Mechanical Properties

The mechanical properties of zinc alloy diecastings depend not only on alloy composition but also on casting conditions  that affect the cooling rate. Conditions that promote rapid cooling in the die give the highest strength and hardness. Hence thin wall sections are proportionally stronger than thick. The tensile properties in the table of “Nominal Properties” below represent typical values for a sound casting around 2mm thick. Properties at different thicknesses can be judged from subsequent tables. The nominal properties are “original” values measured about five to eight weeks after casting. Properties change over time; very slowly at ambient temperatures, or more quickly at elevated temperatures. For alloys ZP3, ZP5 and ZP8 tensile strength and hardness decline while elongation increases. The elongation figure for ZP2 tends to fall over the first few weeks or months of room temperature ageing, after which it begins to increase slowly.

The relative performance of zinc alloys and other common materials are shown in the “Comparison with Alternative Materials” section.

The properties in the various tables below were obtained from a variety of different studies using test specimens of differing designs made under differing conditions. Caution should be taken when comparing the results from different sets of results unless the test specimens and test conditions are identical. Nevertheless despite the potentially wide differences in the casting conditions used for the various specimens tested, the as cast tensile strengths are surprisingly consistent for specimens of similar thickness. It should be born in mind however that the various investigators would have made sure that the test specimens were “sound”. Practically all zinc alloy diecastings contain some porosity, and “sound” is therefore not an absolute term. In zinc pressure die-casting, the porosity originates from two sources. Firstly, from trapped air that is unable to escape from the runner system plus mould which are themselves sealed and filled by molten metal in milliseconds. Secondly, shrinkage porosity is naturally formed as the hot liquid metal turns to its cooled solid state. Provided the porosity level is between, say, 1% and 5% and the individual pores are very small it is not significantly weakening. The actual distribution of porosity is at least as important as the nominal percentage porosity. As a result designers and diecasters should attempt to ensure soundness in strength critical areas of castings, porosity elsewhere is not so important, at least not from a structural integrity point of view.

With the increasing use of computer-aided engineering has come a demand for closer specification of materials properties. In the past simple statements of strength values were considered adequate without statement of the specimen dimensions and the test conditions. As a result of this change in demand the properties of zinc diecasting alloys and many other materials are currently undergoing re-evaluation. In the tables below those property values that are related to a fully specified test are shown in bold font.

Because zinc is a low melting point material, zinc alloy castings are liable to creep. (Creep is defined as the time-dependent strain that takes place under constant load.) Where zinc alloy diecastings are to be employed in situations where they experience long term loading, especially at elevated temperatures, the effects of creep may need to be taken into account. The creep resistance of alloys ZP8 and ZP2 are superior to that of ZP5, which in turn is superior to ZP3. Information that will allow estimation of the amount of creep deformation that will occur to a proposed casting design, under a given set of load, temperature and design life parameters is included in the Creep Properties section.

Reference 2

Nominal Mechanical Property Values

As Pressure Diecast, Tested at 20^o Celsius.
Property values that are related to a fully specified test are shown in bold.

Alloyother names	ZP3Alloy 3, Zamak3, ZP0400, ZnAL4	ZP5Alloy 5, Zamak5, ZP0410, ZnAl4Cu1	ZP2Alloy 2, Zamak2, ZP0430, ZnAl4Cu3	ZP8ZA8, ZP0810, ZnAl8Cu1	GDSLZP0730, ZnAl7Cu3	Acuzinc5ZP0350, ZnAl3Cu5	EZAC	HF alloy
Mechanical Properties
Ultimate Tensile Strength: (MPa) (1) (6)	315	331	397	386,8	387	407,79	414	276
Yield Strength – 0.2% Offset: (MPa) (1) (6)	276	295	360	318,6	315	337,84	393	234
Elongation: % in 70mm (1) (2) (6)
ε at UTS	2,37	3,09	2,21	2,53	1,4
ε at rupture	7,73	3,43	5,99	3,41	1,5	6	1	5,3
Modulus of Elasticity (Young): (GPa) (1)(4)(6)	84,3	84,5	84,4	82,7	84		85	84,3
Shear Strength: (MPa) (7)	214	262	317	275	245	280
Poison coefficient (8)	0,27	0,27	0,27	0,29
Hardness: Brinell (1) (6)	97	114	130	95-110	119	115	140	93
Impact strength
Charpy Unnotched 20°C: J/cm2 (6)	116	131	96		162,5			95
Charpy Notched 20°C: J/cm2 (6)	4	5	9
Fatigue Strength Rotary Bend (MPa) (3) (7)
5×108 cycles	48	57	59	103
1×107 cycles	81	86	104	63		139
Compressive Yield Strength: MPa (5)
0.2% Offset: MPa	274	199	257	233

