Life cycle inventory study on magnesium alloy substitution in vehicles

Life cycle inventory study on magnesium alloy substitution in vehicles

ARTICLE IN PRESS Energy 32 (2007) 1352–1360 www.elsevier.com/locate/energy Life cycle inventory study on magnesium alloy substitution in vehicles Ma...

196KB Sizes 6 Downloads 38 Views

ARTICLE IN PRESS

Energy 32 (2007) 1352–1360 www.elsevier.com/locate/energy

Life cycle inventory study on magnesium alloy substitution in vehicles Masataka Hakamadaa,, Tetsuharu Furutaa, Yasumasa Chinob, Youqing Chena, Hiromu Kusudaa, Mamoru Mabuchia a

Department of Energy Science and Technology, Graduate School of Energy Science, Kyoto University, Yoshidahonmachi, Sakyo, Kyoto 606-8501, Japan b Materials Research Institute for Sustainable Development, National Institute of Advanced Industrial Science and Technology, 2266-98 Anagahora, Shimo-shidami, Moriyama, Nagoya 463-8560, Japan Received 24 March 2006

Abstract Magnesium (Mg) alloys are suitable materials for weight reduction in vehicles because of their low density of 1.7 g/cm3 and high specific strength. The effect of Mg substitution for conventional steel parts in a vehicle on total energy consumption and CO2 emissions was evaluated through life cycle inventory calculation. The Mg substitution reduces the total energy consumption by weight reduction, although the production energy of a Mg-substituted vehicle is higher than those of conventional and Al-substituted vehicles. The Mg substitution can save more life cycle energy consumption than the Al substitution. Recycling of Mg parts is indispensable for efficient CO2 reduction, because the CO2 emissions during new ingot production of Mg are much higher than those of conventional steel and Al. Strengthening of the Mg parts also can reduce the total energy consumption and CO2 emissions. If the main body and hood are made of Mg alloy and the ratio of recycled ingot is sufficiently high, the life cycle energy consumption and CO2 emissions will be markedly reduced. r 2006 Elsevier Ltd. All rights reserved. Keywords: Magnesium; Life cycle inventory; Forged alloy; Weight reduction

1. Introduction The motor vehicle industry has been attempting to improve fuel efficiency by weight reduction in automobiles, and thereby to reduce emissions of greenhouse effect gas. To attain the weight reduction, the improvement of steel strength and the substitution of aluminum (Al) parts for conventional steel parts have been conducted. Some vehicles have been equipped with Al parts (such as engine piston, wheel and suspension). Recently, the demand for magnesium (Mg) substitution has been increasing because of its low density and high specific strength (¼ strength divided by density) [1–3]. Mg parts have been introduced in a smaller scale than Al; however, the automotive application of Mg alloy is currently drawing much attention of motor vehicle industries because legislation limiting emisCorresponding author. Tel.: +81 75 753 5421; fax: +81 75 753 5428.

E-mail address: [email protected] (M. Hakamada). 0360-5442/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.energy.2006.10.020

sion has triggered the serious requirement of weight reducing [1,2]. Life cycle assessment (LCA) is a method of evaluating environmental impact such as energy consumption and CO2 emissions, based on the life cycle of a product [4]. To date, many LCAs on vehicles (including Al-substituted ones) have been conducted [5–11]. It was shown that the use of Al in automotive applications gives rise to less energy consumption during its life cycle than the use of conventional steel, and that recycling of Al scrap leads to further life cycle energy savings [5,6]. It is important to estimate the environmental impact by Mg substitution in vehicles through LCA; however, there are few LCAs of Mg substitution. Koltun et al. [7] shows the life cycle environment impact of Mg converter housing in vehicles; however, Mg is expected to be applied to more components for weight reduction in vehicles. Full vehicle LCA of the Mg-substituted vehicle has not been conducted thus far. In this study, we show the life cycle inventory (LCI) study of a vehicle equipped with Mg alloy parts (LCI is a

