Process forces during remote laser beam welding and resistance spot welding – a comparative study

Process forces during remote laser beam welding and resistance spot welding – a comparative study

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Procedia CIRP 00 (2017) 000–000 Procedia CIRP 74 (2018) 669–673 www.elsevier.com/locate/procedia th 10 CIRP 10th CIRPConference Conferenceon onPhotonic PhotonicTechnologies Technologies [LANE [LANE 2018] 2018]

Process forces28th during remote laser beam welding resistance spot CIRP Design Conference, May 2018, Nantes,and France welding – a comparative study A new methodology to analyze the functional and physical architecture of a,b, a,b Schlather Felix Theurer , Florianproduct Oefeleb, Michael Zaeha existing Florian products for an*,assembly oriented familyF.identification a

Technical University of Munich, Institute for Machine Tools and Industrial Management, Boltzmannstrasse 15, 85748 Garching, Germany b BMW AG, Knorrstrasse 147, 80788 Munich, Germany

Paul Stief *, Jean-Yves Dantan, Alain Etienne, Ali Siadat

* Corresponding author. Tel.: +49-151-601-33811. E-mail address: [email protected] École Nationale Supérieure d’Arts et Métiers, Arts et Métiers ParisTech, LCFC EA 4495, 4 Rue Augustin Fresnel, Metz 57078, France

* Corresponding author. Tel.: +33 3 87 37 54 30; E-mail address: [email protected]

Abstract

Welding of sheet metal in the automotive industry involves inflexible and expensive joining fixtures to properly position and fasten the parts Abstract that are to be joined. Feature-based fixturing is an approach to reduce fixtures in the assembly process. The approach relies on part-inherent fastening features that realize the functions of positioning and fastening. The proper design of these fastening features requires knowledge of Inforces today’s environment, theprocess trend towards variety and customization is unbroken. Due to this development, the need of thatbusiness result from the joining and thusmore needproduct to be compensated. This paper describes the investigation of process forces during agile andlaser reconfigurable production systems emerged spot to cope with various To design and optimize production remote beam welding and during resistance welding. This isproducts achievedand byproduct using afamilies. thermo-mechanical simulation model and systems as wellstudies. as to choose the optimal product matches, product analysis methods Indeed, mostdifferent of the known methods aimthe to experimental The comparison shows significant differences in process forces are thatneeded. result from the two processes. Finally, analyze a product or one product family on the physical level. Different product however, may differ largely in terms of the number and results allow for the design of fastening to reduce joining fixtures in thefamilies, automotive body shop. features nature ofThe components. This fact by impedes anLtd. efficient and choice of appropriate product family combinations for the production © 2018 Authors. Published Elsevier This iscomparison an open access article under the CC BY-NC-ND license © 2018 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license system. A new methodology is proposed to analyze existing products in view of their functional and physical architecture. The aim is to cluster (http://creativecommons.org/licenses/by-nc-nd/3.0/) (https://creativecommons.org/licenses/by-nc-nd/4.0/) these productsunder in new assembly oriented product families for the optimization Peer-review under responsibility of the the Bayerisches Bayerisches Laserzentrum GmbH. of existing assembly lines and the creation of future reconfigurable Peer-review responsibility of Laserzentrum GmbH. assembly systems. Based on Datum Flow Chain, the physical structure of the products is analyzed. Functional subassemblies are identified, and a Keywords: functionallaser analysis is performed. Moreover, a hybrid functional and fastening physicalfeature architecture graph (HyFPAG) is the output which depicts the beam welding; resistance spot welding; process force; fixture, similarity between product families by providing design support to both, production system planners and product designers. An illustrative example of a nail-clipper is used to explain the proposed methodology. An industrial case study on two product families of steering columns of thyssenkrupp Presta France is then carried out to give a first industrial evaluation of the proposed approach. Introduction ©1.2017 The Authors. Published by Elsevier B.V. Peer-review under responsibility of the scientific committee of the 28th CIRP Design Conference 2018. 2. Forces during welding