Notes:
(1) Flat test specimen, thickness 1,5mm; Property measured 8 weeks after casting. Test speed 10mm/minute.
(2) Strain measured with extensometer. L0 =70 for alloy 2, 3, 5, ZA8 and GDSL. L0=50mm for other alloys.
(3) Fatigue test sample diameter = 5,55mm. Fatigue limit determined with staircase method
(4) Young modulus of elasticity calculated from stress strain graph.
(5) The test specimen is a cylinder of 0.24” diameter and 0.48” height. The cylinders were machined from diecast impact test samples. Test speed: 1,1 MPa/sec. Strain is based on crosshead displacement.
(6) Umicore R&D (ref.5)
(7) ILZRO (ref.3)
(8) Umicore R&D/Brescia Univ. (ref. 6)

Stress vs Strain Curves

Reference 5

Mechanical Properties vs Test Temperature

Tensile Strength (MPa) vs Test Temperature
Thickness 1.5 mm, Ageing 8 weeks, Lo 70mm,
Test speed 10mm/min, Specimen cooled in air

TemperatureoCelsius	ZP3	ZP5	ZP2	ZP8
80	218	232	296	260
60	260	277	338	308
40	293	317	376	356
20	308	331	397	387
0	340	347	431	406
-20	345	365	449	412
-40	360	368	443	422

The above table is illustrated graphically below

Reference 5

The Effect of Test Temperature on the
Mechanical Properties of Alloys ZP3 and ZP5

Property	TemperatureoCelsius	ZP3	ZP5
UTS MPa	95	200	250
	40	250	300
	20	280	340
	0	300	380
	-40	320	380
Elongation % F break	95	15	12
	40	8	6
	20	5	4
	0	4.5	4
	-40	2.5	1.5
Impact Strength J/cm26.18mm square bar Un-notched	95	135	143
	40	140	150
	20	140	145
	10	100	135
	0	25	130
	-10	11	60
	-20	8	12
	-40	7	8
Impact Strength J/cm26.18mm square bar 2mm 45o V notch	70	11	12
	20	2.9	4.1
	0	1.7	2.2
	-25	1.4	1.5
	-55	1.2	1.1
Hardness VPN	95	43	62
	40	68	89
	20	82	90
	0	82	100
	-20	87	105
	-40	91	107

Reference 7

The Effect of Test Temperature on the Impact Strengths of
Alloys ZP3, ZP5 and ZP2

Property	TemperatureoCelsius	ZP3	ZP5	ZP2
Impact Strength J/cm2 6.35mm square bar Unnotched (0.40cm2)	80	112	103	116
	40	106	111	104
	20	116	131	96
	0	44	130	55
	-10		25
	-20	11	13	13
	-40	6	6	5
Impact Strength J/cm2 6.35mm square bar 2mm 45o V notch	80	13.3	13.6	12
	40	11.5	10.9	8.2
	20	5.1	6.9	4.2
	0	2.4	2.4	3.2
	-20	2.1	1.6	1.75
	-40	1.6	1.1	1.5

The Effect of Test Temperature on the Impact Strengths of
Alloys ZP3, ZP5 and ZP2

6.35mm square bar un-notched (0.40cm²)

6.35mm square bar 2mm 45^o V notch

Reference 5

Tensile Properties vs Wall Thickness

Tensile Strength (MPa) vs Wall Thickness

		ZP3	ZP5	ZP2	ZP8
0.76mm	As Cast	333	374	416	421
	Aged	296	327	345	394
1.52mm	As Cast	301	337	387	374
	Aged	274	301	356	360
2.14mm	As Cast	276	312	353	369
	Aged	251	275	329	353

