Vijay Bhalerao1,a, B. Dr. Anurag Nema2,b
1Tata Motors Passenger Vehicles Ltd.,DGM-Quality, Materials Engineering, Chikhali, Pune-411062, Maharashtra, India.
1Tata Motors Sponsored Student of Final Year M.Tech.- EV Technology, MIT SoES, MIT Art Design Technology University, Loni Kalbhor, Pune-412201, Maharashtra, India
2 Assistant Professor, MIT SoES, MIT Art Design Technology University, Loni Kalbhor, Pune-412201, Maharashtra, India
avkb793615@tatamotors.com, banurag.nema@mituniversity.edu.in
Abstract
The future of India’s electric vehicle (EV) market is set to soar, according to the Economic Survey 2023, projecting a noteworthy 49% compound annual growth rate (CAGR) from 2022 to 2030. This rapid growth is expected to result in an impressive 10 million annual sales by 2030. While the potential for success is evident, there remains a critical challenge in dealing with the widespread fear of inadequate battery range among potential consumers.
To address this concern and improve overall energy efficiency, the key lies in the practice of light weighting EVs. Even a small 10% reduction in curb weight can result in a significant 6-8% decrease in energy consumption, making EVs not only more attractive, but also more environmentally sustainable. Cutting-edge materials such as Ultra High Strength steels for automobiles, Aluminium alloys, for vehicle frames, and advanced composites like carbon fibre reinforced plastics for key components such as the roof, firewall, battery tray, and covers are key to accomplishing our objective of reducing weight.
Incorporating state-of-the-art materials into electric vehicles calls for cutting-edge joining methods to seamlessly introduce them. This not only allows for the creation of lightweight components with exceptional strength-to-weight ratios, but it is also essential for maximizing performance. Embracing lightweight materials in the automotive field brings a twofold benefit: shedding vehicle weight and improving aerodynamics. As a result, this results in reduced fuel usage, ultimately reducing the release of harmful pollutants and particles.
In this paper, we present study of workshop trials and qualification of advanced joining processes such as laser welding, laser welding, adhesive bonding of polymers
and electric resistance spot welding of the new generation materials. Finally, a discussion is made on the different applications of these joining processes. Hence, the details presented in this study should be useful for automobile companies to align with requirements of new generation joining processes for light weighting.
Keywords: Light Weight EVs, Aluminum Welding, High Strength Steel & Ultra High Strength Steels, Adhesive Bonding, Composites & Fiber Reinforced Plastics.
Introduction
Presently steel is the major material used in automobile followed by plastics. Material-related lightweight construction involves replacing the original material by a different material with better weight-specific properties. Lightweight construction can be achieved with many different materials, such as high strength steels, aluminium or reinforced polymers. High-performance fibre-reinforced polymers offer the biggest potential for saving weight but are also very expensive, both as a material and in terms of manufacturing. The subject paper covers optimization of weight of the EVs in BIW, Chassis & Trim Parts, using new generation materials and particularly development of suitable new joining processes. In TATA MOTORS particularly for cabin and chassis parts, the existing material joining options were limited to only two conventional processes namely Resistance Spot Welding and GMAW- CO2 welding. The CO2 welding was done using only one grade of welding wire ER70S6. We will discuss the trials and qualification of new joining processes. The new joining processes are compatible with new material grades for light weighting of EVs. The materials include High Strength steels, Aluminium alloys,Titanium alloys and adhesive bonding for polymers
1 Materials
1.1 Steels
Steel amount to 80 % part of the cars. Conventionally used steels are low carbon and low alloy steels. Micro alloyed steels are also widely used. The raw material form is mainly sheets, plates, tubes, channels for cabin and chassis parts. The material can be broadly classified as given below:
2.1.1 CRCA- Cold Rolled Cold Annealed.
The CRCA sheets are used for skin panel cabin and Internal panels. Thickness range of material is ranging from 0.7 mm to 2.4 mm. The conventional grades of Deep Drawn and Extra Deep Drawn are getting replaced by advanced high strength grades such as BH Steel, HSLA steels and IFHS grade steels.
2.1.2 HR- Hot Rolled Plates
Hot Rolled plates are low carbon steel, low alloy steel plates. The thickness range is 2.4 mm thk to 7.0 mm thk. These are used for chassis parts, long runner, load bodies, long channels, Support brackets. The strength is increased by adding alloying elements like Manganese up to 1.8 % and small additions of Niobium, Titanium and Vanadium up to max 0.22 %.