Positioning and fastening of parts is necessary during process and Welding of metals leads to thermodynamic, mechanic and a product that meets the requirements with regard to joint metallurgic interactions in the workpiece [4,5]. These strength and dimensional accuracy. This process involves interactions, in turn, induce forces to the parts. In this paper, joining fixtures to properly position and fasten the parts. As these arerange distinguished in direct and indirect impact and 1. Introduction of the forces product and characteristics manufactured and/or these fixtures are expensive and inflexible [1], various assembled in part-related and fixture-related forces. Non-tactile, thermal in this system. In this context, the main challenge in approaches optimization orinreduction of fixtures joining technologies such remote lasertobeam only Due to deal the with fastthedevelopment the domain of modelling and analysis is as now not only copewelding with single in the sheet metal joining process, especially in the automotive have an indirect, thermal impact on the workpieces (see Fig. communication and an ongoing trend of digitization and products, a limited product range or existing product families, of the laser beam is partially right). The energy P body shop [1,2]. A proper design of fixture systems with 1, T,L digitalization, manufacturing enterprises are facing important but also to be able to analyze and to compare products to define absorbed and leads It to can temperature gradients ΔT(t)existing in the regard to the essential for these approaches. This new challenges in demands today’s is market environments: a continuing product families. be observed that classical material. Thus, a transient temperature field propagates requires, among other aspects, knowledge of forces that result tendency towards reduction of product development times and product families are regrouped in function of clients or features. through the workpieces andproduct the temperature gradients lead to from the joining andInthus need to be compensated. In However, shortened product process lifecycles. addition, there is an increasing assembly oriented families are hardly to find. �⃗ (t). These heatand locally different thermal expansions 𝜺𝜺𝜺𝜺 this paper, process forces are determined that emerge during demand of customization, being at the same time in a global On the product family level, products differ mainly in two transformation-induced, time- of and position-dependent remote laser weldingalland resistance spot main competition withbeam competitors overduring the world. This trend, characteristics: (i) the number components and (ii) the �⃗ (t) and reaction forces expansions finally lead to stress 𝝈𝝈𝝈𝝈 welding. These joining technologies are among the main used which is inducing the development from macro to micro type of components (e.g. mechanical, electrical, electronical). �𝑭𝑭𝑭𝑭⃗R,CL(t) at the clamping elements. Stress and reaction forces in the automotive shop [3].lot sizes due to augmenting markets, results inbody diminished Classical methodologies considering mainly single products disappear at ambient temperature if the expansions are in the product varieties (high-volume to low-volume production) [1]. or solitary, already existing product families analyze the elastic range. To cope with this augmenting variety as well as to be able to product structure on a physical level (components level) which identify possible optimization potentials in the existing causes difficulties regarding an efficient definition and 2212-8271 ©system, 2018 Theit Authors. Published by Elsevier Ltd. This is an opencomparison access article of under the CC BY-NC-ND license Addressing this production is important to have a precise knowledge different product families. Keywords: Design method; Family identification joining ofAssembly; sheet metal structures to ensure a stable

(http://creativecommons.org/licenses/by-nc-nd/3.0/)

Peer-review of Published the Bayerisches Laserzentrum 2212-8271 ©under 2018responsibility The Authors. by Elsevier Ltd. ThisGmbH. is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0/) 2212-8271 © 2017 The Authors. Published by Elsevier B.V. Peer-review under responsibility of scientific the Bayerisches Laserzentrum GmbH. Peer-review under responsibility of the committee of the 28th CIRP Design Conference 2018. 10.1016/j.procir.2018.08.047

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Florian Schlather et al. / Procedia CIRP 74 (2018) 669–673 Author name / Procedia CIRP 00 (2018) 000–000

Fig. 1. Welding tool impact and reaction forces at clamping elements during resistance spot welding (left) and laser beam welding (right) [6]