ARTICLE IN PRESS M. Hakamada et al. / Energy 32 (2007) 1352–1360

fundamental part of LCA). Many existing steel parts are assumed to be replaced by Mg parts. The environmental impact (energy consumption and CO2 emissions) of the Mg-substituted vehicle is compared with those of conventional and Al-substituted vehicles. The LCI studies of the Mg-substituted vehicle are also provided assuming several possible cases. In the assumption, we varied (1) the weight ratio of recycled ingot to whole incoming ingot, (2) the strength of the Mg alloy and (3) application range of the Mg parts. 2. Goal and scope 2.1. Goal The goal of this LCI study is to estimate the effectiveness of Mg substitution for vehicle parts by calculation of the energy consumption and CO2 emissions of a conventional vehicle, a vehicle with Al substitution and a vehicle with Mg substitution. 2.2. Vehicle data We assume in this LCI study that the vehicle is a typical passenger car with an engine displacement of 2000 cc. Based on statistical data, the duration of use and total operating distance of a vehicle are assumed to be 10 years and 100,000 km, respectively, in this LCI study [8,12] (Refs. [8,12] are primary LCA studies on a vehicle, which are conducted in Japan). The overall merit of a weight reduction via material substitution is dependent on the total operating distance. The value of 100, 000 km is common in Japan [8,12], although shorter than those in Europe and North America [9]. The typical weights of the vehicle parts are shown in Table 1 [13] (Ref. [13] describes how the materials are applied to current automobiles and asserts the importance of material selection in automotive Table 1 Weight of vehicle parts and replacement ratio in this LCI study Vehicle parts and materials

Weight (kg)

Replacement ratio (%) Normal case

Potential case

Steel Main body Doors and hoods Wheel Wheel hub Sheet frame Steering Transmission Engine Other parts Total steel products

260.7 81.9 30.8 57.9 45.0 21.3 70.7 141.4 90.3 810.0

0 50 100 100 100 100 0 0 0 —

100 100 100 100 100 100 0 0 0 —

Other materials Other parts Total other products

404.0 1214.0

— —

— —

1353

weight reduction). The total weight of the vehicle is 1214 kg. The steel parts, which will be replaced by Mg or Al alloy parts in the following analyses, constitute 67% of the total weight. 2.3. Mg or Al substitution In a normal case, which is mainly applied through this LCI study, Mg or Al alloy is applied to doors, wheels, wheel hubs, sheet frames, and steering as substitutes for the steel parts. These parts have already been made of Mg and Al in some cases [13], and can be easily made of Mg or Al alloy. Additional analyses were conducted under the assumption that Mg or Al parts are also substituted for the main body and hoods in a later discussion (denoted as a potential case). The assumed Mg or Al replacement ratios of each part are also listed in Table 1. The subject Mg or Al alloy in the present LCI study is assumed to be only a forged alloy, not a cast alloy, to avoid confusion between the cast alloy and the forged alloy. Hence, the transmission, engine and their surrounding parts are excluded from this LCI study because it is difficult to make them by forging. In the normal case, the total weight of the steel parts, which are replaced by Mg or Al parts, is 196 kg, which is 24% of all the steel parts. The powertrain efficiency is assumed to be unchanged by the material substitution because the transmission, engine and their surrounding steel parts are not replaced by Mg or Al parts. 2.4. Calculation of substitution In general, various properties and factors such as strength, Young’s modulus (rate of change of stress with strain), heat resistance, fatigue resistance, toughness, corrosion and cost, must be taken into consideration in material selection of automotive parts. The most important properties for material selection depend on the parts. However, it is too complicated to strictly analyze each part from the various properties and factors. For simplicity, in the present analysis, strength is considered for Mg or Al alloy substitution. Namely, when the product of the specific strength and weight of a Mg part is equal to that of a steel part, the steel part is replaced by the Mg part. Therefore, when steel parts are replaced by Mg parts, the total weight of the substituted Mg parts (MMg) is given by M Mg ¼

sFe =rFe M Fe , sMg =rMg

(1)

where MFe is the weight of the Fe (steel) parts to be replaced, sMg is the strength of the Mg alloy, sFe is the strength of the steel, rMg is the density of the Mg alloy and rFe is the density of the steel. Similar logic holds for Al substitution. The strength and density of a steel (SPFC440) [14], a Mg alloy (AZ31) [15] and an Al alloy (5000 series) [16] are listed in Table 2.

ARTICLE IN PRESS M. Hakamada et al. / Energy 32 (2007) 1352–1360

1354

Table 2 Strength and density of steel (SPFC440), Mg alloy (AZ31) and Al alloy (5000 series) Material

Tensile strength (MPa)

Density (g/cm3)

Specific strength (MPa cm3/g)

Automotive steel (SPFC440) Mg alloy (AZ31) Recycled Mg alloy (AZ31) Al alloy (5000 series) Recycled Al alloy (5000 series)

440 260 234 275 248

7.85 1.78 1.78 2.71 2.71

56.1 146.1 131.5 101.5 91.3

We assume that, when Mg and Al alloys are recycled, their strength is decreased to 90% because of contamination. Taking the recycling into consideration, Eq. (1) should be corrected as

