpiece material was thin carbon steel sheet (JIS; S50C, area: 30 mm×30 mm, thickness: 0.5 mm - 1.0 mm). Figure 7 shows the experimental set up of laser hardening. Laser hardening was carried out by the diode laser and Ytterbium-doped near-IR fiber laser. The laser scanning speed was varied from 100 mm/min to 500 mm/min. We aimed at through-thickness hardening by one laser scanning path for the thin material.
Fig. 2. Industrial life cycle considering secondary tool.
Resource Material Manufacturing Usage Disposal
Resource Material Manufacturing Disposal
Cycle of primary tool Cycle of secondary tool
Reuse Remanufacturing
Recycle
Reuse Remanufacturing
Recycle
Usage
Resource Material Manufacturing Usage Disposal
Resource Material Manufacturing Disposal
Cycle of primary tool Cycle of secondary tool
Reuse Remanufacturing
Recycle
Reuse Remanufacturing
Recycle
Usage
Further, power consumption was measured by a clamp power meter (Yokogawa M&C, CW120). The energy efficiencies were calculated by using this data.
Laser hardening with low power laser
Figure 8 shows the variation of hardened depth with laser scanning speed for the diode laser. As expected, the hardened depth increased at lower scanning speeds. In particular, the hardened depth increased drastically at 200-300 mm/min and through-thickness hardening was obtained at speeds lower than 200 mm/min in the case of 0.5 mm sheet thickness. Figure 9 shows the laser irradiated cross section of 0.5 mm sheet thickness. It can be seen that the heat affected zone extends from the irradiation side to the back side of the sheet. However, the dark lines could be seen even though martensite was formed in the center. Figure 10 shows the Vickers hardness in the width direction (at 0.1 mm depth line) for the diode laser treated sample. The hardness was about 800 HV and was hardened enough in the center of the part. However, the hardness decreased at 0.3 mm from the center. This is to be expected since untempered martensite formation is known to occur close to the center of the laser treated material due to the high temperatures generated there, whereas only tempering of the original material occurs closer to the dark lines that mark the edge of the heat affected zone. As a result, through-thickness hardening of the thin material was possible at speeds lower than 200 mm/min with 30 W low power diode laser. Next, the optimal condition was identified from a viewpoint of sheet thickness. Figure 11 shows the variation of hardened depth with sheet thickness. The red-dotted line in the graph represents the limitation that the hardened depth cannot exceed its sheet thickness. Almost the same hardened depth was obtained at 0.9 mm-1.0 mm thickness. Decreasing the sheet thickness, the hardened depth increased in the 0.4 mm-0.8 mm thickness range. However, the depth was not the same value with sheet thickness and through-thickness hardening couldn’t be achieved at higher sheet thicknesses. However, for the 0.5 mm sheet thickness, through-thickness hardening could be obtained.
Further, the hardened depth cannot exceed its sheet thickness
Table 1. Specifications of lasers used.
Fig. 3. Diode laser.
Fig. 4. CO2 laser. Fig. 5. Ytterbium-doped near-IR fiber laser.
Fig. 6. Electric furnace.
Fig. 7. Experimental setup.
Laser scanning direction
Workpiece Laser scanning direction
Workpiece
Laser type Power Wave length
CO2 laser 100 W 10.6 μm
Ytterbium-doped fiber laser 30 W 1.07 μm
Diode laser 30 W 0.808 μm
and that follows the red-dotted line as shown in Fig. 11. Based on this, it is thought that 0.5 mm sheet thickness is almost optimal thickness for the laser conditions used.
Efficiency of heat treatment process and environmental impact
In this section, efficiency of energy usage is discussed based on the hardening result, power consumption and general conversion efficiency. The purpose of this approach is to compare the different heat treatment methods from a viewpoint of the efficiencies of heat transfer, conversion and energy usage, and consequently clarify the environmental friendliness of laser hardening with the low power laser. Figure 12 shows the flow chart of efficiency calculation. Here, Q1 is the electrical energy (wall-plug) which is input to the heat treatment equipment, Q2
is the optical energy, and Q3 is the heat energy which is absorbed to the object. The wall-plug efficiency E1 and the total efficiency E are defined as given by Eqs. (1) and (2). The absorptions E2, which is Q3/Q2, are approximately 5% for the 10.6 μm wavelength (without coating), 30% for the 1.07 μm wavelength, and 50% for the 0.808 μm wavelength in the case of steel material. These values were used for each laser. Further, the efficiency of microwave was also compared from the viewpoint of the same electromagnetic wave. This was picked up as a familiar example of wall-plug to object heating. Here, E1 is 52% and the absorption E2 is minimum 48% for the 122 mm wavelength for food in the case of microwave 5).