If the yield strength is exceeded, residual stress and distortion remain in the parts. In contrast to remote laser beam welding, a weld gun directly affects the parts in the weld zone by its mechanical �⃗M,WG during resistance spot impact of the welding electrodes 𝑭𝑭𝑭𝑭 welding (see Fig. 1, left). Depending on the parts and the �⃗ R,CL evolve at the clamping conditions, reaction forces 𝑭𝑭𝑭𝑭 clamping elements. Additionally, the welding current generates thermal stress and reaction forces as described for laser beam welding. In the literature, there are several examples of evaluating forces during welding. In the case of spot welding, however, they focus on the electrode force and its influence on the joint strength [7 – 9] or the contact resistance [10]. Research on process forces during laser beam welding mainly focuses on the compensation of residual stress and distortion in dependency of the clamping condition. These investigations are based on simulation models with different modeling approaches such as the ‘inherent strain approach’ [11,12], ‘thermal shrinkage models’ [13], the ‘local-globalapproach’ [14] or thermo-mechanical structure simulations [15 – 17]. However, they do not explicitly consider required clamping forces to properly design joining fixtures. 2.1. Determination of process forces during remote laser beam welding An approach to calculate required clamping forces during remote laser beam welding was developed and validated in[6].

The model was designed for a joining process with parameters listed in Tab. 1 and is based on a thermo-mechanical finite element simulation. The material used was a mild steel CR240LA-GI50/50-U with 1 mm sheet metal thickness. 2.2. Determination of process forces during remote laser beam welding An approach to calculate required clamping forces during remote laser beam welding was developed and validated in [6]. The model was designed for a joining process with parameters listed in Tab. 1 and is based on a thermomechanical finite element simulation. The material used was a mild steel CR240LA-GI50/50-U with 1 mm sheet metal thickness. Table 1. Weld parameters for the laser beam welding experiments Parameter name Wave length λL

Value

Unit

1070

nm

Welding speed vW

10

mm/s

Laser output power PL

900

W

Spot radius rS

0.2

mm

Attack angle αA

36

°

The software JMatPro v.10 was used to calculate the temperature-dependent material data, including heat of fusion.

Fig. 2: Simulation model (left), experimental validation (middle) and exemplary force at clamping point mp 2 (bottom) [6]



Florian Schlather et al. / Procedia CIRP 74 (2018) 669–673 Author name / Procedia CIRP 00 (2018) 000–000

The energy input through the laser radiation was represented by means of a Gaussian distributed surface heat source. This allows to model a heat conduction mode welding, which is present at these welding parameters (see Tab. 1). The simulation model considers heat transfer through convection, conduction and radiation. The model consists of areas with different meshing (see Fig. 2, left). The areas around the keyhole, at the heat affected zone and at the clamping elements have a fine mesh with an element size of 0.2 – 0.7 mm. Other areas have a coarse mesh (1.7 – 8.7 mm) to fulfil the requirements of the heat transfer and structural mechanics calculation. Unstructured tetrahedron elements were used for meshing. To calibrate the thermal simulation model, the crosssection polish and the global temperature field during welding of five samples were used. In a subsequent thermo-mechanical simulation, expansions and reaction forces that result from the temperature differences were calculated. The forces at the clamping points were measured with an experimental setup at various points and in different spatial directions (i.e. along the x, y and z-axis). The measured forces and the corresponding results from the calculations were used for the validation of the simulation model. The simulation model, the setup for the experimental validation and representative forces at the clamping points are displayed in Fig.2. This model was used in the following (see section 3) to determine process forces at a representative sheet metal structure for the comparison with forces during resistance spot welding. 2.3. Determination of process forces during resistance spot welding

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Fig. 3. Test bench design to measure process forces during spot welding; Left: Force-torque sensor with specimen adapter; Right: Sensor and sheet metal specimens attached to the test bench during the weld gun impact

the same as in the laser beam welding experiments in section 2.1. Table 2. Weld parameters for the resistance spot welding experiments; Two different studies (V1 and V2) were performed Parameter name