Mining and material production Production stage

Parts manufacture

Recycling

Vehicle assembly

M Mg

sFe =rFe ¼ M Fe , ð1  rÞsMg;new =rMg;new þ rsMg;rc =rMg;rc

(2)

where r is the weight ratio of recycled Mg ingot to whole incoming Mg ingot, sMg,new is the strength of the new Mg alloy, sMg,rc is the strength of the recycled Mg alloy (¼ 0.9sMg,new), rMg,new is the density of the new Mg alloy and rMg,rc is the density of the recycled Mg alloy. The recycled ingot weight ratio (r) is typically set to 50%, although it will be varied in Section 4.2. The strength of the Mg (sMg,new) alloy will be also hypothetically improved in Section 4.3. The substitution factor f of Mg for steel is expressed as [6] DM Mg f ¼ , DM Fe

(3)

where DMMg and DMFe are the variations in the weights of Mg and steel parts, respectively (note that the DMFe is negative value). In the present analysis, f of Mg is calculated to be 0.21–0.38, on the basis of Eq. (2). In the same manner, f of Al is 0.55–0.60. These substitution factors are comparable to those employed in the study by Sullivan et al. [6]. 2.5. System boundary and life cycle stages Fig. 1 shows the outline of the life cycle flow and system boundary for the LCI of a vehicle. The life cycle of a vehicle consists of four stages: (1) production stage including extraction of raw materials from the earth, (2) use stage, (3) end-of-life stage and (4) transportation among these stages. The energy for recycling is included in the production stage because it is regarded as the production energy of a recycled ingot. All stages and processes in the system are linked to one another by balanced material flows. The life cycle total energy consumption (Etot) and CO2 emissions (Ctot) are calculated by the following equations: E tot ¼ E prd þ E use þ E eol þ E trp

(4)

Gasoline production Use stage

Driving and maintenance

Disassembly End-of-life stage

Shredding and sorting Controlled land fill site

Fig. 1. Life cycle flow and system boundary for LCI of vehicle.

and C tot ¼ C prd þ C use þ C eol þ C trp ,

(5)

where E and C are the energy consumption and the CO2 emissions at each stage respectively, and subscripts prd, use, eol and trp mean the production stage, use stage, endof-life stage and transportation stage, respectively. A typical entire flow chart of Mg products in a vehicle is shown in Fig. 2, where the figures beside each cell indicate the weight of each product. Once the recycled ingot weight ratio (r) and the strength of the new Mg ingot (sMg,new) are determined, the weight of the substituted Mg is firstly calculated according to Eq. (2) (79.2 kg in Fig. 2 as an example). Weight of other Mg product which comes in and goes out of a process is calculated on the basis of the yield rate (or production efficiency, which is the ratio of outgoing material to incoming material) of the process. The yield rates of each process are discussed in the following section. 3. Life cycle inventory calculation 3.1. Production stage Production stage consists of mining, ingot production, material production, part production and vehicle assembly. The energy consumption and CO2 emissions at the production stage are thus given by the sum totals of those

ARTICLE IN PRESS M. Hakamada et al. / Energy 32 (2007) 1352–1360

Dolomite

New ingot production (Pidgeon process)

1355

New ingot 63.4 kg Rolling

Recycled ingot production (remelt recycling)

Recycled ingot 63.4 kg

Waste

Waste

5.0 kg 15.9 kg

Rolled sheet 121.8 kg Parts manufacture

In-house scrap 40.2 kg

Scrap for vehicle 79.3 kg

Scrap for recycling

Mg parts

waste 2.4 kg

79.2 kg Vehicle assembly

115.4 kg Vehicle

Scrap for other products 36.1 kg

Automobile shredder residue

79.2 kg 75.2 kg

Driving and maintenance

Disassembly, shredding and sorting

End-of-life vehicle 79.2 kg

3.9 kg

Fig. 2. Typical life cycle flow of Mg product in vehicle in normal case, where recycled ingot weight ratio is 50% and strength of Mg alloy is 260 MPa.

production substages, that is E prd ¼ E mm þ E prt þ E asm

(6)

and C prd ¼ C mm þ C prt þ C asm ,

Table 3 Energy consumption, CO2 emissions and yield rate during material production Process

Energy (MJ/kg)

(7)

where E and C are the energy consumption and the CO2 emissions at each substage respectively, and subscripts mm, prt and asm mean the mining and material production substage, part production substage and vehicle assembly substage, respectively. Emm and Cmm are the sum totals of the values of individual material production (steel, Mg (or Al) and other materials). 3.1.1. Mining and material production The yield rate of individual production processes and energy consumption, CO2 emissions of the material production are listed in Table 3. The yield rate of part production of automobiles is usually 60–70% [12,17] (Ref. [17] written in Japanese treats the production and processing of vehicle parts by welding, and information on yield rate is available). Hence, we assume it to be 65% for Mg and Al alloys. The total yield rate of Mg parts production from ingot is a product of the yield rate of the part production and that of rolling,