1 2
1
Q
E = Q
(1)
E = E
1⋅ E
2 (2)Fig. 10. Vickers hardness in the width direction (Diode laser, laser scanning speed 100 mm/min,
sheet thickness 0.5 mm).
Fig. 11. Variation of hardened depth with sheet thickness
(Diode laser, laser power 30 W, laser scanning speed 100 mm/min).
Irradiated side Irradiated side
Fig. 8. Variation of hardened depth with laser scanning speed for diode laser.
Fig. 9. Laser irradiated cross section
(Diode laser, laser power 30 W, laser scanning speed 100 mm/min, sheet thickness 0.5 mm).
0.0 0.2 0.4 0.6
0 200 400 600
Laser scanning speed (mm/min)
Hardened depth (mm) 1.0mm sheet thickness
0.5mm sheet thickness
0 200 400 600 800 1000
-0.6 -0.4 -0.2 0 0.2 0.4 0.6
Distance from the center (mm)
Hardness (HV)
0 0.2 0.4 0.6
0 0.5 1
Sheet thickness (mm)
Hardened depth (mm)
Q
1E
1Q
2E
2Q
3Q
1E
1Q
2E
2Q
3Fig. 12. Flow chart of efficiency.
Figure 13 shows the wall-plug efficiency E1 for the various laser types used and microwave. Here, the use of CO2 laser was not appropriate for on-machine laser hardening among various laser types, but it was still considered for efficiency comparison. The Ytterbium-doped fiber laser and the diode laser exhibited higher wall-plug efficiencies than the CO2
laser among various lasers. It was thought that the original power consumption was low in the case of above two lasers. Further, the efficiency of microwave was higher compared to laser heat sources. This was thought to be caused by differences of wavelength and mechanism. Figure 14 shows the total efficiency E. In this figure, the diode laser is seen to be most efficient among lasers, largely due to the higher absorption obtained at smaller wavelengths. Further, heating
efficiency of microwave is efficient for the material which can be heated by microwave wavelength. In other words, improvement of heating efficiencies for steel material is still necessary. As a result, it was found that the diode laser had the most effective conversion/transfer efficiency.
Next, energy efficiency and the environmental impact of laser hardening were compared with conventional furnace hardening. Figure 15 shows the energy usage per unit hardened depth with various hardening methods. A hardness of ~800 HV was obtained by furnace hardening and the workpiece was totally hardened. These values were calculated as
follows: 30 mm depth 0.5 mm 5 pieces
hardening for 1.5 hours furnace usage, 30 mm depth 0.2 mm hardening for 18 min (100 mm/min laser Fig. 13. Wall-plug efficiency E1
for various laser types and microwave.
1 10 100
CO2 laser Ytterbium- doped fiber
laser
Diode laser Microwave Wall-plug efficiency E1 (%)
0 1 10 100
CO2 laser Ytterbium- doped fiber
laser
Diode laser Microwave
Total efficiency E (%)
Fig. 14. Total efficiency E with various laser types and microwave.
0 500000 1000000 1500000 2000000 2500000
Electric furnace Ytterbium-doped fiber laser
Diode laser
Energy usage per unit hardened depth (J/mm)
Fig. 15. Energy efficiency for hardened depth with various hardening methods.
0 20 40 60 80
Electric furnace Diode laser Carbon dioxide emission (kgCO2)
Fig. 16. Carbon dioxide emission by furnace hardening and diode laser hardening.
scanning speed) Ytterbium-doped fiber laser usage, and 30 mm depth 0.5 mm hardening for 18 min (100 mm/min laser scanning speed) diode laser usage. It depends on the number of pieces placed in the furnace at one time. However, in above calculation, the Ytterbium-doped fiber laser and the diode laser were energy-efficient methods to harden the same depth of material. Figure 16 shows carbon dioxide emission by furnace hardening and diode laser hardening, which have through-thickness hardening potential. The coefficient of carbon dioxide emission was set as 0.555 kg-CO2/kWh, which was the default value in Japanese electric industry. The conditions were set to be the same as mentioned above and 100 workpieces were assumed to be hardened. The temperature of furnace hardening was set as 850°C. It was demonstrated that the carbon dioxide emission of furnace hardening was 67 kg larger than that of the diode laser. This indicates that the diode laser has the potential to harden small parts with low environmental impact.
Hardening of small parts with low power lasers was carried out for the purpose of identifying an eco-friendly laser surface treatment process for thin material. As a result, it was
found that through-thickness hardening was obtained at laser scanning speeds less than 200 mm/min for 0.5 mm steel sheet thickness with 30 W diode laser. Further, it was demonstrated that the diode laser used in this study was energy-efficient among various laser types and had the lowest environmental impact during hardening of small parts.
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