Technical Articles

Laser Welding of Dissimilar High Carbon Materials for the Automotive Applications

By Dr. Mohammed Naeem

The industrial interest for welding dissimilar metals has been increasing because the materials chosen for the component’s application tend to be the most efficient material to meet the performance/cost parameters desired for the component, regardless of the corresponding joinability problems. One such example is laser welding of powertrain components used in the automotive industry.

For example, the differential gear is a functional component of the automobile powertrain and is composed of the differential case, drive gear, shafts, and ring gear. Materials used for this application should have good torsional stiffness, wear resistance and fatigue strength. Since grey cast iron can satisfy these requirements and has low material cost as well as excellent castability, it is widely used for the differential case of a powertrain. High carbon steel is also used for the ring gear of a powertrain. However, since these two base materials contain high carbon content, the weldability is not very good because both of these materials have high equivalent carbon content (CE). The CE can be calculated by the following equation:

CE = C + Mn/6 + (Cr + Mo + V)/5 + (Ni + Cu)/15

For this equation the weldability based on a range of CE values can be defined as follows.

Table 1: Carbon equivalent values to weldability table

Carbon equivalent (CE) Weldability
Up to 0.35 Excellent
0.36–0.40 Very good
0.41–0.45 Good
0.46–0.50 Fair
Over 0.50 Poor

Most of the material combinations used for powertrain applications, i.e. grey cast iron to high carbon steels are considered unweldable, using conventional laser parameters. Currently laser welding of these materials is carried out either by:

  1. Inductive pre-heating, laser welding and followed by inductive annealing (similar joint between high carbon steels). This method produces crack free welds with reduced hardness and no distortion.
  2. Welding of grey cast iron to high carbon steels with addition of high nickel based wire

Laser welding of dissimilar high carbon steels and grey cast iron to high carbon steels was performed in this study without pre-heating and without the addition of the filler material. The material combinations were hardened SAE 4130 to SAE 8620 and hardened SAE 4130 & SAE 8620 steels to grey cast iron. Welding tests were carried out to developed laser parameters which produce defect-free (no cracking or porosity) welds and some of the results from the finding are highlighted in the below sections.

Results:

Figure 1 shows dissimilar material joint between harden SAE 4130 and SAE 8620 produced with continuous wave output. The weld metal (fusion zone) exhibits both cracking and porosity. Temporal pulse shape was developed to produce defect free welds (see Figure 2).

thick butt joint between harden SAE 8620 and SAE 4130

Figure 1: 4.5 mm thick butt joint between hardened SAE 8620 (CE 0.60) and SAE 4130 (CE 0.64) steel; with continuous wave output; no shield gas.

Thick butt joint between harden SAE 8620 and SAE 4130

Figure 2: 4.5 mm thick butt joint between hardened SAE 8620 (CE 0.60) and SAE 4130 (CE 0.64); with temporal pulse shape; no shield gas no cracks or porosity.

Cast iron typically has a carbon content of 2-4%, roughly 8-10 times as much as most carbon steels. The high carbon content causes the carbon to form flakes of graphite. This graphite gives gray cast iron its characteristic appearance when fractured. Because grey cast iron contains flakes of graphite (Figure 3), carbon can readily be introduced into the weld pool. This causes weld embrittlement. Grey cast iron welds are subject to the formation of porosity and the cold cracking susceptibility of the weld joints. During the laser welding of grey cast iron, the rapid cooling partly or completely suppresses graphitization, leads to the formation of cementite in the fusion zone making the weld joint very hard and prone to cracking.

Work has been carried out to laser weld grey cast iron to high carbon steel. Since the weld cracking and porosity are the major concerns with these material combinations, the focus of this work was to develop welding parameters which allow some control over the composition of the resulting weld. Figures 4 and 5 show the results from controlling the mixing of the molten materials which produced defect free welds. The fusion zone mainly consisted of martensitic (needles) structure (Figure 6) for both material combinations, i.e. grey cast iron to SAE 8620 steel and grey cast iron to SAE 4130 steel, respectively.

Flake graphite in as-cast gray iron

Figure 3: Flake graphite in as-cast gray iron (3.5%C; 2.45%Si; 0.40%Mn; 0.08%P; 0.13%Ni; 0.15%Cu; Balance Fe); unetched.

butt joint between SAE 8620 harden steel and grey cast iron

Figure 4: 4.5 mm thick butt joint between SAE 8620 hardened steel and grey cast iron; no shield gas.

butt joint between SAE 4130 harden steel and grey cast iron

Figure 5: 4.5 mm thick butt joint between SAE 4130 hardened steel and grey cast iron; no shield gas.

Fusion zone, martensitic (needles) structure

Figure 6: Fusion zone, martensitic (needles) structure; etched in picric acid.

Summary

Prima Power Laserdyne has developed a range of laser parameters which can significantly change the manufacturing methodologies and processes for the welding of high carbon steels and cast iron which are prone to cracking and porosity during welding.

Fiber Laser and processing parameters have been developed to produce crack and porosity free welds for materials used in powertrain components.

Dissimilar material joints between hardened high carbon steels as well as hardened steels and cast irons were produced without any pre heating or addition of filler material.

The new fiber weld parameters enables the Engineers to further optimize the component materials for performance and cost, while supporting the weld assembly processes.

If you are interested in the learning more about using a fiber laser for cast iron or dissimilar materials for your welding application, please contact lds.sales@primapower.com.

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