Rolled automotive steel productiona Mg new ingot production Mg recycled ingot production Al new ingot production Al recycled ingot production Rolling (Al and Mg) Process

Nonferrous metal production Plastic production Other material production

CO2 emissions (kgCO2/kg)

33.7

2.7

284.0 11.4 153.8 5.7 22.0

42.0 1.7 9.6 0.3 0.8

Energy (MJ/ vehicle) 5.2 3.5 5.5

CO2 emissions (kgCO2/vehicle) 350 240 440

a The values are per kg-product (rolled steel sheet), not per raw materials.

which is 62% ( ¼ 65%  96%). The yield rate of rolling of Mg is assumed to be the same as those of Al [18]. The yield rate of recycling Mg or Al ingot is assumed to be 80%.

ARTICLE IN PRESS 1356

M. Hakamada et al. / Energy 32 (2007) 1352–1360

The values of energy consumption and CO2 emissions from steel, Mg alloy and Al alloy production are available in the form of per kg-material in Table 3. The values for rolled steel production are indicated in ‘per kg-product’, while those for Mg and Al are in ‘per kg-raw (incoming) material’ in Table 3. The values need to be multiplied by the weight of the materials for calculation of energy consumption and CO2 emissions per vehicle. For example, the energy consumption of Mg part production in a Mgsubstituted vehicle (Emm(Mg)) is calculated as E mmðMgÞ ¼ ð1  rÞeMg;new M Mg =Z þ reMg;rc M Mg =Z,

(8)

where eMg,new and eMg,rc are the energy consumption of new and recycled Mg ingot production per unit weight, respectively, and Z is the yield rate. The values of steel production are represented by those of a cold-rolled steel sheet because it is one of the most commonly used steels in a vehicle [13,14]. Note that the values for steel parts are totals of the primary and recycled steel and are indicated in ‘per kg-product’, not ‘per kg-raw material’, because of data unavailability. The energy consumption and CO2 emissions by the Pidgeon process in China are used in new Mg ingot production [19]. Covering gas (sulfur hexafluoride, SF6) of molten Mg alloy also has the greenhouse effect; however, only CO2 emission is considered in this analysis because the usage of the gas is being phased out after the Kyoto Protocol [19]. The energy for production of Mg ingot by remelt recycling is assumed to be 4% of that of new ingot [20] (Ref. [20] describes the trend in magnesium research from the viewpoint of weight reduction in automobiles, and remelt recycling of Mg is introduced in Section 2–5). The LCI data by the Japanese Aluminum Association is used as the production energy and emission data of Al ingot [21]. Perfluorocarbons (CF4 and C2F6) which emit during Al smelting and possess greenhouse effect are also excluded in this analysis. The energy and CO2 impact by rolling of Mg are assumed to be the same as those of Al [18]. The energy and CO2 emission impacts in the production of nonferrous metals except for Mg and Al, plastics and other materials are quoted from Ref. [10], which deals with LCI study on vehicles and collects extensive data of energy consumption and CO2 emissions for each process. They include total values from raw material extraction to material production. 3.1.2. Parts production and vehicle assembly The energy consumptions and CO2 emissions of the part production and vehicle assembly per vehicle (Eprt, Easm in Eq. (6) and Cprt, Casm in Eq. (7)) are shown in Table 4, where these values are cited from Ref. [8], based on statistics by the Ministry of International Trade and Industry, Japan. The energy consumptions and CO2 emission impacts by part production and vehicle assembly for a Mg- or Al-substituted vehicle are assumed to be the same as those for a conventional vehicle.

Table 4 Energy consumption and CO2 emissions during part production and vehicle assembly Process Tire and tube production Battery production Window production Other automobile parts production Vehicle assembly Total

Energy (GJ/ vehicle) 1.55 0.40 0.34

CO2 emissions (kgCO2/vehicle) 86 20 17

9.11

456

8.63 20.02

459 1038

3.2. Use stage The use stage is divided into two substages: the driving substage including gasoline production and the maintenance substage. The energy consumption and CO2 emissions at the use stage are thus given by the sum totals of those substages, that is E use ¼ E drv þ E mn

(9)

and C use ¼ C drv þ C mn ,

(10)

where E and C are the energy consumption and the CO2 emissions at each substage, respectively, and subscripts drv and mn mean the driving substage and maintenance substage, respectively. 3.2.1. Driving Total driving energy consumption is calculated with a total mileage (L) of 100,000 km and average fuel efficiency (e) as follows. Fig. 3 shows the correlation between curb weight (M) and average fuel efficiency [13]. Curve fitting of the plot in Fig. 3 results in  ¼ 6:4  104 M 1:2 ,