1.1 Aluminium Alloys
Aluminium alloy grades particularly ASTM Series 5XXX and 6XXX are presently used in strategic fields of Defence & Aerospace field due to their advantage of lightweight, excellent formability and good weldability. The non ferrous grade can be used for structural channels like A-Pillar, B-Pillar, Beams and sub frames like engine support members. The weight of aluminium is one third of steel and some of the aluminium alloy grades when suitably heat-treated can provide good strength for structural applications
2.4 Fibre Reinforced Plastics and Composites
The automotive composite materials, fibre-reinforced polymers are among one of the widely preferred alternatives for light weighting of the automobile as they offer enhanced properties such as impact strength, easy moldability, improved aesthetics, and reduced weight as compared to conventional automotive components.
They are largely used for trims in internal and external trims of EVs such as Front and Rear bumper, dashboard, door inner covers, side and upper rails, etc.
Qualification of New Joining Procedures for Metals
3.1 High Strength Steel and Ultra High Strength Steel
High Strength & Ultra High Strength steels are different from the conventional grades as their strength is way higher than the conventional grades. This is achieved either by addition of small amounts of alloying elements. In some grades, the strength is achieved by controlling carbon amount to very low levels i.e. 30 ppm (Interstitial free High Strength Steel). Thermo-mechanically treated fine grain steels such as HSLA 340/ HSLA 420 also display high strength.
Joining of high strength steel calls for separate procedure qualification of welding as they behave differently in welding. The HAZ (Heat Affected Zone) [1]area of weld needs to be controlled for hardness to prevent it from crack. Separate Process Qualification was carried out successfully for Electrical Resistance Spot Welding,
CO2/MAG/ GMAW and Hand Held Laser welding machine.
The implementation of high strength steels resulted up to 20% weight reduction. The hand held laser welding trials were carried out in technology centre of machine dealer. The weld penetration and aesthetic quality was found to be very good.
| Conventional Grades | High Strength Grades | |
| CRCA | D513, EDD 513, BH 180, IF EDD | HSLA 340, HSLA 420, DP 780, RT 580, |
| Thickness range | 0.7 to 2.0 mm thk | 0.7 to 2.0 mm thk |
| Welding Process | MAG/ GMAW | MAG/ GMAW |
| Welding Wire | AWS ER 70S6 | AWS ER 90SD2 |
| wire size | Dia 0.8 mm | Dia 0.8 mm |
| Qualification Tests cleared | Visual, Macro, Hardness and Tensile test | Visual, Macro, Hardness and Tensile test |
3.2 High Strength Steel Plate HSS 800 grade
The HSS 800 grade is having high strength up to 900 N/mm2 which is twice of the low carbon mild steel hence it can replace low carbon steel grades in heavy structural applications like cargo body, frame, etc.
| Conventional Grades | High Strength Grades | |
| CRCA | Fe 410, BSK 34, BSK 38 & BS K 46 | HSS 800 |
| Thickness range | 4 to 8.0 mm thk | 4 to 8.0 mm thk |
| Welding Process | MAG/ GMAW | MAG/ GMAW |
| Welding Wire | AWS ER 70S6 | AWS ER 100 S-G |
| Wire size | Dia 1.2 mm | Dia 1.2 mm |
| Qualification Tests cleared | Visual, Macro & Hardness | Visual, Macro & Hardness |
Although HSS 800 is readily weldable, care needs to be taken for correct grade of wire. Preheating is recommended of minimum 150 °C. Slight edge preparation or 45° bevel is useful for 5 thk and above plates for good weld penetration. [2]
Figure 3 Butt Weld Qualification [5] Trials, Root Run tested for Dye penentrant Test
Figure 4 Part failed in field due to lower strength welding wire used for HSS 800 grade material
3.3 Aluminium
Aluminium material [6] can be very good option for cargo body panels and battery tray. Successful riveting trials were carried out to join Aluminium cover sheet with steel frame. The steel frame supports the structure and aluminium sheet is used for closing of the box assembly, particular side panels where load levels are quite low compared to floor panel of the cargo body.