V1

Numbers of experiments

19 2.1 kN

Weld time 1

30 ms

Weld current 1

0 kA

13 kA 350 ms

Weld current 2

0 kA

Holding time

9.4 kA 200 ms

Electrode tip diamter

6 mm F

200

M,WG,x

FM,WG,x

F F force

100

N 0

-100 -200 0

20

40

t F 100

M,WG,y

60 s

80

0

10

20

i /

FM,WG,y

F F force

50

N 0

-50 -100 0

20

40

F 200

M,WG,z

60

s

80

0

80

0

10

20

10

20

FM,WG,z

100

N

F F force

Design of experiments Two different experiments (V1 and V2) were performed. They are summarized in Tab. 2. In V1, the welding process was performed at a total of 19 samples with no welding current applied. The same pair of sheet metals was used during the repetitions. In V2, the welding process was performed at 6 samples with a welding current as in Tab. 2. The pair of sheet metals was replaced by new ones after each sample. The gun compensation mode (for force balancing during gun impact) was active for both V1 and V2, as it is common in industrial applications. During the experiments, the forces were measured by the sensor with a nominal sampling rate of 100 Hz. The sheet metal material used was

6

Weld force

Weld time 2

Test bench design The force impact during resistance spot welding was measured by means of a ROBOTIQ FT-150 force-torque sensor with an attached clamping mechanism that holds a pair of sheet metal samples (see Fig. 3). A servo pneumatic, Xtype weld gun was attached to a KUKA KR 240 R2500 prime industrial robot for the welding operation. The sheet metal plate was in a vertical orientation and worn electrodes were �⃗M,S was used which is a common case in industry. The force 𝑭𝑭𝑭𝑭 measured in all Cartesian directions 𝐹𝐹𝐹𝐹𝑀𝑀𝑀𝑀,𝑆𝑆𝑆𝑆,𝑥𝑥𝑥𝑥 , 𝐹𝐹𝐹𝐹𝑀𝑀𝑀𝑀,𝑆𝑆𝑆𝑆,𝑦𝑦𝑦𝑦 , and 𝐹𝐹𝐹𝐹𝑀𝑀𝑀𝑀,𝑆𝑆𝑆𝑆,𝑧𝑧𝑧𝑧 �⃗𝑴𝑴𝑴𝑴,𝑾𝑾𝑾𝑾𝑾𝑾𝑾𝑾 at the welding position at the sensor whereof the force 𝑭𝑭𝑭𝑭 could be deduced. The forces that result from the thermal impact during resistance spot welding are not considered in this case.

V2

0

-100 -200 0

20

40

time t

60

s

frequency

Fig. 4. Forces from experiment V1 plotted over time; The histograms show the value of the maximum positive and negative forces measured during the sequence of 19 weld spots

Florian Schlather et al. / Procedia CIRP 74 (2018) 669–673 Author name / Procedia CIRP 00 (2018) 000–000

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Results The recorded values of force �𝑭𝑭𝑭𝑭⃗M,WG are plotted over time in Fig. 4. In the histograms, the amount of the maximum positive and negative forces is displayed, which was measured during each repetition. For the dimensioning of fixture systems, the absolute force values along the x-, y- and z-axis are relevant [18]. Therefore, the direction (positive or negative) of the forces in the histograms was neglected and summarized in boxplots as in Fig. 5. The average forces with weld current applied (V2) are between 1.4% (FM,WG,x) and 49.5% (FM,WG,z) higher than without welding (V1). To design fixture systems with regard to the required clamping force, the maximum values were considered. They are 139.4 N (FM,WG,x), 101.5 N (FM,WG,y) and 182.3 N (FM,WG,z).

as in Tab. 1. The results are presented and discussed in the following section. 3.2. Results and discussion The results of the comparative study are displayed in Tab. 3. The maximum force of 108.82 N during spot welding occurs at clamping element c4 in the x-direction. For remote laser beam welding, the maximum force of 49.61 N emerges at clamping element c4 in the y-direction. The maximum forces at the clamping elements are higher for resistance spot welding than for remote laser beam welding. The only exception is clamping element c3.