(11)

where e is expressed in km/L and M is in kg/vehicle. The fuel efficiency of the conventional and Mg- or Alsubstituted vehicles can be derived using Eq. (11), once their curb weights are calculated. Then, the total fuel consumption during use of the vehicle (F) is equal to L/e (in liter). The energy consumption and CO2 emissions in this substage are thus evaluated using on the assumption that the unit higher heating value (eguse) and CO2 emissions (cguse) by the use of gasoline are 35.16 MJ/L and 2.36 kg/L, respectively, and that the unit energy (egprd) and CO2 emissions (cgprd) for gasoline production are 2.77 MJ/L and 0.50 kg/L, respectively [8], that is E drv ¼ ðeguse þ egprd ÞF ¼ ðeguse þ egprd ÞL=

(12)

and C drv ¼ ðcguse þ cgprd ÞF ¼ ðcguse þ cgprd ÞL=.

(13)

ARTICLE IN PRESS M. Hakamada et al. / Energy 32 (2007) 1352–1360 Table 5 Energy consumption and CO2 emissions at end-of-life stage

30

Fuel efficiency,  (km/l)

1357

20

Disassembly Shredding and sorting Controlled landfill site Total

Energy (GJ/vehicle)

CO2 emissions (kgCO2/vehicle)

— 0.56 0.09 0.65

— 0.024 0.004 0.028

10

energy between steel products and Mg products is negligible, it is disregarded in the present LCI study. 0 800

1000

1200

1400

1600

1800

4. Results and interpretation

Curb weight, M (kg) Fig. 3. Relationship between fuel efficiency and curb weight of vehicle.

3.2.2. Maintenance For maintenance of a vehicle, some parts (for example, tires and engine oil) are replaced regularly, and damaged parts are repaired. Around 5.1 GJ/vehicle energy is needed for the maintenance [10] and assumed to be the same for all kinds of vehicle, because the values for the replacement and repair are essentially the same, regardless of the materials. 3.3. End-of-life stage Table 5 shows energy consumption (Eeol) and CO2 emissions (Ceol) at the end-of-life stage [10]. There is an established end-of-life vehicle treatment system in Japan. First, valuable parts (such as the engine), parts that contain toxic substances (such as the battery), and liquids (such as engine oil) are taken out. Then we shred the remaining hulk, sort and recover metals (such as steel, Al, Mg and copper), and the final waste, which is called automobile shredder residue, is buried in a landfill site [22]. The recovery rate of Mg is assumed to be approximately the same as Al, which is 95% [12]. At this recovery rate, the scrap from automobiles can supply the whole amount of recycled ingot. It is assumed that the use of Mg or Al products has no influence on energy and emissions at the end-of-life stage. 3.4. Transportation stage It is difficult to estimate the transportation energy because transported products and distance depend on situations such as automobile manufacture. In this paper, the data in Ref. [8], which is based on an interindustry relation table, is used as the transportation energy data. The energy consumption for average transportation (Etrp) is 6.64 GJ/vehicle, and the CO2 emissions (Ctrp) are 437.1 kg/vehicle. Because the difference in transportation

4.1. Comparison of Mg alloy substitution with conventional steel and Al alloy The resulting total energy consumptions (Etot) and CO2 emissions (Ctot) are listed in Table 6, where the weight ratio of recycled Mg or Al ingot to whole incoming ingot (r) is 50% and the specific strengths of the alloys are shown in Table 2. The values obtained here are roughly in agreement with other LCI studies collected in Ref. [9]. The most energy-consuming and CO2-emitting stage is the use stage, and the second most energy-consuming and CO2-emitting stage is the production stage. The trend also agrees with other LCIs [9]. Mg substitution results in a total life cycle energy saving of 5.1% for a conventional car. The Alsubstituted vehicle also saves 3.2% energy. Mg and Al substitution consume more energy than conventional steel at the production stage because of the high energy requirement for the new ingot production from mined ores. However, ‘‘weight reduction’’ with Mg or Al increases fuel efficiency sufficiently to reduce overall energy use. Mg substitution saves more energy than Al substitution because of its higher specific strength, although its production energy is larger than that of Al. Table 7 shows detailed results at the use stage in the normal case. When Mg parts are substituted for steel parts, curb weight decreases and fuel efficiency is improved more than those in Al-substitution case, because of the high specific strength of Mg. The Mg- and Al-substitution also reduce gasoline production energy and driving energy per vehicle, and the former exceeds the latter in energy saving. In spite of large energy savings by Mg substitution, only slight reduction (1.5%) in CO2 emissions from a Mgsubstituted vehicle is attained. The reduction in CO2 emissions from an Al-substituted vehicle is larger than the case of Mg substitution. The small effect on CO2 emissions by the Mg substitution is due to high CO2 emissions at the production of the new Mg ingot. In this analysis, we assume that the new Mg ingot is produced by the heat reduction method in China, where coal is widely used [19]. Hence, it is suggested that an improvement in