MIG/GMAW trials carried out on AA 6063 T6 temper grade plates. The alloy strength is almost 80 % of that of low carbon steel and density is only 35 % of low carbon steel makes it a good choice for weight reduction.
| Aluminium Material | Aluminium Material | |
| Grades | AA 6063 T6 | AA 6063 T6 |
| Thickness Range | 3.0 to 8.0 mm thk | 3.0 to 8.0 mm thk |
| Welding Process | GTAW welding | MAG/ GMAW/ Laser welding |
| Welding Wire | AWS ER 4043/ 5356 | AWS ER 4043/ 5356 |
| Wire size | Dia 3.0 mm | Dia 1.2 mm |
| Current & Polarity | AC GTAW | DCEP |
| Qualification Tests cleared | Visual, Macro, Tensile & Bend test | Visual, Macro, Tensile & Bend test |
Figure 5 Cargo load body side panels with steel frame and aluminium plates
Figure 6 Mechanical fastening Rivetting used for joining aluminium closing plates with steel frame.
Figure 7 Aluminium material GTAW and GMAW welding coupons
GMAW & GTAW procedures were successfully carried out. For GMAW special pulse MIG welding [3] machines are required. Whereas for GTAW specialized AC TIG welding machines are welding.
Aluminium material having dense oxide layer on surface That is why it is crucial you to use alternating current (AC) polarity when TIG welding aluminium. This polarity provides a cleaning action that helps remove and break up the oxide layer on aluminium, creating a nice flowing weld puddle.
Figure 8 Special GTAW and GMAW welding machines
Challenge faced in Aluminium welding: –
Suitable welding machines are not available presently in our company. Trials were taken at technology centre workshop of machine manufacturers. The machines are costly. Further good quality welding consumable required for shielding gas
Argon and welding wire . LPG gas burner or similar heating set up is required as preheating is must for aluminium welding. Welder skill requirements are high hence training is very essential
Figure 9 Porosity open to surface in welding joint of Aluminium
Figure 10 Internal Porosity open in welding joint of Aluminium
Hand Held Laser Welding
Hand held laser welding is relatively new however it is fast getting popular. The major advantage is aesthetics, ease of welding and required less welder skill. Welding filler wire not required up to thk 1.6 mm.
Figure 11 hand held laser welding without and with filler wire [7] fillet weld joint
The welding machine cost is high. Presently TATA MOTORS does not have the machine however we have proposed to buy one for our proto type weld shop.
Figure 12 Hand held laser welding power souce, 10 times costly than conventional processes
The same machine can be used to weld different materials i.e. steels, stainless steels, aluminium.
Figure 13 Suitability for various material types
The welding speeds are high 3 to 4 times than conventional welding, The welding penetration is deep and produces uniform weld quality. This will be very useful method for EVs considering that it can be used with wide variety of materials. Also the radiation level is low.
Figure 14 Very good, Uniform welding penentration
3.6 Resistance Spot Welding of HSS Steels
The carbon content of High Strength Steel grades is ensured low so that Weldability is not affected. [8]
| Conventional Grades | High Strength Grades | |
| CRCA | D513, EDD 513, BH 180, IF EDD | HSLA 340, HSLA 420, DP 780, RT 580, |
| Thickness range | 0.7 to 2.0 mm thk | 0.7 to 2.0 mm thk |
| Welding Process | Resistance Spot Welding | Resistance Spot Welding |
| Qualification tests | Visual, Macro, Hardness, Peel test and Shear Tensile test | Visual, Macro, Hardness, Peel test and Shear Tensile test |
Minor changes were required in welding current and welding time for welding of HSS steels. Peel test and Shear tensile test results were found to be satisfactory. [9] [3]
Figure 15 Peel test and Shear tensile Test [4] for Spot weld test coupons
Figure 16 Macro test @ 10 X of Electric Resitance Spot Weld joint
4 Qualification/ Development of New Joining Procedures for Plastics, Metals and dissimilar substrate
TEROSON PU 6700 Polyurethane adhesive: –
It is 2-component adhesive which cures at room temperature. For accelerated curing it is recommended to increase the temperature up to 60 to 70 °C. It can be painted with standard commercial car repair refinishing paints. It is found useful for EVs which require joining of different materials such as Steel panel/ Aluminium panel with Plastics and Composites.
| Product Name | Teroson PU 6700 |
| Type of substrate | Metal and Plastic |
| Thickness | 1.0 mm thk D513 and 1.5 mm thk PU Plastic |
| Type of adhesive | Two component polyurethane |
| Mix Ration | by volume 1 : 1 |
| Curing time at 23 °C | approx. 6 hr(s). |
| Shore A hardness | approx. 90 |
| Shear strength | After 2 d (23 °C, 50 % rh) approx. 13 MPa |
Figure 17 Shear tensile test of Adhesive Bonded Plastic strips, Plastic to Metal strips
The shear strength [10] is found comparable with Resistance Spot Welding. Therefore, it can be used for structural applications, particularly the cabin body. Also it seals the joinery hence water entry and corrosion can be prevented for metal panel joineries. It also provides leak proofing of cargo bodies to prevent rain water seepage into cargo body which may make the goods wet and ruin the same.