Aged = 4hrs at 100^oC

Reference 3

Alloy ZP3, Tensile Properties vs Wall Thickness
Specimens cooled in air, Aged 8 weeks, Lo 70 mm, Test speed 10 mm/min, 10 tests

Thickness		0.2% Yield Strength MPa	σ	UTSMPa	σ	Elongation% F Break	σ	Elongation% F max	σ
0.75mm (200C)	max	295	6.22	337	5.74	10.4	1.53	2.72	0.24
	min	276		317		6.36		2.01
1.5mm (200C)	max	272	5.65	312	4.37	8.15	1.59	2.40	0.1
	min	253		296		4.22		2.16
3mm (200C)	max	225	2.8	292	5.9	8.75	2.2	4.7	0.8
	min	216		270		2.23		2.1

σ = standard deviation

Alloy ZP5, Tensile Properties vs Wall Thickness
Specimens cooled in air, Aged 8 weeks, Lo 70 mm, Test speed 10 mm/min, 10 tests

Thickness		0.2% Yield Strength MPa	σ	UTSMPa	σ	Elongation% F Break	σ	Elongation% F max	σ
0.75mm (200C)	max	342	4.6	387	4.7	4.4	0.7	2.4	0.2
	min	327		371		2.0		1.7
1.5mm (200C)	max	303	7.0	336	3.1	5.05	1.05	4.46	0.88
	min	288		317		2.17		2.07
3mm (200C)	max	280	2.9	333	8.2	7.1	1.6	6.5	1.4
	min	272		311		1.8		1.7

σ = standard deviation

Alloy ZP2, Tensile Properties vs Wall Thickness
Specimens cooled in air, Aged 8 weeks, Lo 70 mm, Test speed 10 mm/min, 10 tests

Thickness		0.2% Yield Strength MPa	σ	UTSMPa	σ	Elongation% F Break	σ	Elongation% F max	σ
0.75mm (200C)	max	338	15.4	430	14.8	9.5	2.4	1.8	0.2
	min	336		386		1.6		1.1
1.5mm (200C)	max	378	9	403	4	8.51	1.3	4.84	1.4
	min	351		392		4.10		1.2
3mm (200C)	max	330	7.4	376	10.8	4.5	2.6	4.3	1.2
	min	313		345		1.0		1.0

= standard deviation

 Alloy ZP8, Tensile Properties vs Wall Thickness
Specimens cooled in air, Aged 8 weeks, Lo 70 mm, Test speed 10 mm/min, 10 tests

Thickness		0.2% Yield Strength MPa	σ	UTSMPa	σ	Elongation% F Break	σ	Elongation% F max	σ
0.75mm (200C)	max	357	11	430	7	6.2	1.1	2.0	0.1
	min	329		408		2.2		1.7
1.5mm (200C)	max	331	7	397	5	4.2	0.5	2.8	0.2
	min	307		378		2.5		2.2
3mm (200C)	max	304	16	357	13	3.3	0.6	3.0	0.6
	min	251		318		1.4		1.4

σ = standard deviation

Reference 5

The strength values in the four tables above are illustrated graphically below

Effect of Ageing at Ambient and Elevated Temperatures

Effect of Ageing at Room Temperature on Tensile Strength (MPa)
1.5 mm thickness, Lo 70 mm, Test speed 10mm/min

Time	ZP3	ZP5	ZP2	ZP8
8 weeks	308	331	397	387
3 years	281	314	471
4 years	281	320	477
5 years	282	314	474
10 years		293
12 years				314
13 years			356
14 years	274

Reference 5

The data in the table above is displayed graphically below

Effect of Ageing at Elevated Temperature

Properties After 10 Days at 95^oC, tested at 20^oC
Lo 70 mm, Speed 10mm/min, Specimen thickness 1.5mm

Short Designation	ZP3	ZP5	ZP2	ZP8
Tensile Strength MPa	271	291	326	303
Yield Stress (0.2% offset)	218	241	249	230
Elongation % F break	8.6	7	1.5	6.1
Elongation % F max	2.6	3.8	1.5	2.4
Hardness Brinell (500-10-15)	74	83	97