182.3 139.4

F force F

N

129.3

119.6 101.5 57.6

V1

V2

FM,WG,x

V1

V2

FM,WG,y

V1

V2

FM,WG,z

Fig. 5. Boxplots with min, max, mean and standard deviation of the absolute maximum forces of V1 (no weld current) compared to V2 (with weld current applied)

3. Comparative study 3.1. Study design A simple sheet metal structure was used as a reference to compare required clamping forces during remote laser beam welding and during resistance spot welding (see Fig. 6). Two parts of mild steel CR240LA GI50/50 U with 1 mm sheet metal thickness need to be joined. Four clamping locations ci were defined to fasten the parts for this operation. These clamping locations would require proper dimensioning of the clamping force in the fixture design process [18]. Two different joining studies were performed at this structure. In study no. 1, the two parts were joined with two resistance weld spots. The force profile displayed in Fig. 6 was applied to the weld spots in a mechanical finite element simulation model in Comsol Multiphysics v.4.4 to calculate the forces that result from this joining operation. This profile is derived from the experimental studies in section 2.2. It represents a typical force impact during resistance spot welding (see Fig. 5). In study no. 2, the two parts were joined at two seams with remote laser beam welding (see Fig. 6). The force that results at the clamping elements was calculated using the model introduced in section 2.1. The weld parameters were the same

Fig. 6. Study design (top) and load profile applied (bottom); Two sheet metal plates are to be joined using (a) resistance spot welding (at the weld spot location) and by (b) remote laser beam welding (at the weld seam location)

Table 3. Resulting forces at clamping elements 1 – 4 during remote laser beam welding (LBW) and resistance spot welding (RSW) calculated in the simulations; Force values in N, maximum forces in bold letters Clamping point number

Max. force x

Max. force y

Max. force z

LBW

RSW

LBW

RSW

LBW

RSW

c1

6.78

8.48

4.54

9.85

8.12

11.7

c2

1.88

108.35

48.02

65.42

5.54

59.42

c3

8.56

8.38

6.37

9.67

5.07

8.37

c4

1.49

108.82

49.61

61.43

7.92

56.08

The differences in required clamping forces are ascribed, among others, to three reasons: • Worn electrodes were used in the experiments. They can lead to asymmetric electrode impact on the sheet metal. • In contrast to LBW, there is always an initial hit on the sheet metal through the electrode impact when the weld gun closes (even if the gun compensation mode is active). • The weld parameters applied for LBW (see Tab. 1) induce comparatively little energy to the weld zone.



Florian Schlather et al. / Procedia CIRP 74 (2018) 669–673 Author name / Procedia CIRP 00 (2018) 000–000

Finally, the comparative study indicates loads that can be expected during LBW and RSW. These loads and the approaches for their calculation can be used to properly design joining fixtures as used in the automotive body shop. 4. Conclusion and Outlook This paper presents an approach to determine process forces during remote laser beam welding and resistance spot welding. This allows for the design of fastening features and conventional joining fixtures in the automotive sheet metal assembly process. The determination of process forces is achieved by means of a thermo-mechanical simulation model and experimental studies. The forces that result from the two different joining processes are compared using a reference sheet metal structure. It could be shown that required clamping forces strongly depend on the used joining technology. As a result of an exemplary study, resistance spot welding requires stronger clamping than laser beam welding. However, only short laser weld seams with little energy were applied. This finding can be used to design fixture systems with regard to the requirements and thus to reduce cost and increase flexibility of production systems. The developed models can now be applied to perform parameter studies to investigate the influence of different clamping conditions and welding strategies on required clamping forces. This is subject to further work of the authors. References [1] Bi Z.M, Zhang W.J. Flexible fixture design and automation: review, issues and future directions. International Journal of Production Research 2001;39(13):2867–94. [2] Schlather F, Oefele F, Zaeh M.F. Toward a feature-based approach for fixtureless build-up of sheet metal structures. International Journal of Engineering and Technical Research 2016;5(4). [3] Tikhomirov D, Rietman B, Kose K, Makkink M. Computing welding distortion: comparison of different industrially applicable methods. Advanced Materials Research 2005;6-8:195–202.

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