Application Areas:
It can be used in the automotive industry and in motor vehicle body repair shops mainly for bonding plastic parts as well as metal parts. The parts to be bonded must be free from oil, grease, moisture, dirt and release agents. Metals may either be primed or they should be suitable grinded.
LOCTITE UK 2073/2173 Polyurethane Adhesive: –
It is a solvent free, elastic, 2-component adhesive/sealant based on Polyurethane which is designed for fast cohesion build-up and long open time. It has got excellent adhesion properties to various substrates like different kinds of plastics and specifically low surface energy types such as PP Talc, PP LGF, PC/ABS, PC/PET, etc.
Trials conducted on Plastic and Plastic
| Product Name | Loctite UK 2073/ 2173 |
| Type of substrate | Plastic with Plastic |
| Thickness | 1.5 mm thk PU Plastic and 1.5 mm thk PU Plastic |
| Type of adhesive | Two component polyurethane |
| Mix Ration | by volume 2 : 1 |
| Curing time at 23 °C | approx. 4 hr(s). |
| Shore A hardness | approx. 60 |
| Layer thickness | 1.5 mm |
| Bonding area | 25 mm X 12.5 mm |
| Shear strength | After 7 d (23 °C, 50 % rh) approx. 3 to 4 MPa |
Application Areas:
It can be used for elastic bonding of automotive plastic parts like spoilers, tailgates, roof system, bumpers, luggage racks, dashboards, trim parts and EV battery tray housings.
5 Conclusion
Earlier our steel welding procedures were limited to steels of YS up to 460 N/mm2 . Now we have upgraded procedures for steels up to YS 800 N/mm2 in two types of welding processes namely ‘Electrical Resistance Spot Welding’ and ‘GAW-CO2 Welding’. The use of High Strength, Ultra High Strength Steels with reduced thickness can reduce the weight by 20% compared to conventional steels.
Now we have developed two new procedures for Aluminium AA 6063 T6 grade material in two types of welding processes GTAW & GMAW. AA 6063 T6 Aluminium grade is having almost 80% of the strength of low carbon steel and 35 % lighter compared to carbon steel.[11]
We have developed hand held laser GMAW welding process for steel as well as aluminium grade. For thickness up to 1.6 mm, welding wire consumable is not required. Without filler wire welding can reduce the weight by approx. 4 to 6 % of the assembly.
High-performance fibre-reinforced polymers offer the biggest potential for saving weight up to approx. 50 % when compared with steels. With the adhesive bonding qualification, we can join structural welding between body panels and join metal with plastics/ composites and reduce the weight.
6 References
- EN ISO 5817: – Acceptance norms for visual welding quality
- AWS handbook Volume 1: -Welding technology.
- EN ISO 3834-2: – Comprehensive welding quality requirements
- Shibata and Sakamoto: ‘New joining methods required for aluminium car body’. Conf proc ‘Present state and future of aluminium alloy joining techniques for automobiles’, Light Metal Welding and Construction Association 2001 97–106.
- Larsson: ‘Advanced welding methods for future car body structures’. IIW Doc. XII-1677-01, 151–170.
- ASME Sec IX: – Qualification of welding; Boiler & Pressure Vessel Code
- ASME Sec II C: – Welding consumables
- ASM handbook Volume 6: -Welding & Brazing
- Henkel adhesive product data sheet
- ASME Sec IX: – Welding Qualifications
- AWS handbook Volume IV: – Materials &
Applications-Part 1 - Nava, M. The Road Ahead for Electric Vehicles 2017.Availableonline:https://www.bbvaresearch.com/wpcontent/uploads/2017/02/170213_US_ElectricVehicles.pd
- Matsumoto and Sasabe: ‘Study of friction stir welding for lap joints’. Conf proc. ‘3rd International Friction Stir Welding Symposium’, Kobe, Japan 2001.
- Graf and Staufer: ‘Laser Hybrid process at Volkswagen’. IIW Doc. XII-1730-02, 249–257.
- Harris: ‘Arc welding automotive aluminium’. Advanced Materials and Processes 2001 159 (9) 52–54.
- Editorial Japan Aluminium Association, Standardisation Committee: ‘Aluminium Handbook’ (6th Edition). Japan Aluminium Association, 2001 9–10.