ARTICLE IN PRESS M. Hakamada et al. / Energy 32 (2007) 1352–1360

1358

Table 6 Life cycle energy consumption and CO2 emissions of a vehicle with or without substitution in normal case, where weight ratio of recycled ingot to whole incoming ingot is 50% No substitution

Mg Substitution

Al Substitution

61.5 302.8 0.7 6.6 371.7 —

76.4 268.8 0.7 6.6 352.5 (–5.1)

73.5 278.9 0.7 6.6 359.7 (–3.2)

CO2 emissions (tCO2/vehicle) Production, Cprd Use, Cuse End of life, Ceol Transport, Ctrp

5.1 22.8 0.0 0.4

7.2 20.2 0.0 0.4

5.4 21.0 0.0 0.4

Total, Ctot (Change from a conventional vehicle, %)

28.3 —

27.9 (–1.5)

26.8 (–5.3)

Energy consumption (GJ/vehicle) Production, Eprd Use, Euse End of life, Eeol Transport, Etrp Total, Etot (Change from a conventional vehicle, %)

Table 7 Curb weight and fuel efficiency of vehicle with or without substitution, and energy consumption and CO2 emissions at individual substages in use stage in normal case, where weight ratio of recycled ingot to whole incoming ingot is 50%

Curb weight, M (kg/ vehicle) Fuel efficiency, e (km/L)

No Substitution

Mg Substitution

Al Substitution

1214

1097

1132

12.7

Energy consumption (GJ/vehicle) 21.7 Gasoline production, Egprd Gasoline use, 276.0 Eguse Maintenance, Emn 5.1 Total, Euse 302.8 CO2 emissions (tCO2/vehicle) Gasoline 3.9 production, Cgprd Gasoline use, 18.5 Cguse Maintenance, Cmn 0.3 22.8 Total, Cuse

14.4

13.9

19.2

20.0

244.5

253.8

5.1 268.8

5.1 278.9

3.5

3.6

16.4

17.0

0.3 20.2

0.3 21.0

new ingot production is needed for CO2 reduction through Mg substitution.

Table 8 Effect of recycled ingot weight ratio on life cycle energy consumption and CO2 emissions of Mg-substituted vehicles in normal case Mg Substitution Recycled ingot ratio, r (weight%)

0

50

75

Energy consumption (GJ/vehicle) Production, Eprd Use, Euse End of life, Eeol Transport, Etrp Total, Etot (Change from a conventional vehicle, %)

91.8 267.7 0.7 6.6 366.7 (–1.3)

76.4 268.8 0.7 6.6 352.5 (–5.1)

68.1 269.4 0.7 6.6 344.9 (–7.2)

CO2 emissions (tCO2/vehicle) Production, Cprd Use, Cuse End of life, Ceol Transport, Ctrp Total, Ctot (Change from a conventional vehicle, %)

9.5 20.1 0.0 0.4 30.1 (+6.3)

7.2 20.2 0.0 0.4 27.9 (–1.5)

6.0 20.3 0.0 0.4 26.7 (–5.7)

energy of new Mg ingots offsets the reduction in energy consumption at the use stage by the substitution. The CO2 emissions even increase markedly if Mg parts are used without recycling. Hence, as for Mg, recycling plays an important role in reducing energy consumption and CO2 emissions at the production stage. 4.3. Strength improvement

4.2. Ratio of recycled ingot Table 8 shows the total energy consumption and CO2 emissions under different weight ratios of recycled Mg ingot to the whole incoming ingot (r). As r increases, total energy consumption and CO2 emissions decrease. Without recycling, the benefit of Mg substitution in total energy consumption is negligible, because the large production

Recent extensive studies on Mg alloys indicate the possibility of further strength improvement [3,23]. Even recycled Mg alloy can possess higher strength than ordinary Mg alloy if proper processing is applied [24]. We analyzed the effect of strength improvement of Mg alloy on total energy consumption and CO2 emissions. The results are summarized in Table 9. In this analysis, the

