Preparation for welding of magnesium alloys

Preparation for welding of magnesium alloys

3 Preparation for welding of magnesium alloys L. LIU, Dalian University of Technology, China Abstract: In this chapter, the preparation for Mg welding...

481KB Sizes 0 Downloads 20 Views

3 Preparation for welding of magnesium alloys L. LIU, Dalian University of Technology, China Abstract: In this chapter, the preparation for Mg welding is introduced. Surface treatment is important for Mg welding, and lack of treatments may lead to defects. Typical processes of surface treatment are introduced. In welding with filler wires, the welding groove types are listed in the chapter. As the special characters of Mg, preheating and postweld treatments can influence the weld strength, the common methods are introduced. Key words: magnesium alloy, surface treatment, groove, preheating, postweld treatments.

3.1

Introduction

Mg alloys are usually protected by oil coating, acid-pickled surface, or chromate conversion coating, but surface oxide films or contaminants will cause weld defects. Thus before welding, surfaces of Mg structures and welding wires should be cleaned to remove oxide film and hydride layers, grease/releasing agents and surface coatings. The groove types, preheating and postweld treatment methods are also important to Mg welding processes due to their specific characteristics.

3.2

Surface treatment of magnesium alloys

For corrosion protection, Mg and Mg alloys usually need oxidation treatment so that the surface is coated by chromate film. The film is a major obstacle in welding processes and it can result in some welding effects including cracking (nonfusion), porosity and slag, as shown in Fig. 3.1. The films on sheet surfaces and wires easily absorb moisture, which results in pores in the fusion zone. In filler wire welding, the film on the wires can lead to the formation of hydrogen in the droplets, resulting in an increased hydrogen content in the molten pool. In addition, the film may remain at the weld root and can also lead to pores when the heat input is low. Therefore, the film and oil on all surface and edges of the structures and wires must be completely removed prior to welding. Different methods can be used to clean the surface according to the conditions of amount of oil and film on the structures to be welded. If the amount of oil is small, mechanical methods can be used with the help of stainless steel brushes. When the amount of oil is large, a combination of degreasing and abrasion methods can be used, which, in addition, result in good performance in removing oil and chromate film. Table 3.1 shows the chemical treatment methods before welding Mg alloys.1 16 © Woodhead Publishing Limited, 2010



Preparation for welding of magnesium alloys

17

3.1  Defects of Mg alloys welding. (a) Cracks and pores in fracture, and (b) slag.

Table 3.1  Chemical pretreatment methods Type

Component

Process

Use

Alkaline cleaning solution

Sodium carbonate, 84.9 g Caustic soda, 56.6 g Water, 3.8 dm3 Temperature, 361–73K pH value > – 11

Immerse for 3–10 minutes, rinse with cold water and dry

For degreasing or removing chromate film

Bright cleaning solution

Chromic acid, 0.675 kg Ferric nitrate, 150 g Potassium fluoride, 14.2 g Water, 3.8 dm3 Temperature, 289–311K

Immerse for 0.25–3 minutes, rinse with cold and warm water, and dry

For bright surface by removing chromate film after degreasing

Cleaning solution for spot welding

1. Concentrated sulfuric acid, 36.8 g   Water, 3.8 dm3  Temperature, 294–305K

Immerse for 0.25–1 minutes in (1), and immerse in (2) or (3) after rinsing with cold water. If in (2), immerse for 3 minutes, rinse with cold water and dry.

For removing chromate film after degreasing

2. Chromic acid, 0.675 kg  Concentrated sulfuric acid, 2.0 g   Water, 3.8 dm3  Temperature, 294–305K 3. Chromic acid, 9.3 g   Water, 3.8 dm3   Temperature, 294–305K Chromic acid cleaning solution

Chromic acid, 0.675 kg Concentrated nitric acid, 75 g Water, 3.8 dm3 Temperature, 294–305K

If in (3), immerse for 0.5 minutes, rinse with cold water and dry. Immerse for 0.5–2 minutes in solution, keep for 5 seconds in air, rinse with cold or warm water and dry

© Woodhead Publishing Limited, 2010

For priming or surface prevention

18

Welding and joining of magnesium alloys

Table 3.2  Pretreatment methods for wire No. Process

Component (g L–1)

Temperature (ºC)

  1 Degreasing NaOH (10–25) 60–90 Na3PO 4 (40–60) Na2PO 3 (20–30)   2 Flush with flowing warm water 50–90   3 Flush with flowing cold water Room   4 Alkali treatment NaOH (350–450) MB8 70–80 MB3 60–65   5 Flush with flowing warm water 50–90   6 Flush with flowing cold water Room   7 Chromic acid treatment CrO3 (150–250) Room SO 4 (<0.4)   8 Flush with flowing cold water   9 Flush with flowing warm water 50–90 10 Dry 50–70

Time (minutes) 5–15

4–5 2–3 2–3 5–6 2–3 2–3 5–10 2–3 1–3

3.2  (a) Wires and (b) weld of Mg alloys. (Chen 2004)

The welding wire surface also needs chemical or mechanical cleaning before being used. In the mechanical method, the oxidation film can be removed by bit tools or brushes. In a chemical method, weld wire is usually immersed in the 20–25% nitric acid solutions for two minutes, then flushed with 50–90 °C water and finally dried. Some other methods are shown in Table 3.2. The cleaned wire should be used on the same day (within 10 days in dry weather). These pretreatment methods can be used to make a satisfactory weld, and Fig. 3.2 shows the wires and weld pretreated by the processes above.