Reference 5

The figures in the above table are presented graphically below

Effect of Ageing at Room Temperature on
Tensile Strength vs Test Temperature (Mpa) of ZP3 and ZP5

Test Temperature oC	ZP3		ZP5
	Aged 6months	Aged 3 years	Aged 6 months	Aged 3 years
70	207.5	205.5	255.1	246.8
20	268.9	261.3	316.5	304.1
0	290.3	284.1	338.5	330.3
-25	310.3	302.7	356.5	347.5
-55	330.3	324.1	358.5	355.1

Reference 3

Vickers Hardness of Listed Alloys as a Function of Ageing Time at 100^oC
for Three Thicknesses

Reference 3

Reference 8

Reference 8

Creep Properties

Creep is defined as the time-dependent strain that takes place under a given constant load. Three stages of creep can be distinguished.

1. Primary Creep -the creep strain that occurs at a diminishing rate.
2. Secondary Creep -the creep strain that shows a minimum and almost constant rate.
3. Tertiary Creep -the creep strain that exhibits an accelerating rate, usually leading to rupture.

The elongation (strain) behaviour of zinc alloys is a non-linear function of stress, service temperature, and time under stress. When zinc alloys are subjected to continuous loads, for any considerable period of time, actual tests or empirical mathematical relationships between elongation, stress, time at the given load, and service temperature must be used to predict the strain behaviour of each alloy. The elastic (Young’s) modulus of an engineering alloy, defined as stress divided by strain, is often used in design calculations. For zinc alloys, the value of the modulus depends upon stress, strain rate, and temperature, and is determined from short-term tensile tests. The value of the elastic modulus is valid only for the stress, strain rate, and temperature conditions under which it was measured. When short-term stress versus strain curves are available, the elastic modulus can be determined easily and elastic deflections estimated. However, under sustained loads, zinc alloys experience additional irreversible extension and the use of formulae, as set forth in this section is required to estimate the strains that result at given stresses.

The creep behavior of pressure die cast alloys ZP3 and ZP8 with 2 mm wall section can be modeled using the equation below. It is valid over the temperature range of 25°C to 120°C for these alloys. The formula, which relates the maximum design stress σ (MPa) to temperature T (°K), service life t (time, seconds), and strain, was found to be:

Ln σ = C’+Q/RT – Ln t
n

Here, n is the stress exponent = 3.5
Q is the activation energy = 106kJ/mol
R is the gas constant = 8.3143 x 10^-3 kJ/mol °K
C’ = a constant for the given allowable strain (varies with strain for each alloy, as shown below)

Using this equation, it is possible to calculate the permissible combinations of stress, temperature, and creep life for any creep strain within the range of 0.2 to 1%. The equation is valid for stresses up to 50 MPa and temperatures up to 120°C. (The equation has been found to give times to a given elongation that correlate well with all known experimental data).

Creep extension in most alloy systems can be highly inconsistent. Zinc is no exception, and apparently identical samples can vary from the mean creep life (or strain) by a factor of two. Thus whether the creep life to a given strain is determined from actual tests or using the formula described previously, an additional factor of at least two should be applied to either the life or the strain to take into account such individual variations.

Service at elevated temperatures results in aging of all zinc alloys. When the effect of room temperature aging on creep strength of alloy 3 was investigated, it was found that creep strength decreased under both conditions. Testing under high loads showed that the creep strength decreased as much as 16% within two years of aging at room temperature or by heating at 75°C to 95°C for three days. It thus has been suggested that a factor of 0.8 be applied to permissible stress values computed from the equation given above.

The creep rates for alloy ZP5, up to 100°C, can be estimated by using the figures shown in this section for alloy ZP3 and adding 10°C to temperatures given. That is, alloy ZP5 provides approximately equivalent creep behavior at temperatures 10°C higher than those shown for Alloy ZP3.

Limited creep testing undertaken on alloy ZP2 indicates that its performance is equivalent to alloy ZP8. Research is continuing to identify C’ vs strain curves for alloys ZP5 and ZP2.

Reference 3 and Reference 2