ARTICLE IN PRESS M. Hakamada et al. / Energy 32 (2007) 1352–1360

energy consumption and CO2 emissions are assumed to be unchanged even if the strength is improved. The assumption seems proper, taking the results in the recent studies on Mg [3,23,24] into consideration. The strength improvement reduces curb weight in accordance with Eq. (2), and thus, the energy consumption and CO2 emissions at the use stage decrease. As a result, total energy consumption and CO2 emissions are effectively reduced through strength improvement of the Mg alloy. 4.4. Potential case: Mg body and hood It is difficult to produce a Mg body because of the low workability of Mg in a cold press, which is now being solved to some extent [25]. Hence, as a potential case, we analyzed the environmental impact assuming that the main body (which holds 21% weight of a conventional vehicle) and hoods are made of Mg alloy without change in energy consumption and CO2 emissions. Table 10 summarizes the LCI study results in the potential case. The total weight of steel parts to be replaced by Mg parts is 497.6 kg and curb weight decreases by approximately 24%. In the potential case, more than 10% reduction in the total energy consumption is achieved at both recycled ingot weight ratios to the whole incoming ingot; however, CO2 emissions decrease by only 3.5% at a recycled ingot weight ratio of 50%, which is attributed to the high CO2 emission of new Mg ingot production. The recycle ingot weight ratio as high as 75% makes it possible to reduce the total CO2 emissions efficiently. Thus, it is suggested that high-grade recycling process must accompany the Mg application to the body and hood to lower the life cycle CO2 emissions.

Table 9 Effect of Mg alloy strength on life cycle energy consumption and CO2 emissions of Mg-substituted vehicles in normal case, where weight ratio of recycled ingot to whole incoming ingot is 50% Mg Substitution Strength, sMg,new (MPa)

260

300

400

Curb weight, M (kg)

1097

1087

1070

Energy consumption (GJ/vehicle) Production, Eprd Use, Euse End of life, Eeol Transport, Etrp Total, Etot (Change from a conventional vehicle, %)

76.4 268.8 0.7 6.6 352.5 (–5.1)

73.6 265.8 0.7 6.6 346.6 (–6.7)

68.9 260.8 0.7 6.6 337.0 (–9.3)

CO2 emissions (tCO2/vehicle) Production, Cprd Use, Cuse End of life, Ceol Transport, Ctrp Total, Ctot (Change from a conventional vehicle, %)

7.2 20.2 0.0 0.4 27.9 (–1.5)

6.8 20.0 0.0 0.4 27.3 (–3.7)

6.2 19.6 0.0 0.4 26.3 (–7.2)

1359

Table 10 Life cycle energy consumption and CO2 emissions of Mg-substituted vehicles in potential case, where recycled ingot weight ratio is taken to be 50% and 75% Mg Substitution Recycled ingot ratio, r (weight%)

50

75

Curb weight, M (kg)

917

923

Energy consumption (GJ/vehicle) Production, Eprd Use, Euse End of life, Eeol Transport, Etrp Total, Etot (Change from a conventional vehicle, %)

99.4 217.8 0.7 6.6 324.5 (–12.7)

78.3 219.4 0.7 6.6 305.0 (–17.9)

10.5 16.4 0.0 0.4

7.4 16.5 0.0 0.4

27.3 (–3.5)

24.3 (–14.1)

CO2 emissions (tCO2/vehicle) Production, Cprd Use, Cuse End of life, Ceol Transport, Ctrp Total, Ctot (Change from a conventional vehicle, %)

5. Conclusions The environmental impact of Mg substitution in vehicle parts was evaluated through LCI study, and compared with Al substitution. Several hypothetical analyses were also conducted to examine the potential of the Mg substitution. The results are concluded as follows. (1) Mg substitution can save more life cycle energy consumption than the Al substitution, although Mg ingot production consumes more energy than Al and steel productions. The reduction in use energy by weight reduction overwhelms the increase in energy consumption at the production stage. (2) The use of recycled Mg ingot in a high weight ratio is needed in keeping the life cycle energy consumption and CO2 emissions low because the new Mg ingot production requires high energy and discharges much CO2. In particular, the Mg substitution without recycling leads to the increase in CO2 emissions. Thus, effective recycling of the Mg alloy should accompany the Mg substitution. (3) Strength improvement in the Mg alloy decreases total energy consumption and CO2 emissions. (4) If the body and hood are made of Mg alloy and the ratio of recycled ingot is sufficiently high, the total energy consumption and CO2 emissions will be markedly reduced.

References [1] Mordike BL, Ebert T. Magnesium properties—applications—potential. Mater Sci Eng A 2001;A302(1):37–45.