3.3

Welding groove for magnesium alloys

Groove type is very important both in welding and in repairing welding. Table 3.3 shows the groove types in welding2 and Fig. 3.3 shows the groove types

© Woodhead Publishing Limited, 2010

Table 3.3  Groove types for welding No. Joint type

Groove type

Thickness (T/mm)

Dimension a/mm

c/mm

b/mm

p/mm

Welding method

a

© Woodhead Publishing Limited, 2010

1

Square ≤3.0 0–0.2 — — — — groove butt joint

2

Corner >1.0 0.2 — — — — Manual or automatic joint tungsten arc welding (with filling wire)

3

Lap joint

4

Single 3–8 0.5–2.0 — — 0.5–1.5 50 º–70 º V-groove butt joint

>1.0





3–4



Manual or automatic tungsten arc welding

— Manual or automatic tungsten arc welding

5 Double ≥20 1.0–2.0 — — 0.8–1.2 60 º V-groove butt joint

Manual or automatic tungsten arc welding

Manual or automatic tungsten arc welding with filling wire

19

20

Welding and joining of magnesium alloys

3.3  Groove types for repairing welding.

in repairing welding. When the magnesium sheet is less than 6 mm thick, ‘activating welding’ of Mg alloys can be used without groove.

3.4

Preheating and postweld treatments for magnesium alloys

There are often some defects in the castings, such as cracks, misruns or blowholes, and these defects have to be welded for repairing. In complex structures, especially in the structures with different thickness sections, the restraint on a weld can be high; therefore it is necessary to supply some preheating process to prevent weld cracking, especially in high-Zn Mg alloys. Local area preheating and entire preheating can be used in different conditions. In local area preheating, the torch can be used to heat a local area. In entire preheating, all the structures can be put into a furnace and be heated at a proper temperature. There are some considerations in preheating Mg alloys: first, the heating temperature must be lower than the maximum temperatures for various alloys; the temperatures are shown in Table 3.4.3 Second, the structures require protection from the atmosphere during preheating, and argon shield is usually used in preheating process. Castings made from alloys are usually heat-treated to T4 or T6 tempers. The weld metal freezes so rapidly that the grains in the bead metal become fine. The time required for complete solution of the second phase in the weld metal is shorter than the time required for the as-cast metal in the casting itself. Grains of the weld metal grow and the mechanical properties decline if heating is continued after solution is completed. Solution heat treating of the entire welded casting structures is not necessary if the casting that has been welded is in the as-cast condition before welding. It is necessary only to weld on castings that have already been raised to temperature to either T4 or T6 temper. After welding, castings and the weld metals can be put into the solution-heat-treated state in a

© Woodhead Publishing Limited, 2010



Preparation for welding of magnesium alloys

21

Table 3.4  Preheat and postweld treatments for castings Alloy

Temper

Before weld After weld

Max preheat temperature (ºC)

AZ63A T4 T4 380 T4 or T6 T6 380 T5 T5 260 AZ81A T4 T4 400 AZ91C T4 T4 400 T4 or T6 T6 400 AZ92A T4 T4 400 T4 or T6 T6 400 AM100A T6 T6 400 EK30A T6 T6 260 EK41A T4 or T6 T6 260 T5 T5 260 EZ33A F or T5 T5 260 HK31A T4 or T6 T6 260 HZ32A F or T5 T5 260 K1A F F None QE21A T4 or T6 T6 450 QE22A T4 or T6 T6 260 QH21A T4 or T6 T6 450 ZE41A F or T5 T5 315 ZH62A F or T5 T5 315 ZK51A F or T5 T5 315 ZK61A F or T5 T5 315 T4 or T6 T6 315

Postweld treatment Temperature (ºC)

Time (hours)

385 385 +220 220 415 415 415 +215 410 410 +260 415 +220 200 200 200 345 +215 315 +200 315 None 510 +200 530a +200 510 +200 330 +175 330 +175 330 +175 150 500 +130

0.5 0.5 5 5 0.5 0.5 0.5 4 0.5 0.5 4 0.5 5 16 16 16 2 5 1 16 16

a Quench

0.5–1 8 8 8 0.5–1 16 2 16 2 16 2 16 48 2–5 48

in water at 60–85ºC before second heat treatment. Note: T4, solution treatment; T6, solution treatment and artificial aging; F, as-cast condition.

© Woodhead Publishing Limited, 2010

22

Welding and joining of magnesium alloys

short time at the heat-treating temperature, and the recommended procedures are shown in Table 3.4.

3.5

References

1. Chen Z (2004). Mg Alloys. Beijing: Chemical Industry Press. 2. Xu H, Liu J, Xie S (2007). Mg alloy fabrication and processing technology. Beijing: Metallurgical Industry Press. 3. American Society for Metals (1983). Welding, brazing and soldering: metal handbook, 9th edn, Vol. 6. Metals Park (OH): American Society for Metals.

© Woodhead Publishing Limited, 2010