ARTICLE IN PRESS 1360

M. Hakamada et al. / Energy 32 (2007) 1352–1360

[2] Luo AA. Magnesium: current and potential automotive applications. JOM 2002;54(2):42–8. [3] Inoue A, Kawamura Y, Matsushita M, Hayashi K, Koike J. Novel hexagonal structure and ultrahigh strength of magnesium solid solution in the Mg–Zn–Y system. J Mater Res 2001;16(7):1894–900. [4] LCA Jitsumu Nyuumon Henshuu Iinkai. LCA practical guide (LCA Jitsumu Nyumon). Tokyo: Japan Environmental Management Association for Industry; 1998 (in Japanese). [5] Das S. The life-cycle impacts of aluminum body-in-white automotive material. JOM 2000;52(8):41–4. [6] Sullivan JL, Hu J. Life cycle energy analysis for automobiles. SAE technical paper 951829. Warrandale, PA: Society of Automotive Engineers; 1995. [7] Koltun P, Tharumarajah A, Ramakrishnan S. Life cycle environmental impact of magnesium automotive components. In: Neelameggham NR, Kaplan HI, Powell BR, editors. Magnesium technology 2005. Warrendale: TMS; 2005. p. 43–9. [8] Kobayashi O. Automobile LCA study (Jidosha no LCA Kenkyu). In: Proceedings of the second international conference on EcoBalance. Japan: Tsukuba; 1996. p. 76–9 (in Japanese). [9] Sullivan JL, Cobas-Flores E. Full vehicle LCAs: a review. SAE technical paper 2001-01-3725. Warrandale, PA: Society of Automotive Engineers; 2001. [10] Funazaki A, Taneda K. Life cycle assessment on end-of-life vehicles (Shiyouzumi jidosha no LCA). Journal of JSAE (Jidosha Gijutsu) 2002;56(7):57–63 (in Japanese). [11] Sullivan JL, Williams RL, Yester S, Cobas-Flores E, Chubbs ST, Hentges SG, et al. Life cycle inventory of a generic US family Sedan overview of results USCAR AMP Project. SAE technical paper 982160. Warrandale, PA: Society of Automotive Engineers; 1998. [12] Funazaki A, Taneda K. A study of inventories for automobile LCA (4) – A life cycle material flow of a vehicle (Jidosha LCA no tameno inventory sakusei no kangaekata (4)—life cycle ni okeru sharyo kousei zairyo no busshitsu flow). JARI Res J 2001;23(10):46–53 (in Japanese). [13] Kou Y. Aluminum vs. steel body—secrets of automobile materials (Arumi vs tetsu body—jidosha you zairyo no himitsu). Tokyo: Sankaido; 2002 (in Japanese). [14] Japanese Industrial Standards Committee. Cold rolled high strength steel sheets with improved formability for automobile structural uses. JIS G 3135. Tokyo: Japanese Standards Association; 1986.

[15] Japanese Industrial Standards Committee. Magnesium alloy sheet and plates. JIS H 4201. Tokyo: Japanese Standards Association; 1998. [16] Japanese Industrial Standards Committee. Aluminium and aluminium alloy sheets, strips and plates. JIS H 4000. Tokyo: Japanese Standards Association; 1999. [17] Sadamura K. Stamping a laser-welded blank (Laser yousetsu usuita no press seikei). J JSTP 1993;34(391):917–24 (in Japanese). [18] Japan Aluminum Association. Summary of LCI data of aluminum rolled sheet (Aluminum atsuenhin no LCI data no gaiyou). Tokyo: Japan Aluminum Association, 2003. See also: http://www.aluminum. or.jp/whats_new/030326/atuen.pdf (in Japanese). [19] Ramakrishnan S, Koltun P. Global warming impact of the magnesium produced in China using the Pidgeon process. Resour Conserv Recycl 42 2004;42(1):49–64. [20] Watai H. Research and development trends of magnesium alloy— From the viewpoint of weight reduction in structural materials for automobiles (Magnesium goukin no Kenkyu Kaihatsu Doko— jidosha you kouzou zairyo no keiryoka no shiten kara). Sci Tech Trends 2005;53(8):20–9 (in Japanese). [21] Japan Aluminum Association, Summary of LCI data of aluminum new ingot and recycled ingot for expanded materials (Aluminum shin jigane oyobi tenshinzai you saiseijigane no LCI data no gaiyou). Tokyo: Japan Aluminum Association, 2003. See also: http:// www.aluminum.or.jp/whats_new/030326/chikin.pdf (in Japanese). [22] Funazaki A, Taneda K, Tahara K, Inaba A. Automobile life cycle assessment issues at end-of-life and recycling. JSAE Rev 2003;24(4): 381–6. [23] Yamashita A, Horita Z, Langdon TG. Improving the mechanical properties of magnesium and a magnesium alloy through severe plastic deformation. Mater Sci Eng A 2001;A300(1–2): 142–7. [24] Chino Y, Lee JS, Nakaura Y, Ohori K, Mabuchi M. Mechanical properties of Mg-Al-Ca alloy recycled by solid-state recycling. Mater Trans 2005;46(12):2592–5. [25] Chino Y, Lee JS, Sassa K, Kamiya A, Mabuchi M. Press formability of a rolled AZ31 Mg alloy sheet with controlled texture. Mater Lett 2006;60(2):173–6.